Many aspects of sleep change gradually from infancy to old age. A variety of primary sleep disorders that are relatively uncommon in younger age groups become more common in old age, and many medical conditions that increase in prevalence with age also disrupt sleep. Even those sleep changes that are typical of healthy older people may be problematic for many. Thus, there is a high probability that older people will complain about the quality of their sleep. Addressing these problems requires an understanding of normal sleep patterns at different phases of the life span, of the pathologies of sleep that arise during aging, and of the treatment options available.
The structure of sleep
Sleep can be defined in many ways—behavioral, subjective, physiological—but the standard definitions of sleep and of its internal structure are derived from the patterns of electrical activity in the brain, which are recorded as an electroencephalogram (EEG) using surface electrodes on the head. EEG recordings during sleep reveal gradual, cyclic changes during the night in both the background frequencies and transient electrical events. These, in combination with recordings of muscular electrical activity, are used to define a number of standardized stages of sleep.
The five stages of sleep include the rapid eye movement (REM) stage and stages 1–4, which are the non-REM (NREM) stages. REM sleep dominates during the earliest stages of human development, but NREM increases during childhood, with the gradual emergence of the deepest NREM stages (3 and 4), characterized by the presence of slow EEG waves (delta waves). Newborn babies spend about 50 percent of their sleep time in REM sleep. REM sleep is typically stabilized by adolescence and accounts for 20 to 25 percent of sleep time. The precise age at which the deepest stage of NREM (stages 3 and 4) occurs has not been conclusively determined but evidence suggests a decline starting by age 20. Stages 3 and 4 are collectively called delta- or slow-wave sleep (SWS). In healthy, young adults, there is an orderly progression during sleep from the shallowest (stage 1) to deepest (stages 3 and 4) stages of NREM, followed by a period of REM sleep. The sleep pattern cycles back through stage 2 and then to REM regularly four to six times during the night, with a cycle length of about 90–120 min. NREM sleep, including SWS, dominates the first half of the night, while the second half includes more REM sleep and little or no SWS. In young adults, stage 2 occupies about 50 percent of the sleep period, and REM about 25 percent.
SWS is considered a deep-sleep stage because quite intense external stimuli are needed to arouse an individual from SWS, and reports of preceding mental activity after awakening from SWS are very limited. It is relatively easier to arouse people from REM sleep, and dream reports after arousal are vivid and often bizarre. NREM stages are characterized by reductions in physiological activity and more difficult arousal, while REM presents a picture of chaotic physiological activity and easier arousal. Typical REM features include rapid or unstable heart rate, respiration and temperature change; rapid horizontal eye movements; twitching of the extremities and facial muscles; and penile erection in males. The EEG pattern resembles that of an awake, aroused individual, despite continued sleep and a profound loss of muscle tone in the major postural muscles.
Sleep changes during aging
After middle age, there is a decline in the duration of SWS, especially in men, from 20 percent to 5 percent or less of total sleep time. There is some debate as to whether this change reflects a significant physiological process, or whether it reflects the inappropriateness of using the standard criteria to define SWS in older people. Amounts of stage 1 sleep increase at this age, perhaps reflecting more nighttime arousals and lighter sleep. REM duration remains relatively constant after early childhood, but its timing changes with age. After about fifty years of age, there is a shortening of the latency (delay from sleep onset) to the first REM episode of the night, perhaps associated with the reduction in SWS durations early in the night.
Although the total duration of sleep may not change dramatically, older people redistribute their sleep throughout the twenty-four-hour day-night cycle. Naps increase in frequency, and sleep during the night tends to become more fragmented and interrupted by longer periods of waking. In contrast to healthy young adults, healthy elderly people may spend only 80 percent of their bedtime at night asleep. Older people also show a preference for both earlier bedtimes and awakening times. It remains unclear to what degree increased napping reflects reduced social pressure to stay awake, compensation for disrupted sleep at night, or a spontaneous change in the daily rhythms of sleep.
An internal daily (circadian) clock regulates the expression of daily rhythms, including the rhythm of sleep and waking. Changes in clock function with aging, including reduced strength (amplitude) of the circadian signal and disrupted rhythm organization, may contribute to changes in sleep habits. These alterations may reflect anatomical changes in the hypothalamic mechanisms that are responsible for circadian rhythm generation.
Sleep disorders during aging
One of the most common sleep disorders is sleep apnea —the cessation of breathing during sleep for periods from seconds to minutes. Apnea may result from a loss of respiratory effort (central sleep apnea) or, more commonly, by an upper airway obstruction (obstructive sleep apnea), usually accompanied by loud snoring. An apneic episode usually ends with arousal to wakefulness and a gasping intake of breath. The resulting sleep disruptions lead to poor sleep and excessive daytime sleepiness (EDS). The incidence of apnea increases with age in both sexes, but it is more common in men, especially if they are overweight. Among people over age sixty-five, 24 percent have sleep apnea.
Sleep apnea and snoring have been implicated as secondary causes of morbidity and mortality in patients with cardiac and cerebrovascular disease, probably because of increased hypertension, lowered brain oxygen levels, and irregular heartbeats. EDS secondary to sleep apnea may be a serious and important risk factor for motor vehicle and other accidents. Use of sedative or hypnotic agents in undiagnosed apnea patients may exacerbate breathing problems and may even be fatal. Treatments for sleep apnea include continuous positive airway pressure (CPAP) to open up collapsed airways, weight loss, reduction of alcohol and sedative use, surgery, and dental devices.
Behavioral disorders associated with sleep are common, but increase further with age. A major concern is that these disorders disrupt sleep and lead to EDS, as well as decrements in daytime performance, social interactions, and physical and psychological health. Restless legs syndrome (RLS), is a disorder marked by a restless, ‘‘crawling’’ sensation in the legs that creates an irresistible urge to move them. Walking, massage, leg movements, or cold-water immersion may temporarily relieve the symptoms, but these are incompatible with sleep. Most RLS patients also have periodic limb movement (PLM) disorder, though this disorder can also occur independently. While PLM disorder is rare in those under thirty years of age, it occurs in approximately 45 percent of those over age sixty-five. PLM disorder involves repetitive movements of the feet and legs in bouts lasting several minutes. These occur frequently in stage 2 and often disrupt sleep. PLMs have been observed in patients with medical conditions such as uremia or diabetes, in patients with sleep apnea and narcolepsy, and in relation to the use or withdrawal of some drugs.
REM-sleep behavior disorder involves agitated movements during sleep in response to vivid dreams, resulting from a lack of the normal inhibition of muscle tone during REM. Trigger dreams often include themes of fleeing or fighting, resulting in the sleeper showing vigorous punching, kicking, and other movements, which may lead to injury to the sleeper or bed partner. While this condition is relatively rare, it increases in prevalence in males over age sixty. Pharmacological treatments may reduce but not eliminate these behavioral disorders of sleep.
Insomnia, or insufficient sleep, is characterized by self-reports of unsatisfactory sleep, daytime fatigue, and social or work impairment. Many factors (medical, psychological, environmental) contribute to insomnia, and it takes several different forms. Women complain more often of insomnia than men, especially during and after menopause. While younger adults typically show initial insomnia (difficulty falling asleep), older people tend to have difficulty with early awakening and sleep maintenance during the night. Among adults over sixty-five, 29 percent complain of problems maintaining sleep.
Medical conditions and sleep disruption
Medical and psychiatric conditions may disrupt sleep, and pharmacological treatments for these conditions are often unrecognized contributors to sleep disruption. Pain is a common symptom of many acute and chronic illnesses that increase in frequency with aging. Conditions such as arthritis, cancer, cardiovascular disease, and musculoskeletal degeneration or injury may be accompanied by pain. Pain can disturb sleep if it is not adequately controlled, and it may be exacerbated by the postures usually adopted during sleep.
Patients with heart disease may awaken out of REM sleep suffering from angina (chest pain) or chest tightness because of changes in heart rate and breathing that occur in this sleep stage. Symptoms of respiratory illnesses (asthma, emphysema) and gastrointestinal conditions (acid reflux) typically worsen during the night and contribute to sleep disruption. Stroke, a common condition in elderly people, may lead to insomnia or daytime drowsiness, depending on a variety of antecedent conditions and the location of the brain damage. In addition, infectious diseases may have a greater impact on older people and may affect sleep patterns.
Many of these serious medical conditions are accompanied by anxiety or depression, both of which can further disrupt normal sleep patterns. Independent of specific medical conditions, both anxiety and depression have important impacts on sleep quality and quantity. Anxiety, and accompanying muscle tension and pain, may make it difficult to initiate and to sustain sleep. Depression often leads to early morning awakening, awakenings during the night, reduced levels of SWS, and a shortened latency to the first nightly REM period.
Several progressive, dementing illnesses (e.g., Alzheimer’s, Parkinson’s, and Huntington’s diseases) increase in both prevalence and severity with increasing age, and they can amplify sleep disruptions in older people. Alzheimer’s disease is the most common of these conditions, and is characterized by disturbances in daily rhythms, including disrupted sleep, daytime napping, and periodic agitation, especially in its later stages. Some studies indicate that the amounts of both SWS and REM sleep decrease in patients with Alzheimer’s, but other studies have not confirmed this observation. Disruption of sleep and daily rhythms in patients with dementia places a further burden on caregivers at home, who may consequently become sleep deprived. The occurrence of nocturnal activity and wandering is often a major consideration in the decision to institutionalize Alzheimer’s patients.
Behavioral treatment of sleep disorders
A number of behavioral approaches should be considered first in treating sleep disturbances in any age group. These can be summarized as maintaining appropriate ‘‘sleep hygiene.’’ Included in this concept are:
- avoidance of caffeine, alcohol, and nicotine (all of which disrupt sleep), especially in the second half of the day
- assessing the effects of prescription drugs on sleep and modifying these as appropriate
- maintaining a regular bedtime and wake time throughout the week
- avoiding daytime napping, except for a regularly scheduled, early nap, which may be beneficial for some in reducing daytime sleepiness
- maintaining a relaxing evening routine in preparation for bedtime, which may include reading, meditation, work on a quiet hobby, and a warm bath (except for those for whom warm baths are contraindicated)
- use of the bedroom only for sleep or sex
- regular, moderate exercise, but not within four hours of bedtime
- reduced fluid intake late in the day to avoid frequent awakening to urinate during the night
- reduced noise or light in the bedroom, if these disturb sleep, or separate bedrooms if a bed partner’s snoring or movements disturb sleep
- an extra pillow to elevate the head in order to reduce symptoms of nocturnal acid reflux
If sleep has been chronically disrupted for whatever reason, people may develop conditioned responses to the bedroom environment that preclude sleeping. Worrying about whether one will be able to sleep is itself a common cause of poor sleep. A deconditioning approach can be used to learn to associate the bedroom with sleep rather than with anxiety about sleep. An individual undergoing deconditioning is instructed to go to bed only when sleepy and to get out of bed and go to a different room if sleep does not follow within a short, fixed interval, returning to the bed only when sleepy.
Pharmacological treatment of sleep disorders
When sleep disruptions are severe and unresponsive to behavioral and environmental strategies, a pharmacological approach may be warranted, but these must be acknowledged to be symptomatic and not a cure for the underlying causes of sleep disturbances. Because of the high incidence of insomnia among older people, they are frequent consumers of both prescription and nonprescription sedatives. However, since they typically have increased sensitivity and reduced ability to metabolize these same drugs, physicians must monitor these treatments carefully to avoid overdoses and increased risks of falls, accidents, and daytime confusion or cognitive problems.
Barbiturates are rarely used today to aid sleep because of high risks of toxicity, tolerance, dependence, and the potential for life-threatening interactions with other medications. Benzodiazepines are currently the most commonly prescribed medications for sleep problems. Relative to barbiturates, they have a wider safety margin between effective and toxic doses and less potential for development of tolerance (the need for a progressively larger dose to achieve equivalent effects). They can, however, pose serious health threats when used in conjunction with sedating medications or alcohol, especially in older people. Benzodiazepines increase stage 2 sleep but reduce amounts of SWS; thus, they may aid sleep onset and/or maintenance but alter its characteristics.
Different benzodiazepines may be short-, intermediate-, or long-acting. Short-acting benzodiazepines can reduce the time it takes to fall asleep, but may not aid sleep maintenance, and thus may exacerbate early-morning insomnia. Intermediate-acting drugs have effects that last throughout the night and into early morning. Long-acting benzodiazepines are also effective in both initiating and maintaining sleep, but may lead to daytime sedation and impaired cognitive and psychomotor performance. These residual or hangover effects may contribute to increased confusion, falls, and memory disturbances. Another serious problem associated with prolonged use of benzodiazepines is rebound insomnia—an increase in sleep disruption after withdrawal of the drug—which may be worse than the original sleep complaint. Thus, benzodiazepines are best used as a short-term sleep aid, rather than as a long-term maintenance strategy for sleep.
Benzodiazepines are not the sedative of choice in two situations. When insomnia arises from depression, an antidepressant with sedative properties may be a preferred treatment. The antidepressant effect is typically delayed for two weeks or more, but the sedative property can improve sleep shortly after initiating treatment. Patients with symptoms of sleep apnea should avoid sedating medications because these have respiratory depressant properties, which may worsen the symptoms of sleep apnea.
Nonbenzodiazepine sedatives, which have been marketed in some countries, act on similar brain mechanisms, but in a different way. These drugs (zopiclone and zolpidem) are short-acting and have been reported to have fewer unwanted effects than benzodiazepines. They cause less residual daytime anxiety than some short-acting benzodiazepines, fewer cognitive effects than longer-acting drugs, appear to have little abuse potential, and are reported not to generate rebound insomnia after discontinuation. Because of their short action, they are useful for sleep initiation, but they are less useful for treating early morning insomnia.
Benjamin Rusak Peggy Ruyak
See also Alzheimer’s Disease; Brain; Depression.
Ashton, H. ‘‘The Effects of Drugs on Sleep.’’ In Sleep. Edited by R. Cooper. London: Chapman and Hall, 1994. Pages 175–211.
Bliwise, D. ‘‘Dementia.’’ In Principles and Practice of Sleep Medicine, 3d ed. Edited by M. Kryger, T. Roth, and W. Dement. Philadelphia: W. B. Saunders, 2000. Pages 1058–1071.
Bliwise, D. ‘‘Normal Aging.’’ In Principles and Practice of Sleep Medicine, 3d ed. Edited by M. Kryger, T. Roth, and W. Dement. Philadelphia: W. B. Saunders, 2000. Pages 26–42.
Carskadon, M. A., and Dement, W. C. ‘‘Normal Human Sleep: An Overview.’’ In Principles and Practice of Sleep Medicine, 3d ed. Edited by M. Kryger, T. Roth, and W. Dement. Philadelphia: W. B. Saunders, 2000. Pages 15–25.
Chokroverty, S. ‘‘Sleep Disorders in Elderly Persons.’’ In Sleep Disorders Medicine: Basic Science, Technical Considerations, and Clinical Aspects. Edited by S. Chokroverty. Boston: Butterworth-Heinemann, 1994. Pages 401–415.
Foley, D. J.; Monjan, A. A.; Brown, L. S.; Simonsick, E. M.; Wallace, R. B.; and Blazer, D. G. ‘‘Sleep Complaints Among Elderly Persons: An Epidemiological Study of Three Communities.’’ Sleep 18, no. 6 (1995): 425–432.
Ohayon, M. M.; Caulet, M.; and Priest, R. G. ‘‘Violent Behavior during Sleep.’’ Journal of Clinical Psychiatry 58 (1997): 369–376.
Rechtschaffen, A., and Kales, A., eds. A Manual of Standardized Terminology, Techniques and Scoring System for Sleep Stages of Human Subjects. Los Angeles: UCLA Brain Information Services/ Brain Research Institute, 1968.
SLEEP , as a periodic, recurrent state of inactivity and altered consciousness, marks a boundary line in human experience. However sleep is culturally evaluated and understood, it is a state quite different from ordinary, waking life. As such, it has been a universal object of religious interest and imagination. The various traditions, symbolisms and rituals of sleep are closely related to religious understanding of night and the role of dreams, to assessments of death, and to themes associated with its apparent opposites: dawn and awakening.
Mythologies of Sleep
Figures and notions associated with sleep appear in numerous mythologies and folk traditions.
Personifications of sleep
While personified figures of sleep appear in mythology, poetry, and artistic representations, they are rarely themselves the focus of cultic activity. The personifications often reveal the ambiguous assessment of sleep: it is peaceful and restorative, and it is like death. For example, while the early Iranian Vispe Ratavo (7.4) can enjoin the worship of sleep, sleep is more usually understood within that tradition to be a negative force controlled by the demon Bushyasta; in India, the ambivalent deities Rudra (Kaivalya Upaniṣad 6–9) and Śiva (Uṇādi Sūtra 1.153) are identified as lords of sleep.
The most developed personification of sleep occurs within the Greco-Roman tradition with the archaic figure of Hypnos (Lat., Somnus). Schematically, the poet Hesiod (eighth century bce) located Hypnos and his brother, Death (Thanatos), along with the race of Dreams, as the asexually produced children of Night. Sleep is the friend of man; Death, his pitiless adversary (Theogony 211–213, 756–766). In other poetic and artistic materials, the fraternal relations of Sleep and Death are further developed; they are twin brothers (Iliad 14.231). Hypnos or Somnus is personified as a small winged bird, an infant, or a young warrior. In some traditions, Sleep carries a horn or a poppy-stalk from which he drips a liquid that causes slumber. The only recorded instance of a regular cult of sacrifice to Hypnos is at Triozen (Pausanius, 2.31.3); rather it is Hermes, in his role as conductor of dreams, who was the object of nightly libations and petitions for a good sleep.
At its most complex level, the notion of sleeping gods is tied to cosmic cyclical patterns of the periodic dissolution and recreation of the world. Thus Viṣṇu falls asleep on the back of the cosmic serpent (Śeṣa or Ananta) at the end of each world-age. In late Puranic texts, Nidrā, the personified goddess of sleep, is depicted as entering Viṣṇu's body. Visnu sleeps until Brahmā commands Nidrā to depart so that Viṣṇu might awake and recreate the cosmos (Mārkaṇḍeya Puraṇā 81.53–70). Viṣṇu also is represented as undergoing an annual period of sleep. During the monsoon months, beginning in June and July, Viṣṇu—and therefore the world—sleeps (Padma Puraṇā 63, 125). In late traditions, Indra is thought to perform Viṣṇu's functions during this period (Harivaṃsá 50.26).
The notion of a deity undergoing periodic durations of sleep is not uncommon and may be expressed in ritual as well as myth. Thus there was an annual Syrian ritual of the "awakening" of Melqart-Herakles (Josephus Flavius, Jewish Antiquities 8.146), while, from earliest times through the Roman period, the daily Egyptian temple service began with a hymn awakening the sun (Pyramid Text 573).
However, gods are most commonly celebrated for being sleepless and hence all-seeing. YHVH (Psalm 121:3–4), the Adityas (Ṛgveda 2.27.9), Mitra (Ṛgveda 3.59.1), Ahura Mazdā (Vidēvdāt 19.20), Sraosha (Yasht 11.10–12), and others are praised for never sleeping. A special motif is that of a god with multiple eyes, some of which are always open, while others are asleep, thereby guaranteeing their omniscience (for example, Argos and El-Kronos in the account of Philo of Byblus). The opposite motif, that of the sleeping and therefore powerless deity, is represented by Elijah's taunts to the priests of Baal (1 Kgs. 18.27–28) and east European Christian dualistic creation myths that have the Devil working while God sleeps during the sabbath that followed creation.
Sleep in heroic tales and folklore
One of the more persistent folkloristic themes is that of a lengthy sleep, a Rip Van Winkle motif in which sleep serves as a sort of suspended animation for a period of years. While occasionally the emphasis is on perpetual sleep, conferred as either a punishment or a boon (both are claimed in different accounts of the Greek hero Endymion), there is, more frequently, a terminus. Best known is the widely distributed eschatological tale of the Sleeping Emperor, asleep within a cave or mountain. Frederick Barbarossa is held to be asleep within the Kyffhäuser, seated at a marble table surrounded by his knights, awaiting his awakening, when he will lead Germany in a glorious battle and usher in a golden age. The same sort of claim is made for Charlemagne asleep in a hill near Paderborn, Wittekind in Westphalia, Siegfried in Geroldseck, Henry I in Goslar, Thomas of Erceldoune in the Eildon Hills, and others. These beliefs may at times fuel apocalyptic movements, as in the case of the flagellants of Thuringia, led by Konrad Schmid in the fourteenth century. The same motif occurs in the hagiography of wise men: Epimenides sleeping fifty-seven years in the Dictaean cave near Knossus, Shimʿon bar Yoḥʾai in Palestine for twelve, Zalmoxis in Thrace for three.
The widespread legend of the Seven Sleepers of Ephesus, first recorded in a Western language by Gregory of Tours in the sixth century, has also found its way into the Qurʾān (18.8–25); the legend narrates how the seven Christian anti-idolators slept in a cave for 367 years before being awakened during the reign of Theodosius II. Subsequently, they returned to sleep, not to be awakened again until the general resurrection. Related, but different from this motif of cave hibernation, are the Jewish legends of two figures who sleep during a period of tribulation: Abimelech the Ethiopian, who slept for sixty-six years from the destruction of the Temple by the Babylonians; Ḥoni ha-Meʿaggel ("the circle maker"), who slept for seventy years, in one account from the destruction of the Temple, in another, from the period of the conflict between Aristobulus II and Hyrcanus II.
Equally persistent, although not well organized into a narrative scheme, is the motif of magical sleep most familiar from tales such as that of Sleeping Beauty, wherein a potion, a spell, or an object causes unnatural sleep that either cannot be undone or must be undone by countermagic or by an act of accidental or innocent intervention.
In heroic quest-sagas, the hero is often put to a test, one of which is that sleep is forbidden until the quest is accomplished. The best known instance is a negative example: Gilgamesh's failure to stay awake. In such sagas, the hero frequently confronts a sleepless adversary, such as the dragon in the Argonautica who guards the Golden Fleece until it is overcome by Jason through the intercession of Medea and her magic song invoking "Sleep, highest of gods" (Apollonius Rhodius, 4.146).
Soul loss and transformation
A tradition of exceedingly wide distribution is that the soul becomes separated from the body during sleep and that death will result if the individual is awakened or the body moved before it returns. So ubiquitous is this belief that E. B. Tylor thought it one of the basic human experiences that gave rise to religion and James G. Frazer devoted the third volume of The Golden Bough to its ramifications.
Closely related to the theme of the separable soul is the notion that sleep is a time of shape-changing (often expressed in folklore in accounts of were-animals). These traditions are most fully elaborated in shamanism. The shaman is, among other things, an expert in the retrieval of lost souls by being able to achieve a sleeplike trance and pursuing them. In South America, one of the tests for shamanic abilities is that the candidate frequently experiences lengthy periods of deep sleep. Throughout shamanism, the return of the shaman from an ecstatic journey is most commonly described as an "awakening." During his journey, the shaman will frequently change shape. Snorri Sturlson's description of the shamanic attributes of the Norse god Óðinn (Odin) is representative of this related set of elements: while his body lay "as if sleeping or dead," he assumed the form of a bird, animal, fish, or snake and traveled to far lands "on his own or other men's errands" (Ynglingasaga 7).
Rituals of Sleep
Religious practices related to sleep or the interruption of sleep are characteristic of many traditions.
Who sleeps with whom is a central question of religious etiquette. This is not only a matter of sanctioned sexuality. Within many societies, a mark of male adulthood is residential segregation in the men's house, a location of secret rituals forbidden to women. Elements of rituals of initiatory separation focus on this shift in social location. In other societies, the blueprint of a domestic house represents a map of such social and sexual relations. For example, in central Thailand, only certain members of the family sleep in the bedroom; other close relations may enter the room but may not sleep there; guests are restricted to an entrance room, separated from the sleeping room by a clearly demarcated threshold.
Going to a particular sacred place to sleep in order to gain a religious end is common. Most frequently such incubation is for the purpose of gaining a revelation or a cure. The former is to be related to similar phenomena such as the American Indian dream quest; the latter, to the general ideology of shrines. Perhaps the most extensive record of incubation is that by Aelius Aristides in the second century who has left, in his Sacred Teachings, a report of his experiences in a variety of Asklepian shrines over a twenty-seven year period.
Sleep interruption or deprivation
The regular interruption of sleep is a common practice in religious asceticism, especially in monastic communities. Thus the Christian "canonical hours" in which monks would be aroused at either midnight or 2:00 am for prayer. Similar interruptions figure in other spiritual regimens such as the Daoist practice of the "expulsion of the breath" that must be undertaken at least five times during the period of midnight to noon.
Sleeplessness is an often recurring feature of religious asceticism and vision quests. In the lists of five sins that must be "cut off" by a yogin, sleep appears (Mahābhārata 12.241.3). Like the sleeplessness of the hero, sleep deprivation is the mark of a spiritual athlete. The hagiographies of holy persons frequently celebrate either their ability to sleep far less than ordinary people or to do without sleep entirely. For example, the pre-Christian Syrian Stylites were said to resist sleep for seven days while perched on columns. According to legend, if they fell asleep, a scorpion would sting them awake (Lucian, Syrian Goddess 29). In the traditions of the Christian Desert Fathers, preserved in texts such as Palladius's Lausiac History, monks such as Doretheus, Macarius of Alexandria and Pachomius are praised for never sleeping. All-night vigils, spending days and nights immersed in cold water, and sleeping only in a sitting position were frequent ritual means for achieving sleep deprivation. Sleeplessness is also a feature of initiatory ordeals. In Australia, novices are prevented from sleeping for periods of up to three days. The goal in these varied practices appears to be twofold: to transcend the normal bodily processes and to achieve heightened consciousness.
Metaphors of Sleep in Religious Speech
References to sleep in myth and religious literature reflect its metaphorical significance as the state of death, ignorance, or enlightenment.
Sleep as death
In many languages, sleep is a metaphor for death. While this may serve as a euphemism, the connection, as the Greek myth of Death and Sleep as twin brothers suggests, is deeper. At one level, there is the physical resemblance of a sleeping body to a corpse. This association is heightened by burial ceremonies that treat the grave or receptacle as a bed and place the corpse in a position of repose (either prone or sitting). The parallel is stronger in cultures that hold that the soul escapes from the sleeper's body just as it does from the corpse at death. Finally, there is the view that the land of the dead is a land of sleep. To return from the dead is to be awakened.
Some myths of the origin of death lay emphasis on the sleep motif. For example, the Selkʾnam of Tierra del Fuego tell of the repeated attempts of the ancestors, who were tired of life, to achieve a deep sleep. After many failures, another group of ancestors wrap them in blankets and teach them the way of "transformation sleep." After a few days of such sleep (i. e., death), they will either be reborn on earth or, if they do not wish to return, they will be reborn as another kind of being or take up a celestial existence.
Sleep as ignorance
The understanding of sleep as a cessation of consciousness leads readily to the use of sleep as a metaphor for ignorance. In gnostic traditions within a diversity of religions, sleep, forgetfulness and oblivion have become characteristics of earthly existence. Salvation consists of awakening and recollection. In these traditions, the archaic language of the darkness and sleep of the land of the dead has been transferred to ordinary, mundane existence. The waking world of light and consciousness has been transferred to the "beyond."
Sleep as enlightenment
While enlightenment is most frequently expressed in terms of awakening (as in the root budh, "to wake, be awake," which has given rise to such words as Buddha, bodhisattva, and bodhi, "perfect knowledge," all reflecting the sense "to be awake from the slumber of ignorance and delusion"), it can, at times, be expressed in the language of sleep. This is especially the case in mystical systems where lack of consciousness of the world and contact with the supramundane is emphasized. In her various writings, Teresa of Ávila uses terms such as sleep, falling asleep, being numb, repose, languishing, and stupor to describe ecstasy. This builds on the Augustinian tradition that ecstasy is a state midway between sleep and death, more than sleep, less than death, where the soul is withdrawn from the bodily senses (Augustine, De Genesi ad litteram 12.26.53). In the Indic Upaniṣadic tradition, the language of sleep in relation to enlightenment is further developed. The brief Mandukya Upaniṣad presents the common fourfold schematization: (1) waking consciousness, (2) dream consciousness, that is, ordinary sleep, (3) deep sleep (susupti ), a sleep without consciousness or dreams, and, the goal, (4) pure consciousness. Deep sleep is the realm of the "Knower"; the only danger is to confuse this penultimate stage with the true gnosis of the fourth state.
Material on sleep can be gleaned from monographs on related topics—for example, the classic study of incubation by Ludwig Deubner, De incubatione (Leipzig, 1900)—and from the large body of literature on dreams, of which Ernesto de Martino's Il sogno e le civiltà umane (Bari, 1966) is most useful. Still, there is no reliable cross-cultural overview of sleep. The chapter on the interrelationships of sleep, death, and the erotic titled "On the Wings of the Morning: The Pornography of Death" in Emily Townsend Vermeule's Aspects of Death in Early Greek Art and Poetry (Berkeley, 1979) is an exemplary study that needs to be matched for other cultures.
Jonathan Z. Smith (1987)
Historically,there has been concern with two major questions about sleep: Whatspecific mechanisms start, maintain, and terminate sleep? What functions does sleep serve? In addition, there has been considerable interest in the relationship of sleep to learning and performance, aswell as in abnormalities of sleep behavior.
Causes and processes. The oldest theories about the causes of sleep postulatedeit her congestion or anemia of the brain. More recent hypotheses have attributed sleep to arterial anoxemia, endocrine periodicities, functional neural blocks, or the accumulation of fatigue products. Pieron’s idea of a “hypnotoxin” that accumulates during waking and is metabolized away during sleep (1913, p. 520) is one of the best-known explanations. A large number of biochemical studies on blood and other body fluids (reviewed by Kleitman 1939)have failed to turn up evidence in support of any of these hypotheses. The anoxia theory was controverted by the finding thatthe oxygen content of blood was normal during sleep and that cerebralblood flow was actually increased. The toxin theory was seriously challenged by the finding that Siamese twins with a common blood supply failed to show synchronized sleep-wake rhythms. However,knowledge of the biochemical processes taking place within thesleeping brain is not sufficient to accept or reject the causal influence of chemical factors. At present, these and similar theories of the causes of sleep are not in fashion.
Electro physiological mechanisms. Since the 1930s, the development of electrophysiological techniques has facilitated new approaches to the processes of sleep. Interest has focused on the elucidation of the specific brain mechanisms responsible for the onset and termination of sleep. It is now well established that wakefulness results from activity in the ascending reticular system of the brain stem. However, the central mechanisms for the onset and maintenance of sleep are not so well understood.
The reticular activating system, which originates in the reticular formation of thelower brain stem and extends upward to the hypothalamus,subthalamus, thalamus, and cortex, receives collaterals from allsensory path ways as well as from the cortex. Stimulation of thereticular system by any of these sources or by such biological products as nor epinephrine is accompanied by behavioral activationand alertness. Arousal is usually sustained after removal of the stimulus. Lack of stimuli or lack of stimulus variation has so porific effects. [See NERVOUS SYSTEM, article on STRUCTURE AND FUNCTION OFTHE BRAIN.]
Since the publication of Kleitman’sevolutionary theory of sleep, most physiologists have accepted the“stimulus deficiency” explanation of sleep (1939). The onset and maintenance of sleep are said to be a result of reduced afferent stimulation that deprives the cortex of its sensory rawmaterial. The resulting “deactivation” of the reticularsystem is, indeed, an important sleep-inducingmechanism. Everydayexperience indicates that in man and animals the preparations forsleep include the suppression of external stimuli. Destruction of thereticular system produces sleep as well as elec-troencephalographic(EEC) patterns indicative of sleep.
More recent evidence,however, does not fully support this view of sleep as a passivephenomenon. In fact, it strongly suggests the existence of active,sleep-producing mechanisms in the central nervous system.Low-frequency stimulation of several brain locations from the lowerbrain stem to the thalamus produces sleep, in contrast to thearousing effects caused by high-frequency electrical stimulation ofthe reticular system. Barbiturate infusion at most brain levelsinduces sleep, but at some locations induces waking. It is true thatchanges in the internal milieu, such as reduced blood pressure andlow body temperature, contribute to sleep and that familiar orunchanging external stimuli favor sleep, but these conditions are notsufficient to maintain sleep. There is behavioral and physiologicalevidence to support the hypothesis that there are two mechanisms inthe brain for the control of sleep and waking: an activating and adeactivating system. The many degrees of wake-fulness and sleepprobably result from complex interactions between these two systems,involving the interplay of numerous structures in the nervous system.
The sleep—wake rhythms. Most living things show alternatingactivity and quiescence in response to the alternation of day andnight. Higher animals show polyphasic or monophasic and diurnal ornocturnal patterns, depending on their adaptive requirements. These“circadian” (circa, dies) rhythms seem to be determinedprimarily by internal mechanisms but usually can be“reset” by environmental pacemakers, such as night andday. Some biological cycles are remarkably persistent, whereas othersare easily modified by environmental changes.
The cerebral cortexmay have a unique role in the maintenance of sustained sleep andwaking. All animals capable of adjusting their sleep-wake cycles tothe monophasic diurnal rhythm of man have a well-developed cortex.Removal of the cortex in such animals causes reversion to polyphasicsleep cycles.
The sleep-wake cycles of rats, rabbits, puppies,and human infants are polyphasic. Monkeys and higher mammals, as wellas many birds, have mono-phasic diurnal cycles. Attempts to modifythe diurnal rhythm in man include two types of manipulation:alteration of the phase and alteration of the period of thesleep-wake cycle. The phases of sleep and waking can beeasily modified (as in shifts to night work), but attempts to alterthe 24-hour period to days of 12, 48, 21, or 28 hours have had onlylimited success. Some subjects adjusted to 21-hour or 28-hour days,but none adjusted to 12-hour or 48-hour cycles. A practicalrequirement for readjusting the phases of sleep and waking occurswhen travelers move across lines of longitude. World travelers(especially on long-distance flights) experience phase-shiftasynchrony in the day—night cycle, which may cause emotional distressand impaired performance. It seems that about two weeks are neededfor a complete diurnal readjustment of physiological rhythms.
A number of physiological rhythms vary with man’s sleep-wakecycle. Among these are heart rate, skin resistance, and muscle tonus.One measure of considerable interest is the diurnal temperaturecycle. Temperature is normally maximal by day and at its lowest inthe early hours of the morning. Several studies have shown thatperformance efficiency, especially speed, is highly correlated withthe temperature cycle. The diurnal variation of both temperature andefficiency are probably related to the establishment of a circadianrhythm of excitement in the reticular system and hypo-thalamus.Prolonged disregard of normal biological rhythms may lead to genuine,although poorly understood, “rhythm diseases” anchoredin disturbed autonomic functioning. [See TIME, article on PSYCHOLOGICAL ASPECTS.]
Sleep seems to be a parasympatheticphenomenon. During normal sleep, general metabolism slows down. Heatproduction and body temperature decrease, heart rate and respirationdecrease, blood pressure is lowered, and CO2 tension increases.Kleitman (1939) has pointed out, however, that most of these changescould be simply a consequence of the prone position or of musclerelaxation, independent of sleep.
Physiological cycling duringsleep. During the waking state, the resting normal human witheyes closed usually displays a continuous EEG pattern of 8 to 12cycles per second, which is known as the “alpha”rhythm. As he drifts into sleep, the alpha rhythm disappears and isreplaced by a low-voltage pattern with irregular frequency. Duringthis phase, thresholds for responding to environmental stimuli areabout the same as those in the waking state. As sleep continues, thebackground EEG voltage becomes higher and the frequency of therhythms decreases, until the deepest stages of sleep are reached. Inthe deepest stages, the EEG record is composed of low-frequency,high-voltage “delta” rhythms, and it is very difficultto waken the subject. Approximately 90 minutes after the onsetofsleep, the high-voltage patterns disappear and are replaced by alow-voltage irregular phase similar to that seen at the beginning ofsleep. This cycle of alternating slow high-voltage and fasterlow-voltage rhythms recurs with a period of about 90 minutes. Theemergent low-voltage pattern has commonly been believed to be a“light” phase of sleep, but recent experiments showthat thresholds for awakening are often very high during this stage.[See NERVOUS SYSTEM, article on ELECTROENCEPHALOGRAPHY.]
Other physiological measures also suggest a 90-minute period as aunit of sleep time. Changes in such physiological and behavioralvariables as skin conductivity, heart rate, breathing rhythms, eyemovements, and reports of dreaming occur cyclically and in phase withthe EEG cycle. Increased heart rate, rapid eye movements, and reportsof dreaming occur during the low-voltage phase.
The EEG of cats,rats, and monkeys also show alternating slow high-voltage and fastlow-voltage patterns during sleep. Jouvet (1961) has shown that thehigh-voltage phase in cats is controlled by cortical mechanisms, andthe low-voltage phase by pontine and limbic structures. EEGrecordings obtained simultaneously from deep brain structures,cortex, and scalp suggest that in the cat the low-voltage phase ofsleep involves hindbrain “sleep” and forebrain“wakefulness.” The stage is accompanied by inhibitionof motor activity, reduced heart rate and blood pressure, very highthresholds for awakening, and rapid eye movements. There is evidence,then, that during the low-voltage “dreaming” stage ofsleep at least part of the brain is highly activated. But the veryhigh response thresholds to auditory and other stimuli are difficultto interpret. Some studies show that auditory stimuli that warn ofimmediate and severe consequences are responded to in this stage asreadily as in the other low-voltage phases of sleep. Thus, duringdreaming the subject appears to be controlled by intero-ceptivestimuli and seems to block out external signals unless they havepoignant significance.
The cyclical sequence of EEG patterns ofsleep in man appears to be quite stable, both from night to night andfrom subject to subject. Disturbance of these patterns by“deprivation” of either high-voltage or low-voltage EEGphases causes increasing amounts of the disturbed phase to appear onsucceeding nights, as if compensation were necessary. Continueddeprivation of the low-voltage “dreaming” phase causesirritability and unease.
Sleep loss and biological functions. Is sleep a vital function? One way to test this is todeprive an organism of sleep. Several animal studies have shown thatprolonged loss of sleep results in death. A number of humansubjects have stayed awake 200 hours or more without sustainingserious physical damage, but after 120 hours without sleep, mostsubjects developed reversible delirious or psychotic states characterized by visual hallucinations, delusions of persecution, disorientation, and confusion. The psychosis of sleep deprivation, which disappears after normal sleep, is a model mental illness that has considerable significance for experimental psychiatry.
If sleep plays a vital role in biological survival and psychological health, precisely what is this role? It has been suggested that sleephas three main functions: (a) restoring metabolic balance, (b) permitting recovery from muscular fatigue, (c) aiding neuralre organization. With few exceptions, studies of sleep loss have not given positivesupport to any of these hypotheses. Most systemsof the body hold up remarkably well with loss ofsleep.
Metaboliceffects. One recent study of sleep loss, however (Luby et al.1960), has shown striking metabolic effects. After the fourth day of sleep deprivation there was evidence of a drastic breakdown in themanufacture of adenosine triphosphate (ATP), the substance that provides energy for a host of biological processes. Thus, the highenergy transfer systems of cell metabolism showed gross impairment. This metabolic dysfunction was accompanied by rapid deterioration in personality and performance. These findings are congruent with the hypothesis that the catabolic processes that predominate during the waking state are compensated for during sleep; that sleep permits the restoration of energy and an aboliccompensation.
Effect on performance. During sleep loss the subject shows deficits in performance that are similar to thoseobserved with fatigue. The deficit takes the form of lapses (brief periods of poor performance), which in the sleep-deprived subject are accompanied by extreme drowsiness, intrusive thoughts, visualillusions, and dreams. They are preceded and accompanied by a slowing of heart rate, breathing, and EEC rhythms. Lapses increase infrequency, duration, and depth as the period without sleep progresses until the subject reaches a kind of hypnagogic state in which thebackground EEC pattern resembles a light phase of sleep. The frequency of lapses during sleep loss is increased by monotony or by prolonged performance and is decreased by incentive. The main effects of these brief periods of extreme drowsiness are slowed reactions. The subject is usually able to sustain accuracy if given sufficient time. Tasks extremely sensitive to sleep loss are those that areprolonged, repetitive, and work-paced, impose a high speed-load, and provide low incentive (Williams et al. 1959).
As sleep loss increases, other mental functions are affected. For example, the ability to store and retrieve new information is greatly impaired. Finally, when most data-processing functions are seriously debased,the subject becomes psychotic. Some goal-directed, sequential, and rational behavior remains, however. Even after seven days of sleep deprivation, subjects can pull themselves together for short-term problem-solving and can carry out well-learned, perhaps automatic, sensory-motor tasks.
Performance decrement and personality aberrations during sleep loss follow the diurnal temperature cycle. Effectiveness reaches a low point late at night (when body temperature is at its lowest) and improves during the next day, eventhough the amount of sleep loss has increased.
Interaction with other stress. There are at present very few reports on the combinedeffects of sleep loss and other types of stress. The available studies indicate that the effects are not readily predictable from aknowledge of the independent effects of each form of stress. Forexample, Wilkinson (1963) has shown that high-intensity background noise normally causes impaired performance on vigilance and choice reaction-time tasks. But the degrading effect of noise is reduced after a night of sleep loss. Apparently, noise and sleep deprivation produce different types of fatigue. With sleep loss, the level of stimulation required for arousal is too low; with noise, it may betoo high. Similar complexities appear in studies that combine raised body temperature with sleep loss. Recent unpublished studies suggestthat for the sleep-deprived subject a moderate increase in body temperature (say, one degree) improves performance, but that higher temperatures act synergistically with sleep deprivation to causes evere decrement in both speed and accuracy.
Fatigue at skilled tasks. The word “fatigue” usually refers to the changes in muscle metabolism that occur with prolonged exertion. A vast amount of research has been done on this topic, but it will not be discussed in this article. There are, however, many prolonged tasks requiring very little muscular effort, during which performance declines and the subjects report fatigue. The general nature ofimpairment at such tasks was first described by Bills (1931). He observed that performance did not decline as a simple function of time at the task, but instead became increasingly uneven. Adequate performance was interrupted from time to time by“blocks,” or brief periods of no response, which increased in frequency and duration with continued mental work. Bills attributed mental blocks to “a recurrent low condition ofmental functioning.” [See FATIGUE.]
In recent years the prototype task for the study of prolonged performance has been theso-called vigilance test. During such a task, an observer watches orlistens for long periods for critical signals that occur from time to time in a background of neutral signals. Radar and sonar operations are practical examples of such tasks. During the course of anhour’s performance, the proportion of signals missed is low atfirst, but after 15 to 30 minutes increases rapidly. These errors of omission are increased by monotony, by distraction, and by states of physiological depression such as those caused by sleepiness, low oxygen pressure, high nitrogen pressure, and tranquilizing drugs. Performance improves under incentive, and the drug amphetaminelargely prevents the decline in efficiency. Thus the effects ofprolonged performance are similar to those seen with sleep loss.Presumably, the decrement is due to recurrent periods of loweredcerebral vigilance. It is not certain whether these phases representincreased distractibility or simply drowsiness, or both. Very fewstudies have used continuous physiological monitoring duringprolonged performance, but the available reports suggest that thephysiological changes accompanying performance decrement are similarto those that accompany the lapses of sleep deprivation. [See ATTENTION.]
Learning and performance during sleep. Thereis no convincing evidence that learning can occur during sleep. Thesleeping state seems to be incompatible with many forms of cognitivebehavior. Simon and Emmons (1955) in a well-controlled series ofexperiments showed by continuous EEC monitoring that humans failed tolearn items of information during actual sleep. Their resultschallenge all earlier studies purporting to show that learning can beinduced during sleep, and they cast great doubt on the claims ofnumerous commercial enterprises that advertise sleep-learningmethods.
However, negative results in one well-controlled studydo not prove that all forms of learning are impossible in thesleeping state. Failure to induce learning may have been due toprocedural problems. Simon and Emmons used only one night oftraining. There was no effort to “shape” complexresponses from simpler behavior or to elucidate the precise stimulusattributes, incentive conditions, and response requirements thatmight be compatible with the sleeping state. The cyclic rise andfallof EEC activation patterns during sleep suggests that there maybe some phases of sleep in which learning is possible.
Theancient observation that some discrimination is possible in sleep hasbeen confirmed by recent experiments. Cats are capable of remarkablyfine discrimination of auditory signals while asleep. Humans candiscriminate between complex auditory patterns, make appropriatemotor and physiological responses to conditioned stimuli, responddifferentially to warning signals and neutral signals, and performwell-learned motor sequences without awakening.
Sleepdisorders and sleep therapy. The many abnormalities of sleep arewell known. They include excessive sleep, inability to sleep,restless sleep, nightmares, bed-wetting, sleep paralysis, and otherproblems. Neurologists classify sleep disorders under three headings:hypersomnia, insomnia, and nocturnal behavioral symptoms. All threecan occur with brain disease or with psychological etiology.
Hypersomnia. Narcolepsy is a condition of hypersomniacharacterized by sudden sleep seizures at inappropriate times. Thesyndrome also includes sleep paralysis and cataplexy. Theirresistible desire to sleep may occur several times daily,especially after heavy meals, during periods of low body temperature,or during monotonous activity. The duration of sleep varies from afew seconds to several hours. Hypnagogic hallucinations often precedeand accompany the attacks of sleep.
Narcolepsy occurs in bothidiopathic and symptomatic forms and may follow a chronic course foras long as forty years. Recently, a genetic factor has beenidentified in a group of patients with idiopathic narcolepsy (Yoss& Daly 1960). The idiopathic form usually appears duringadolescence and is more common among males. The condition can betreated symptomatically with analeptic drugs.
Insomnia.Insomnia, or hyposomnia, is also associated with a variety ofpathological states including brain tumor, metabolic disease,circulatory disorders, and aging. It is a common precursor of acuteschizophrenic reactions and, of course, accompanies general emotionalupset, depression, or anxiety. The hyposomnia may occur at the onset,during, or at the termination of sleep. Clinical observers havereported that difficulty in falling asleep is a common symptom inanxiety neuroses, whereas frequent and early awakening from sleep isa classic feature of depressive illnesses. Insomnia is usually adifficult treatment problem, although drugs, psychotherapy, andrelaxation-training are often effective.
Nocturnalbehavioral symptoms. Behavioral symptoms during sleep—includingnight terrors, sleep-walking, enuresis, grinding of teeth, delirium,and nightmares—can occur with specific disorders of the nervous system or as transient symptoms in normal individuals. All of these abnormalities may have some connection with aberrations of normal dreaming. Nearly all involve dreamlike activity and tend to occur during low-voltage EEC phases.
Sleep therapy. Sleep can be learned as a response in order to reduce anxiety. Since the excessive sleeper may use sleep to “narcotize” anxiety, it may be supposed that sleep would be useful as an adjunct to psychotherapy. Sleep therapy was first used in Switzerland, but it has achieved greatest popularity in the Soviet Union. As a consequence of Pavlov’s theories, prolonged sleep is used to“protect” the brain during mental illness. It is said to be most effective in cases of catatonic excitement, depression, and acute anxiety, but no controlled experimental studies werelocated. [See MENTAL DISORDERS, TREATMENT OF, article on SOMATIC TREATMENT.]
Conclusion. Since the first comprehensive summary of sleep and wakefulness by Kleitman (1939) there has been aconsiderable increase in scientific interest in the problem. At thattime there were about thirteen hundred references in the literature. There are now more than four thousand. Research activity is much greater, however, in neurophysiology than in psychology or other disciplines. Psychologists for the most part seem to have taken the transition from waking to sleep as a natural boundary for the study of behavior. Sleep, however, is not behaviorally empty. Even in deepest sleep subjects retain responsiveness to external stimuli.This finding that a considerable amount of behavior can be induced during sleep raises some interesting problems for both psychologists and neurophysiologists. If sleep is not, behaviorally, a silentstate, what limits does sleep impose on the organization of behavior? If the pattern and meaning of auditory signals can be recognized by the sleeping subject, how are we to define sleep? How is the central nervous system organized during the sleeping state?
A few answers have been suggested to the fundamental questions raised by the identification of similar cyclic patterns of sleep in man and animals. The discovery of the physiological correlates of dreaming permitted the development of entirely new concepts relating animal to human subjective states. But as research increases, questionsmultiply. Do animals dream? Is the dreaming phase in man under the control of the limbic system? What organizing concept in neurophysiology will account for the simultaneous high activation levels and high response threshold seen in the emergent low-voltage stage of sleep?
The age-old questions of the causes of sleep, the biological functions of sleep, the neural and humoral mechanisms of sleep, the effects of distributed sleep, and the modifiability of thes leepwake cycle are only partially answered. Clearly, research on the psychological, physiological, and social properties of sleep has important implications for all of the life sciences. There are practical reasons, too, for the brisk developments in this field. The requirements of space flight, of modern industry, and of military operations put the subjects of sleep and waking, and of biological rhythms in general, into challenging perspective.
Harold L. Williams
[See alsoDreamsand Fatigue. Other relevant material may be found inHypnosis; Mental Disorders, article onBiological Aspects; Nervous System, articles onStructure Andfunction OF The Brainand Electroencephalography.]
Hartley, Samuel H.; and Chute, E. 1947 Fatigue and Impairment in Man. New York: McGraw-Hill.
Bills, Arthur G. 1931 Blocking: A New Principle of Mental Fatigue. American Journal of Psychology 43:230-245.
Ciba Foundation For The Promotion OF International cooperation IN Medical And Chemical Research 1961 Symposium on the Nature of Sleep. Edited by G. E. W. Wolstenholme and MaeveO’Connor. Boston: Little.
Council For International organizations OF Medical Sciences 1954 Brain Mechanisms and Consciousness. Proceedings of a conference held at Ste. Marguerite,Quebec, Canada, August, 1953. Edited by Edgar D. Adrian et al. Oxford: Blackwell; Springfield, 111.: Thomas.
Henry Ford hospital, Detroit 1958 Reticular Formation of the Brain: Symposium.Edited by Herbert H. Jasper et al. Boston: Little.
Jouvet, M.1961 Telencephalic and Rhombencephalic Sleep in the Cat. Pages 188-208 in Ciba Foundation, Symposium on the Nature of Sleep. Edited by G. E. W. Wolstenholme and Maeve O’Connor. Boston: Little.
Kleitman, Nathaniel (1939) 1963 Sleep and Wakefulness. Rev.& enl. ed. Univ. of Chicago Press.
Lindsley, Donald B.1960 Attention, Consciousness, Sleep and Wakefulness. Volume 3, pages 1553-1593 in Handbook of Physiology. Section 1: Neurophysiology.Edited by H. W. Magoun et al. Baltimore: Williams & Wilkins.
Lobashev, M. E.; and Savvateev, V. B. 1959 Fizio-logiia sutochnogoritma zhivotnykh (Physiology of Circadlan Rhythms in Animals). Moscowand Leningrad: Izdatel’stvo Akademii Nauk Sssr.
Luby,Elliot D. et al 1960 Sleep Deprivation: Effects on Behavior,Thinking, Motor Performance and Biological Energy Transfer Systems.Psychosomatic Medicine 22:182-192.
Oswald, Ian 1962Sleeping and Waking: Physiology and Psychology. Amsterdam and NewYork: Klsevier.
Pieron, Henri 1913 Le probleme physiotogique dusom-meil. Paris: Masson.
Simon, Charles W.; and Emmons, Williamh. 1955 Learning During Sleep? Psychological Bulletin 52: 328-343.
Wilkinson, Robert T. 1963 Interaction of Noise With Knowledge of Results and Sleep Deprivation. Journal of Experimental Psychology 66:332-337.
Williams, Harold L.; Luein, A.; and Goodnow, J. J.1959 Impaired Performance With Acute Sleep Loss. PsychologicalMonographs 73, no. 14:1-26.
Wolf, William (editor) 1962 RhythmicFunctions in the Living Systems. New York Academy of Sciences, Annals98:753-1326.
Yoss, Robert E.; and Daly, David D. 1960 HereditaryAspects of Narcolepsy. American Neurological Association,Transactions [I960]:239-240.
Sleep is a state of physical inactivity and mental rest in which conscious awareness, thought, and voluntary movement cease and intermittent dreaming takes place. This natural and regular phenomenon essential to all living creatures normally happens with the eyes closed and is divided into two basic types: REM (rapid eye movement) and NREM (nonrapid eye movement) sleep. As passive as sleep appears, it is actually a very active and deliberate process in which the brain busily turns off wakeful functions while turning on sleep mechanisms. No one knows exactly why humans, and other animals, must sleep or how it happens, but the quality, quantity, and type of sleep impacts the quality, quantity, and effectiveness of our wakeful mental and physical activities. These, in turn, influence the quality, quantity, and timing of sleep.
At one time, it was believed that the mind simply turned off during sleep, or that the soul left the body during sleep. Greek philosopher Aristotle (384–322 BC) thought that the digestion of food created vapors that naturally rose upward, causing the brain to become drowsy. Dreams—the only part of sleep the sleeper actually experiences—were often interpreted as prophetic revelations. Today, dream interpretation is used in some psychoanalytic and self-awareness activities for personal insight and revelation.
Despite the fact that most people spend more time sleeping than in any other single activity, scientists still lack much knowledge about why humans need sleep or what triggers it. Serious scientific studies only began a little over 50 years ago, and several different theories have been developed, none of which have been proven. It is known, however, that the higher the organism on the evolutionary chain (humans being the highest) the more important sleep becomes.
According to the restorative theory of sleep, body tissues heal and regenerate during non-REM sleep and brain tissue heals during REM sleep. This theory seems generally accepted for brain tissue restoration, particularly in the cerebral cortex, which cannot rest during the waking state. However, some researchers question its validity regarding body tissue restoration, believing that sleep simply acts as an immobilizer, forcing the body to rest, with rest and nourishment being the actual restorative factors. The release during sleep of growth hormones, testosterone, and other anabolic (constructive) hormones leads some experts to support the restorative theory, while others believe this release is coincidental to, and not caused by, sleep.
The energy conservation theory of sleep notes that animals that burn energy quickly and produce their own body heat, such as humans do, sleep more than those with slow metabolisms (energy consumption) or that do not produce body heat (snakes, for instance). This theory is based upon the observation that metabolic rates decrease during slow-wave sleep—the last two stages of the four-stage, NREM sleep cycle and that some researchers believe is the most important stage.
According to the adaptive theory of sleep, sleep encourages adaption to the environment for increased chances of survival. Animals such as cats that spend little time searching for food and have few natural enemies may sleep 15 hours a day for long periods. Grazing animals like buffaloes and horses, which spend many hours foraging and which are at risk from natural predators, sleep only two to four hours a day in short spurts. Proponents of the adaptive theory believe early humans slept in caves to protect themselves from night-stalking animals.
Because instinct plays an important role in the survival of any species, including humans, the instinct theory presumes sleep, like mating or hunger, is a survival instinct.
Studies show that new information is best retained when introduced just before sleep begins and retained less well after waking or if REM sleep is interrupted. These observations lead to the memory consolidation theory of sleep. REM sleep seems to play an important role in storing information.
Enforced sleep-deprivation experiments
In the attempt to understand human need for sleep, experiments in sleep deprivation play an important role. Total sleep deprivation longer than 40 hours proves impossible, however, due to brief, totally unpreventable periods of microsleep that will happen even during physical activity. These microsleeps barely last a few seconds, but they may explain performance lapses in waking activities. They demonstrate the body’s obvious need for sleep and may even have some restorative function.
While sleep deprivation can eventually cause death, sleep deprivation lasting up to ten days shows no serious, prolonged consequences and does not cause severe psychological problems or mental illness as once thought. In 1965, for example, 17-year-old Randy Gardner decided to attempt a new world record for total sleep deprivation as his high school science fair project. He succeeded in staying awake for an incredible 264 hours. When researchers and psychiatrists from Stanford University (California) heard of Gardner’s experiment, they rushed to the scene and monitored his progress. On the last night, one researcher took Randy to an arcade to keep him awake. Randy won every game, indicating that prolonged sleep deprivation did not seriously impair his physical or psychomotor functioning. After his extraordinary vigil, Randy slept just 14 hours and 40 minutes, awoke naturally around 10:00 P.M., stayed awake 24 hours, and slept a normal eight hours. Follow-up over the years has shown that Gardner suffered no adverse effects from his experience. Scientific studies in the 1990s and 2000s show that such sleep deprivation activities are likely dangerous to human health.
Losing more than one night’s sleep does produce a noticeable increase in irritability, lethargy, disinterest, and even paranoia. While not seriously impaired, psychomotor performance and concentration are adversely affected. While autonomic (involuntary) nervous system activity increases during sleep deprivation to keep heart rate, blood pressure, breathing, and body temperature normal, physical fitness cannot be maintained and immunological functions seem to suffer.
Biological determinants of sleep
Another question which remains only partially answered is how sleep onset is determined and why. The factors involved include circadian rhythms (biological time clocks); the degree of stimulation in the wakeful state; the degree of personal sleepiness; the decrease in core body temperature; a quiet and comfortable sleep environment; conditioning arising from bedroom cues; and homeostasis, the automatic attempt by the body to maintain balance and equilibrium (for example, the air temperature may fall to 50°F [10°C], but the human body burns calories to maintain its normal temperature of 98.6°F [37°C]).
The fact that sleep deprivation increases the desire for sleep firmly points to a homeostatic element in sleep. This is intricately linked to highly influential circadian rhythms controlled by centers probably located in the hypothalamus, part of the brain primarily involved in autonomic nervous system functions. Circadian rhythms determine the human approximate 24- to 25-hour sleep-wake pattern and a similar cycle in the rise and fall of core body temperature and other physiological functions.
It is not yet known whether two separate biological clocks influence sleep-wake cycles and temperature levels and, if so, if a single control clock regulates them both. However, body temperature drops slightly in the evening as sleep draws near, reaches its lowest point around 2:00 to 4:00 a.m., rises slightly before awakening, and increases to maximum as the day progresses. This pattern is not a result of being asleep or awake, for body temperature does not drop during daytime naps nor does it rise at night after a sudden change in sleep schedule, such as shift work. It takes about two weeks for circadian rhythms controlling temperature levels to get back into sync with sleep-wake states.
Studies done on human circadian rhythms in situations totally devoid of time cues (such as sunrise, sunset, clocks, etc.) show that these rhythms are controlled completely internally and usually run on a cycle of almost 25 rather than 24 hours. In normal situations, factors called zeitgebers (from the German zeit for time and geber for giver) such as daylight, environmental noises, clocks, and work schedules virtually force humans to maintain a 24-hour cycle. Therefore, human circadian rhythms must phase advance from their normal, approximate 25-hour cycle to an imposed 24-hour cycle.
The body has difficulty adapting to much more than an hour of phase-advance in one day. Drastic time changes—like those caused by rapid longdistance travel such as flying—require either phase-advancement or phase-delay. This is why air travelers experience jet lag. Recovery from east-west travel requiring phase-delay adjustments is usually quicker than in phase-advancement resulting from west-east travel. Some people seem simply unable to phase-advance their biological clocks, which often results in sleep disorders.
Measurement of electrical impulses in the sleeping brain
The greatest contribution to sleep study was the development of the EEG, or electroencephalogram, by German psychiatrist Hans Berger (1873–1941)in 1929. This electrode, attached to the scalp with an adhesive, records electrical impulses in the brain called brain waves. The discovery triggered investigations into sleep in major centers around the world. Specific brain wave patterns became evident and sleep was generally classified into distinct stages.
In 1953, Professor Nathaniel Kleitman and his graduate student Eugene Aserinsky reported their close observations of a sleep stage they called REM-rapid eye movement. An electro-oculogram, or EOG, taped close to the eyelids, recorded both vertical and horizontal eye movement, which became rapid and sporadic during REM sleep. The electromyogram, or EMG, recorded chin and neck muscle movement which, for as yet undetermined reasons, completely relaxed during REM sleep. Kleitman and Aserinsky found that when subjects were awakened from REM sleep they almost always reported a dream, which was seldom the case when awakened from non-REM sleep.
Following the initial REM discoveries, sleep research greatly increased. One important discovery arising from this research was the high prevalence of sleep disorders, some of which now explain problems previously blamed on obscure physical or psychological disorders but which could not be effectively treated by medicine or psychiatry.
Combined, the EEG, EOG, and EMG produce a fascinating picture of sleep’s structure. These monitoring devices transfer electronic stimulus to recording devices, or on to paper. The number of complete brain wave cycles per second (frequency) is measured in units of hertz (Hz) by the EEG. The difference between the highest and lowest point of each wave (the peak and trough) is measured in amplitude, (mil-lionths of a volt, or microvolts [uV]). As sleep approaches and deepens, frequency decreases and amplitude increases.
Very specific rhythms occur in different stages of the sleep-wake cycle. Beta rhythms are fast, low voltage waves (usually above 15 Hz and below 10 uV) which appear in alert, wakeful states. In the quiet, restful wakeful state prior to sleep onset, or in relaxed meditative state with the eyes closed, the brain displays alpha rhythms of about 8 to 11 Hz and 50 uV. Fairly high chin muscle activity and slow, rolling eye movements are recorded. Alpha waves disappear with visual imagery or opening the eyes, which causes alpha blocking.
Non-REM sleep is generally believed to occur in four stages and is characterized by lack of dreaming. As the sleeper enters the drowsy, light sleep of stage 1, theta rhythms, ranging between 3.5 to 7.5 Hz with a lower voltage, appear. The sleeper is generally non-responsive during this stage, which takes up about 5% of the sleep cycle, but is easily awakened. Once again, high chin muscle activity occurs and there is occasional slow, rolling eye movement.
Within a few minutes, the sleeper enters stage 2 sleep. Brain waves slow even further and spindles (short bursts of electrical impulses at about 12 to 14 Hz which increase and decrease in amplitude) appear, along with K-complexes (sharp, high voltage wave groups, often followed by spindles). These phenomena may be initiated by internal or external stimuli or by some as yet unknown source deep within the brain. A few delta waves may appear here. This portion of sleep occupies about 45% of the sleep cycle.
Normally, stage 3 sleep, comprised of 20 to 50% low frequency/high voltage delta waves, follows stage 2 as a short (about 7% of total sleep) transition to stage 4 sleep, which shows slower frequency higher voltage delta wave activity above 50%. There is virtually no eye movement during stages 2, 3, and 4.
In stage 4 sleep, some sleep spindles may occur, but are difficult to record. This stage occupies about 13% of the sleep cycle, seems to be affected more than any other stage by the length of prior wakefulness, and reflects the most cerebral shutdown. Accordingly, some researchers believe this stage to be the most necessary for brain tissue restoration. Usually grouped together, stages 3 and 4 are called delta, or slow wave sleep (SWS), and is normally followed by REM sleep.
The sleep cycle from stage 1 through REM occurs three to five times a night in a normal young adult. Stages 3 and 4 decrease with each cycle and stage 2 and REM sleep occupy most of the last half of the night’s
Alpha/beta/delta/theta rhythms —Brain wave activity occurring in different stages of wakefulness or sleep identified by amplitude and frequency.
Amplitude —Difference between the highest and lowest point of a wave.
Autonomic nervous system —The part of the nervous system that controls involuntary processes, such as heart beat, digestion, and breathing.
Circadian rhythms —The rhythmical biological cycle of sleep and waking which, in humans, usually occurs every 24 hours.
Homeostasis —The body’s automatic attempt to maintain balance and stability of certain internal functions, such as body temperature, influenced by the external environment.
Metabolism —Chemical changes in body tissue which convert nutrients into energy for use by all vital bodily functions.
Phase advance/phase delay —Adjustment of circadian rhythms from their internal, biologically controlled cycle of approximately 25 hours to the 24-hour-a-day cycle imposed by the Sun.
sleep. Time spent in each stage varies with age, and age particularly influences the amount time spent in SWS. From infancy to young adult, SWS occupies about 20 to 25% of total sleep time and perhaps as little as 5% by the age of 60 years. This loss of time is made up in stage 1 sleep and wakeful periods.
The period comprised of the four stages between sleep onset and REM is known as REM latency. REM onset is indicated by a drop in amplitude and rise in frequency of brain waves. The subject’s eyes flicker quickly under the eyelids, dream activity is high, and the body seems to become paralyzed because of the decrease in skeletal muscle tone. After REM, the subject usually returns to stage 2 sleep, sometimes after waking slightly. REM sleep occurs regularly during the night. The larger the brain, the longer the period between REM episodes—about 90 minutes for humans and 12 minutes in rats.
REM sleep is triggered by neural functions deep within the brain, which releases one type of neuro-transmitter (chemical agent) to turn REM sleep on and another to turn it off. Whereas autonomic activity (such as breathing and heart rate) slows and becomes more regular during non-REM sleep, it becomes highly irregular during REM sleep. Changes in blood pressure, heart rate, and breathing regularity take place, there is virtually no regulation of body temperature, and clitoral and penile erections are often reported. Most deaths, particularly of ill or aged individuals, happen early in the morning when body temperature is at its lowest and the likelihood of REM sleep is highest.
REM activity is seen in the fetus as early as six months after conception. By the time of birth, the fetus will spend 90% of its sleep time in REM but only about half that after birth. REM constitutes about 20 to 30% of a normal young adult’s sleep, decreasing with age. These observations support one of several theories about our need for REM sleep which suggests that, to function properly, the central nervous system requires considerable stimulation, particularly during development. Because it receives no environmental stimulation during the long hours of sleep, it is possible that the high amount of brain wave activity in REM sleep provides the necessary stimulation.
See also Biological rhythms.
Billiard, Michel, ed. (translated by Angela Kent). Sleep: Physiology, Investigations, and Medicine. New York: Kluwer Academic/Plenum, 2003.
Horne, James A. Sleepfaring: A Journey Through the Science of Sleep. Oxford, UK: Oxford University Press, 2006.
Lee-Chiong, Teofilo L. Sleep: A Comprehensive Handbook. Hoboken, NJ: Wiley-Liss, 2006.
Mindell, Jodi A. A Clinical Guide to Pediatric Sleep: Diagnosis and Management of Sleep Problems. Philadelphia, PA: Lippincott Williams & Wilkins, 2003.
Moorcroft, William H. Understanding Sleep and Dreaming. New York: Springer, 2005.
Rosen, Marvin. Sleep and Dreaming. Philadelphia, PA: Chelsea House Publishers, 2005.
Steriade, Mircea. Brain Control of Wakefulness and Sleep. New York: Springer, 2005.
Marie L. Thompson
Sleep is a biological imperative critical to the maintenance of mental and physical health. It is a state of lessened consciousness and decreased physical activity during which the organism slows down and repairs itself. The sleep cycle involves two distinct phases that alternate cyclically from light sleep to deep then deeper and deepest sleep throughout the sleep period. There are two main phases of sleep.
- rapid eye movement (REM) sleep, during which dreaming occurs
- non-rapid eye movement (NREM) or slow-wave sleep (SWS)
The timing and progression of the sleep cycle and the total amount of nightly sleep required for optimal health varies from infancy to adulthood, depending on developmental stage and temperament . Children, particularly infants, require the most sleep during a 24-hour period. The natural sleep-wake cycle, governed by an internal "biological clock," tends toward a 25-hour day. It is affected by the relative balance of light and darkness in the environment. As darkness approaches, the hormone melatonin is secreted by the pineal gland and signals the brain that it is time to sleep.
NREM deep sleep
Sleep begins in stage one of the sleep phase known as NREM, or non-rapid eye movement, sleep. NREM sleep has four stages: light sleep, deeper sleep, and two stages of deepest sleep. Stage one is the "drifting off" period of light sleep in the transition between wakefulness and sleep and comprises about 5 percent of the entire sleep period. Stage two sleep involves a change in brain-wave patterns and increased resistance to arousal and accounts for 45–55 percent of total sleep time. Stages three and four are the deepest levels of sleep and occur only in the first third of the sleep period. NREM stage four sleep usually takes up 12 to 15 percent of total sleep time. Sleep terrors, sleep walking, and bedwetting episodes generally occur within stage four sleep or during partial arousals from this sleep stage.
It typically takes about 90 minutes to cycle through the four deepening stages of NREM sleep before onset of the second phase of sleep known as REM or dream sleep.
REM dream sleep
Rapid eye movement (REM) sleep is qualitatively different from NREM sleep. REM sleep is characterized by extensive central nervous system (CNS) activity with an increase in brain metabolism accompanied by the vivid imagery of dreams. During REM sleep the body is nearly paralyzed, a condition called "atonic," that serves to inhibit the dreamer from physical movement during active dreaming.
"Waking and dreaming are two states of consciousness, with differences that depend on chemistry," according to J. Allan Hobson, professor of psychiatry at Harvard Medical School. Physical activity and thought are suppressed in sleep, but the brain nonetheless remains active "processing information, consolidating and revising memory, and learning newly acquired skills." The brain self-activates, radically changing its chemical climate from wakefulness to sleep states.
REM sleep is also known as "paradoxical sleep" because muscle activity is suppressed even as the CNS registers intense brain activity and spontaneous rapid eye movements can be observed. Brain-wave monitoring of REM sleep with an electroencephalograph (EEG) reveals a low-voltage, fast-frequency, non-alpha wave record. Beyond infancy, REM sleep comprises 20–25 percent of the entire sleep period. This sleep phase is concerned with memory and the consolidation of new information.
Newborn infants usually sleep for brief periods at a time around the clock, with the total of day and nighttime sleep roughly equal. A newborn's total sleep need is from 16 to 18 hours in every 24-hour period. Newborns spend approximately 50 percent of their sleep period in the REM phase. Infants are most easily awakened during this phase of sleep that is accompanied by yawning, squirming, and quiet vocalizations.
Infants move through REM and non-REM sleep stages in a 90 minute cycle, and they rise to a near-waking state every three to four hours, more often in breastfed infants. By about six months of age, babies usually will sleep through the night for 12 or more hours and will continue to nap several times throughout the day.
Researchers conducting a 2004 survey for the National Sleep Foundation discovered that children in every age group fail to meet even the low-end requirements for adequate sleep. By the third month of life, a child's sleep requirement is about 14 to 15 out of every 24 hours, a need that continues until about 11 months of age. However, research indicates that children age three months to 11 months sleep only 12.7 hours on average.
Toddlers are far more physically active than infants, and their sleeping behavior and the timing of sleep cycles reflects their maturing brains. A toddler will spend only about 30 percent of her sleep time in REM dream sleep. Toddlers on average require 12 to 14 hours of sleep and may no longer need an afternoon nap to meet this sleep requirement. But research shows that children in the one to three-year-old range may actually average only about 11.7 hours of sleep.
Children in this age group tend to be more troubled with nightmares and night terrors than younger children. They may resist going to bed at night because of fear of the dark or of some monster lurking under the bed. Parental reassurance and comfort and the addition of a night light may alleviate some of these concerns.Preschool children may also feel anxiety around the issue of toilet training and bedwetting.
School-age children require from eight to 10 hours of sleep nightly. Adequate sleep is especially important as school children's lives become busier and stress levels rise. Sleep disruptions such as nightmares tend to increase with this age group as the child has more life experiences and anxieties to process. Parents should also monitor the child's use of caffeinated beverages which can cause sleep difficulties and add to the overall loss of adequate sleep.
Adolescents require at least 10 hours of nightly sleep. This is a busy time when many teens' lifestyles include school, work, sports , and other extracurricular activities , as well as socializing with peers. This increase in activity, together with early-morning school schedules, leaves little time for adequate sleep. Various psychological disorders also may trouble the adolescent, particularly anxiety and depression. Parents should pay attention to a young teen who shows sudden changes in eating habits, loss of interest in usual activities, and other behavioral clues that may indicate onset of depression.
According to the "2004 Sleep in America Poll" published by the National Sleep Foundation, 69 percent of children younger than age 10 experience problems with sleep that may occur as often as several times a week. Sleep disruptions in children are usually a normal symptom of central nervous system development. In older children sleep disruptions may increase and intensify due to external stressors in the home or school environment. Sleep difficulties can also be a sign of physical or mental health problems. They are often present in children with attention-deficit/hyperactivity disorder (AD/HD) and in children who have experienced physical, psychological, or sexual abuse.
Childhood sleep problems and parasomnias include:
- Bedwetting: A common sleep problem characterized by involuntary urination during sleep. This is a routine occurrence in children up to five years of age. Bedwetting is also called "nocturnal enuresis."
- Nightmares: A common parasomnia characterized by dreams with frightening psychological content, a feeling of imminent physical danger, and a sensation of being trapped or suffocated. Nightmares occur during REM, or dream-time, sleep and trigger a partial or full awakening. The word "mare" in Old English means "demon."
- Insomnia: Difficulty falling asleep and remaining asleep, or early-morning awakenings. Insomnia may be short-term, due to stress or physical or psychological problems, or may be due to the lack of a healthy bedtime routine.
- Night terrors: A common childhood sleep disruption characterized by an abrupt arousal from stage 4 sleep within the first hour of the sleep period. The child may sit bolt upright in acute terror, screaming inconsolably. Night terrors are a confusional arousal resulting from immature sleep patterns with an intense activation of the flight or fight emotion. They occur in the deepest stage of slow-wave non-REM sleep. Night terrors are also called "pavor nocturnus."
- Sleep apnea: A serious and potentially life-threatening sleep disruption characterized by brief interruptions of airflow during sleep and frequent partial arousals throughout the night. Sleep apnea is less common than other sleep disturbances, occurring in about 2 percent of children.
- Sleep bruxism: A sleep disturbance characterized by grinding the teeth or clenching of the jaws during sleep. Sleep bruxism is common among children of all ages. This sleep problem usually subsides over time.
- Sleep rocking and head banging: A sleep disturbance characterized by rhythmical movements of the body during sleep. Rhythmical movements may be observed in children as young as six months. More dramatic movements, involving head banging and rocking, occur in as many as 60 percent of nine-month-old children. These sleep disturbances tend to decrease with age, appearing in only about 5 percent of children over two years of age.
- Sleep walking: A sleep disturbance characterized by a partial-arousal involving walking about for a few steps, or for much longer distances, with a glassy, trance-like appearance to the eyes. Sleepwalking occurs in the deepest stages of slow-wave, non-REM sleep within the first few hours of sleep onset. Researchers have found that as many as 15–30 percent of children experience at least one sleepwalking episode. Sleepwalking can be triggered by external stimuli, such as an abrupt noise, or by moving a sleeping child to a standing position. This sleep disturbance tends to run in families. Sleepwalking is also called "somnambulism."
All children need regular and adequate sleep to assure optimal mental and physical health. Sleeping patterns developed in infancy usually persist into adulthood. It is important that parents help the child to establish a healthy bedtime routine that will assure adequate sleep time, minimize bedtime struggles, and help to reduce the occurrence of common childhood sleep problems.
As reported by Steven Reinberg, research by Maria M. Wong of the University of Michigan, published in 2004 in the journal Alcoholism: Clinical and Experimental Research, cautions parents to pay more attention to their children's sleep habits. "Sleep problems are a risk factor for alcohol and drug problems," Wong concluded from data obtained in the first study to link alcohol and drug use with sleep disorders in early childhood. The study obtained sleep data from 257 boys ages three to five years and followed them until they were 12–14 years old. Almost half of the children in the study who experienced childhood sleep problems began using alcohol and drugs by the time they were 14 years old.
In many households, electronic distractions interfere with the establishment of a regular bedtime routine that would help a child to settle down and prepare for restful sleep. Calming-down activities, such as being read to by a parent, have been replaced with electronic stimulation resulting in less sleep time.
As reported in Manchester Online, Luci Wiggs, a research fellow at Oxford University, is co-author of a 2004 poll of more than 1,000 parents with children four to 10 years of age. She found that 67 percent of these children had a television, computer, or game machine in their bedroom. These stimulating diversions, which she calls "digital distractions," resulted in a cumulative sleep deficit for at least one fifth of the children surveyed that may "compromise children's physical health, academic achievements, and mental health."
Children who consume caffeine throughout the day, in soda or iced tea beverages, also lose the sleep required for optimal health and cognitive functioning. A survey by the National Sleep Foundation released in 2004 found that 26 percent of children ages three and older drink at least one caffeinated beverage a day and suffer a loss of about 3.5 hours of sleep each week.
Parents are on a journey of discovery with each child whose temperament, biology, and sleep habits result in a unique sleep-wake pattern. It can be frustrating when children's sleep habits do not conform to the household schedule. Helping the child develop good sleep habits in childhood takes time and parental attention, but it will have beneficial results throughout life. An understanding of the changing patterns of the typical sleep-wake cycle in children will help alleviate any unfounded concerns. Maintaining a sleep diary for each child will provide the parent with baseline information in assessing the nature and severity of childhood sleep problems. Observant parents will come to recognize unusual sleep disruptions or those that persist or intensify.
When to call the doctor
Developmental changes throughout childhood bring differences in the sleep-wake cycle and in the type and frequency of parasomnias that may interrupt sleep. Medical consultation to rule out illness, infection, or injury is prudent if the child's sleep problems prevent adequate sleep and result in an ongoing sleep deficit. As reported by News-Medical in Child Health News, children's sleep problems should be taken seriously as they may be a "'marker' for predicting later risk of early adolescent substance use." In the same article, University of Michigan psychiatry professor Kirk Brower, who has studied "the interplay of alcohol and sleep in adults," stressed that "The finding does not mean there's a cause-and-effect relationship."
Consultation with a child psychologist may be helpful if frightening dreams intensify and become more frequent as this may indicate a particular problem or life circumstance that needs to be changed or one that the child may need extra help working through.
Most childhood sleep disturbances will diminish over time as the brain matures and a regular sleep-wake cycle is established. Parental guidance is crucial to development of healthy sleep habits in children.
Hobson, J. Allan. Dreaming: An Introduction to the Science of Sleep. Oxford: Oxford University Press, 2002.
Moorcroft, William H. Understanding Sleep and Dreaming. New York: Kluwer Academic/Plenum Publishers, 2003.
Schroeder, Carolyn S., and Betty N. Gordon. Assessment and Treatment of Childhood Problems, 2nd ed. New York: Guildford Press, 2002.
"Kids' Sleep Problems Can Portend Alcohol and Drug Use." Connecticut Post, April 15, 2004. Available online at <www.lexis-nexis.com> (accessed October 6, 2004).
Moss, Lyndsay. "Computers and Games 'Keeping Children Awake."' Press Association News, March 26, 2004. Available online at <lexis-nexis.com> (accessed August 3, 2004).
Wilmott, Bob. "Many Children Fall Short of the Sleep They Need." St. Louis Post-Dispatch, April 26, 2004. Available online at <www.lexis-nexis.com> (accessed August 3, 2004).
American Sleep Disorders Association. 1610 14th Street, NW, Suite 300, Rochester, MN 55901–2201. Web site: <www.sleepnet.com/asda.htm>.
National Sleep Foundation. 1522 K Street, NW, Suite 500, Washington, DC, 20005. Web site: <www.livingwithillness.com/id174.htm>.
"Children kept awake by computers and games." Manchester Online, March 26, 2004. Available online at <wwwmanchesteronline.co.uk/business/technology/s/85/85453_children_kept_awake_by_computers_and_games.html> (accessed October 7, 2004).
"Children's Bedtime Routines: Sound Sleeping Advice." Mayo Foundation for Medical Education and Research, September 23, 2003. Available online at <www.mayoclinic.com/invoke.cfm?id=CC00020.> (accessed July 23, 2004).
"Computers 'rob children of sleep."' BBC News. Available online at <newsvote.bbc.co.uk/mpapps/pagetools/print/news.bbc.co.uk/1/hi/health/3568963.stm> (accessed August 4, 2004).
Driver, Helen. "Parasomnias." Canadian Sleep Society. Available online at <www.css.to/sleep/disorders/parasomnia.htm> (accessed July 29, 2004).
Reinberg, Steven. "Kids' Sleep Problems Can Portend Alcohol and Drug Use." Healthfinder News. Available online at <www.healthfinder.gov/news/newsstory.asp?docID=518390> (accessed October 7, 2004).
"Sleep Behavior Problems (Parasomnias)." Kids Health for Parents. Available online at <www.kidshealth.org/parent/general/sleep/parasomnia.html> (accessed July 26, 2004).
"Sound sleep in infants lessens likelihood of drug abuse in later years." News-Medical in Child Health News, April 16, 2004. Available online at <www.news-medical.net/print_article.asp?id=545> (accessed October 7, 2004).
"Tips for Healthy Sleep." American Sleep Disorders Association. Available online at <www.slepnet.com/> (accessed August 4, 2004).
Biological clock —A synonym for the body's circadian rhythm, the natural biological variations that occur over the course of a day.
Parasomnia —A type of sleep disorder characterized by abnormal changes in behavior or body functions during sleep, specific stages of sleep, or the transition from sleeping to waking.
Suprachiasmatic nuclei (SCN) —SCN is that part of the brain that functions as a person's "biological clock" to regulate many body rhythms. The SCN is located on top of the main junction of nerve fibers that connects to the eyes.
Sleep is a state of physical inactivity and mental rest in which conscious awareness, thought, and voluntary movement cease and intermittent dreaming takes place. This natural and regular phenomenon essential to all living creatures normally happens with the eyes closed and is divided into two basic types: REM (rapid eye movement) and NREM (non-rapid eye movement) sleep. As passive as sleep appears, it is actually a very active and deliberate process in which the brain busily turns off wakeful functions while turning on sleep mechanisms. No one knows exactly why we must sleep or how it happens, but the quality, quantity, and type of sleep impacts the quality, quantity, and effectiveness of our wakeful mental and physical activities. These, in turn, influence the quality, quantity, and timing of sleep.
Beliefs, theories, and scientific observations of sleep
At one time, it was believed that the mind simply turned off during sleep, or that the soul left the body during sleep. Aristotle thought that the digestion of food created vapors which naturally rose upward, causing the brain to become drowsy. Dreams—the only part of sleep the sleeper actually experiences—were often interpreted as prophetic revelations. Today, dream interpretation is used in some psychoanalytic and self-awareness activities for personal insight and revelation.
Despite the fact that most people spend more time sleeping than in any other single activity, scientists still lack much knowledge about why we need sleep or what triggers it. Serious scientific studies only began about 50 years ago, and several different theories have been developed, none of which have been proven. It is known, however, that the higher the organism on the evolutionary chain (humans being the highest) the more important sleep becomes.
According to the restorative theory of sleep, body tissues heal and regenerate during non-REM sleep and brain tissue heals during REM sleep. This theory seems generally accepted for brain tissue restoration, particularly in the cerebral cortex, which cannot rest during the waking state. However, some researchers question its validity regarding body tissue restoration, believing that sleep simply acts as an immobilizer, forcing the body to rest, with rest and nourishment being the actual restorative factors. The release during sleep of growth hormones , testosterone, and other anabolic (constructive) hormones leads some experts to support the restorative theory, while others believe this release is coincidental to, and not caused by, sleep.
The energy conservation theory of sleep notes that animals which burn energy quickly and produce their own body heat , such as humans do, sleep more than those with slow metabolisms (energy consumption) or that do not produce body heat (snakes , for instance). This theory is based upon the observation that metabolic rates decrease during slow-wave sleep—the last two stages of the four-stage, NREM sleep cycle and which some researchers believe is the most important stage.
According to the adaptive theory of sleep, sleep encourages adaption to the environment for increased chances of survival. Animals such as cats that spend little time searching for food and have few natural enemies may sleep 15 hours a day for long periods. Grazing animals like buffaloes and horses which spend many hours foraging and which are at risk from natural predators sleep only two to four hours a day in short spurts. Proponents of the adaptive theory believe early humans slept in caves to protect themselves from night-stalking animals.
Because instinct plays an important role in the survival of any species , including humans, the instinct theory presumes sleep, like mating or hunger, is a survival instinct.
Studies show that new information is best retained when introduced just before sleep begins and retained less well after waking or if REM sleep is interrupted. These observations lead to the memory consolidation theory of sleep. REM sleep seems to play an important role in storing information.
Why we sleep and how it is triggered
Enforced sleep-deprivation experiments
In the attempt to understand our need for sleep, experiments in sleep deprivation play an important role. Total sleep deprivation longer than 40 hours proves impossible, however, due to brief, totally unpreventable periods of "microsleep" which will happen even during physical activity. These microsleeps barely last a few seconds, but they may explain performance lapses in waking activities. They demonstrate the body's obvious need for sleep and may even have some restorative function.
While sleep deprivation can eventually cause death, sleep deprivation lasting up to ten days shows no serious, prolonged consequences and does not cause severe psychological problems or mental illness as once thought. In 1965, for example, 17-year-old Randy Gardner decided to attempt a new world record for total sleep deprivation as his high school science fair project. He succeeded in staying awake for an incredible 264 hours. When researchers and psychiatrists from Stanford University heard of Gardner's experiment, they rushed to the scene and monitored his progress. On the last night, one researcher took Randy to an arcade to keep him awake. Randy won every game, indicating that prolonged sleep deprivation did not seriously impair his physical or psychomotor functioning. After his extraordinary vigil, Randy slept just 14 hours and 40 minutes, awoke naturally around 10:00 p.m., stayed awake 24 hours, and slept a normal eight hours. Follow-up over the years has shown that Gardner suffered no adverse effects from his experience.
Losing more than one night's sleep does produce a noticeable increase in irritability, lethargy, disinterest, and even paranoia. While not seriously impaired, psychomotor performance and concentration are adversely affected. While autonomic (involuntary) nervous system activity increases during sleep deprivation to keep heart rate, blood pressure , breathing, and body temperature normal, physical fitness cannot be maintained and immunological functions seem to suffer.
Biological determinants of sleep
Another question which remains only partially answered is how sleep onset is determined and why. The factors involved include circadian rhythms (biological time clocks); the degree of stimulation in the wakeful state; the degree of personal sleepiness; the decrease in core body temperature; a quiet and comfortable sleep environment; conditioning arising from "bedroom cues"; and homeostasis , the automatic attempt by the body to maintain balance and equilibrium (for example, the air temperature may fall to 50°F [10°C], but our body burns calories to maintain its normal temperature of 98.6°F [37°C ]).
The fact that sleep deprivation increases the desire for sleep firmly points to a homeostatic element in sleep. This is intricately linked to highly influential circadian rhythms controlled by centers probably located in the hypothalamus, part of the brain primarily involved in autonomic nervous system functions. Circadian rhythms determine our approximate 24- to 25-hour sleep-wake pattern and a similar cycle in the rise and fall of core body temperature and other physiological functions.
It is not yet known whether two separate biological clocks influence sleep-wake cycles and temperature levels and, if so, if a single "control clock" regulates them both. However, body temperature drops slightly in the evening as sleep draws near, reaches its lowest point around 2:00-4:00 a.m., rises slightly before awakening, and increases to maximum as the day progresses. This pattern is not a result of being asleep or awake, for body temperature does not drop during daytime naps nor does it rise at night after a sudden change in sleep schedule, such as shift work. It takes about two weeks for circadian rhythms controlling temperature levels to get back into sync with sleep-wake states.
Studies done on human circadian rhythms in situations totally devoid of time cues (such as sunrise, sunset, clocks, etc.) show that these rhythms are controlled completely internally and usually run on a cycle of almost 25 rather than 24 hours. In normal situations, factors called "zeitgebers" (from the German zeit for time and geber for giver) such as daylight, environmental noises, clocks, and work schedules virtually force us to maintain a 24-hour cycle. Therefore, our circadian rhythms must "phase advance" from their normal, approximate 25-hour cycle to an imposed 24-hour cycle.
The body has difficulty adapting to much more than an hour of phase-advance in one day. Drastic time changes-like those caused by rapid long-distance travel such as flying-require either phase-advancement or phase-delay. This is why travelers experience "jet lag." Recovery from east-west travel requiring phase-delay adjustments is usually quicker than in phase-advancement resulting from west-east travel. Some people seem simply unable to phase-advance their biological clocks, which often results in sleep disorders .
The structure of sleep
Measurement of electrical impulses in the sleeping brain
The greatest contribution to sleep study was the development of the EEG, or electroencephalogram, by German psychiatrist Hans Berger in 1929. This electrode, attached to the scalp with glue, records electrical impulses in the brain called brain waves. The discovery triggered investigations into sleep in major centers around the world. Specific brain wave patterns became evident and sleep was generally classified into distinct stages.
In 1953, Professor Nathaniel Kleitman and his graduate student Eugene Aserinsky reported their close observations of a sleep stage they called REM-rapid eye movement. An electro-oculogram, or EOG, taped close to the eyelids, recorded both vertical and horizontal eye movement, which became rapid and sporadic during REM sleep. The electromyogram, or EMG, recorded chin and neck muscle movement which, for as yet undetermined reasons, completely relaxed during REM sleep. Kleitman and Aserinsky found that when subjects were awakened from REM sleep they almost always reported a dream, which was seldom the case when awakened from non-REM sleep.
Following the initial REM discoveries, sleep research greatly increased. One important discovery arising from this research was the high prevalence of sleep disorders, some of which now explain problems previously blamed on obscure physical or psychological disorders but which could not be effectively treated by medicine or psychiatry .
Combined, the EEG, EOG, and EMG produce a fascinating picture of sleep's structure. These monitoring devices transfer electronic stimulus to magnetic tapes, or on to paper via mechanical pens. The number of complete brain wave cycles per second are measured in "hertz" (Hz) by the EEG. The difference between the highest and lowest point of each wave (the peak and trough) is measured in" amplitude," (millionths of a volt, or microvolts-uV). As sleep approaches and deepens, hertz decrease and amplitude increases.
Stages of sleep
Very specific rhythms occur in different stages of the sleep-wake cycle. Beta rhythms are fast, low voltage waves (usually above 15 Hz and below 10 uV) which appear in alert, wakeful states. In the quiet, restful wakeful state prior to sleep onset, or in relaxed meditative state with the eyes closed, the brain displays alpha rhythms of about 8-11 Hz and 50 uV. Fairly high chin muscle activity and slow, rolling eye movements are recorded. Alpha waves disappear with visual imagery or opening the eyes, which causes alpha blocking.
Non-REM sleep is generally believed to occur in four stages and is characterized by lack of dreaming. As the sleeper enters the drowsy, light sleep of stage 1, theta rhythms, ranging between 3.5-7.5 Hz with a lower voltage, appear. The sleeper is generally nonresponsive during this stage, which takes up about 5% of the sleep cycle, but is easily awakened. Once again, high chin muscle activity occurs and there is occasional slow, rolling eye movement.
Within a few minutes, the sleeper enters stage 2 sleep. Brain waves slow even further and spindles (short bursts of electrical impulses at about 12-14 Hz which increase and decrease in amplitude) appear, along with K-complexes (sharp, high voltage wave groups, often followed by spindles). These phenomenon may be initiated by internal or external stimuli or by some as yet unknown source deep within the brain. A few delta waves may appear here. This portion of sleep occupies about 45% of the sleep cycle.
Normally, stage 3 sleep, comprised of 20-50% low frequency/high voltage delta waves, follows stage 2 as a short (about 7% of total sleep) transition to stage 4 sleep, which shows slower frequency higher voltage delta wave activity above 50%. There is virtually no eye movement during stages 2, 3, and 4.
In stage 4 sleep, some sleep spindles may occur, but are difficult to record. This stage occupies about 13% of the sleep cycle, seems to be affected more than any other stage by the length of prior wakefulness, and reflects the most cerebral "shutdown." Accordingly, some researchers believe this stage to be the most necessary for brain tissue restoration. Usually grouped together, stages 3 and 4 are called delta, or slow wave sleep (SWS), and is normally followed by REM sleep.
The sleep cycle from stage 1 through REM occurs three to five times a night in a normal young adult. Stages 3 and 4 decrease with each cycle, while stage 2 and REM sleep occupy most of the last half of the night's sleep. Time spent in each stage varies with age, and age particularly influences the amount time spent in SWS. From infancy to young adult, SWS occupies about 20-25% of total sleep time and perhaps as little as 5% by the age of 60. This loss of time is made up in stage 1 sleep and wakeful periods.
The period comprised of the four stages between sleep onset and REM is known as REM latency. REM onset is indicated by a drop in amplitude and rise in frequency of brain waves. The subject's eyes flicker quickly under the eyelids, dream activity is high, and the body seems to become paralyzed because of the decrease in skeletal muscle tone. After REM, the subject usually returns to stage 2 sleep, sometimes after waking slightly. REM sleep occurs regularly during the night. The larger the brain, the longer the period between REM episodes-about 90 minutes for humans and 12 minutes in rats .
REM sleep is triggered by neural functions deep within the brain, which releases one type of neurotransmitter (chemical agent) to turn REM sleep on and another to turn it off. Whereas autonomic activity (such as breathing and heart rate) slows and becomes more regular during non-REM sleep, it becomes highly irregular during REM sleep. Changes in blood pressure, heart rate, and breathing regularity take place, there is virtually no regulation of body temperature, and clitoral and penile erections are often reported. Most deaths, particularly of ill or aged individuals, happen early in the morning when body temperature is at its lowest and the likelihood of REM sleep is highest.
REM activity is seen in the fetus as early as six months after conception. By the time of birth , the fetus will spend 90% of its sleep time in REM but only about half that after birth. REM constitutes about 20-30% of a normal young adult's sleep, decreasing with age. These observations support one of several theories about our need for REM sleep which suggests that, to function properly, the central nervous system requires considerable stimulation, particularly during development. Because it receives no environmental stimulation during the long hours of sleep, it is possible that the high amount of brain wave activity in REM sleep provides the necessary stimulation.
See also Biological rhythms.
anch, a. michael, et al. sleep: a scientific perspective. englewood cliffs, nj: prentice hall, 1988.
ellman, steven j., and john s. antrobus, eds. the mind in sleep: psychology and psychophysiology. new york: john wiley & sons, 1991.
horne, james. why we sleep: the functions of sleep in humans and other mammals. oxford: oxford university press, 1988.
montplaisir, jacques, and roger godbout, eds. sleep and biological rhythms: basic mechanisms and applications to psychiatry. new york: oxford university press, 1990.
moorcroft, william h., and luther college. sleep, dreaming, and sleep disorders: an introduction. lanham: university press of america, 1989.
reite, martin, kim nagel, and john rudd. concise guide to evaluation and management of sleep disorders. washington, dc: american psychiatric press, 1990.
stampi, claudio, ed. why we nap: evolution, chronobiology, and functions of polyphasic and ultrashort sleep. boston: birkhauser, 1992.
Marie L. Thompson
KEY TERMS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
- Alpha/beta/delta/theta rhythms
—Brain wave activity occurring in different stages of wakefulness or sleep identified by amplitude and frequency.
—Difference between the highest and lowest point of a wave.
- Autonomic nervous system
—The part of the nervous system that controls involuntary processes, such as heart beat, digestion, and breathing.
- Circadian rhythms
—The rhythmical biological cycle of sleep and waking which, in humans, usually occurs every 24 hours.
—The body's automatic attempt to maintain balance and stability of certain internal functions, such as body temperature, influenced by the external environment.
—Chemical changes in body tissue which convert nutrients into energy for use by all vital bodily functions.
- Phase advance/phase delay
—Adjustment of circadian rhythms from their internal, biologically controlled cycle of approximately 25 hours to the 24-hour-a-day cycle imposed by the Sun.
Sleep is a state marked by a lowered level of consciousness, decreased movement of the body’s muscles, and slowing of metabolism*.
- * metabolism
- (me-TAB-o-liz-um) includes the chemical processes in the body that convert foods into the energy needed for body functions.
for searching the Internet and other reference sources
While it might sound like the title of a new science fiction movie, this phrase actually describes the typical experience of most people who live to the age of 75. The fact that people spend about one-third of their lives asleep (roughly eight hours out of every twenty-four-hour day) suggests that sleep is very important to how their brains and bodies function.
The desire for sleep is strong. People may be able to deny themselves food or water for a few days, but they cannot go without sleep. Research has shown that severely sleep-deprived people become very uncomfortable and anxious and may even start to hallucinate*. This explains why prisoners-of-war sometimes are forced by their captors to stay awake for long periods of time: the captors know that prisoners are likely to become so desperate for sleep that they will eventually break down and share important information.
- * hallucinate
- (huh-LOO-sin-ate) means to hear, see, or other-wise sense things that are not real.
Researchers have not been able to precisely explain the function that sleep serves. At one time it was thought that sleep provided a time for the brain to rest, but studies have shown that the brain’s neurons, or nerve cells, are just as active during sleep as when a person is awake. Some neurons are even more active during sleep. Another theory about the function of sleep is that sleep is essential for the proper storage of long-term memories by the brain. A study by researchers at Harvard University found that performance of a newly learned task actually improves after a good night’s sleep. Yet another theory is based on the observation that the brain makes proteins at a much faster rate when people are asleep than when they are awake. These proteins are necessary for maintaining the structure and function of neurons, so it may be that sleep gives the brain the chance to replenish its store of these important substances. This would explain why people report feeling “burnt out” when they are not getting enough sleep; the brain literally may be burning through its proteins faster than it can replace them.
While researchers are still trying to figure out why people need to sleep, they have learned a great deal about what happens during the sleep process itself.
Measuring brain waves with electroencephalography (EEG)
Throughout the nervous system, neurons communicate with each other in a language that is both chemical and electrical. Researchers can use specialized equipment to measure the waves of electricity generated by the hundreds of thousands of brain cells. This test, which is called electroencephalography (eh-lek-tro-en-sef-uh-LAH-gruh-fee), or EEG, involves placing electrodes on the scalp and attaching them to a machine that amplifies the electrical changes that are occurring. A special recording pen then translates these changes onto paper, generating waves of various sizes. The appearance of the EEG changes according to whether a person is excited or anxious, calm or relaxed. For instance, when a person is relaxed, the brain generates what are known as alpha waves from the back of the head. People can actually learn how to generate alpha waves by thinking calm, soothing thoughts and entering a state of total relaxation.
“Get a Good Night’s Sleep” May Be Good Advice
Instead of cramming all night for the weekly math quiz, a person might be better off practicing the new concepts until bedtime and then getting a good night’s sleep. A team of researchers at Harvard University found that college students who got a full night’s sleep after they learned a new task were much more likely to improve their performance on the task the next day than those who did not get enough sleep. The researchers taught the students to spot visual targets on a computer screen and to press a button as soon as they were certain they had seen one. With about an hour of practice, students were able to perform the task well. When retested later that same day, the students showed no improvement over their best times. When tested the next day, students who had slept six hours or less showed no improvement. Other students who slept more than six hours performed better than their best times from the previous day.
Did the extra sleep really make the difference? In another study with a different group of students, the researchers allowed half of the students to sleep at the end of the day but kept the other half awake until the next night. Then both groups were allowed to sleep on the second and third nights. On the fourth day, both groups were tested. The researchers found that the students who slept the first night performed better than the students who did not, even though the others had two nights of catch-up sleep. This finding seems to provide even more evidence that sleep plays a key role in helping the brain absorb and retain new material.
Research shows that the brain makes proteins essential for neuron function at a faster rate during sleep than during waking hours. But close to half of all teens do not get as much sleep as they need, leading to feelings of “burn out,” possibly because the brain is burning through proteins faster than it can replace them.
Shallow sleep and deep sleep
Thanks to sleep studies using EEG, researchers have been able to identify four stages of sleep that are distinguished by different wave patterns recorded from the brain: stage four is the deepest, and stage one is the shallowest. Within forty-five minutes of falling asleep, most people descend very rapidly to level four, which is marked by larger, slower brain waves on the EEG. Heart rate and blood pressure decrease, the muscles relax, and it is very difficult to awaken the sleeper. Throughout the night, the person surfaces from level four to levels three, two, and one (progressively shallower states of sleep in which the brain waves get smaller and faster) and then back down to level four again. This cycle happens several times during the night.
The red regions of these PET scans show the difference in brain activity during normal sleep (left) and REM sleep (right). During REM sleep dreams occur, and the brain strengthens memories by processing experiences and new information. The PET scan of the REM brain actually looks similar to a PET scan of a brain that is fully awake. Photo Researchers, Inc.
During the shallow sleep stages, people enter into yet another stage of sleep known as Rapid Eye Movement, or REM. This is when dreaming occurs. The eyes move rapidly behind the closed eyelids, and the person’s muscles become very rigid. In addition to moving through the four non-REM sleep stages, people can experience as many as four or five periods of REM sleep per night; in general, younger people spend more time in REM sleep than do older people. Because dreams are quickly forgotten after moving out of the REM phase, most people probably dream much more than they think they do. Ordinarily, a person will only remember the dreams that he or she had during the REM phase that occurred just before waking.
Research studies have shown that all mammals, not just humans, experience REM sleep, and that the brain knows the difference between this stage and the non-REM stages. Sleep studies have been done in which people are persistently awakened before they can enter REM sleep. After this goes on for a few nights, the people are allowed to sleep normally. Measurements show that they tend to spend more time in REM sleep on successive nights than they usually would. In a sense, the brain seems to be making up for lost REM-sleep time. This holds true for the deep sleep stages (non-REM stages three and four) as well; if people are deprived of these stages, they will spend more time in them on successive nights.
There are a variety of physical, psychological, and environmental factors involved in the sleep process.
Levels of alertness in the brain are controlled by different groups of chemical neurotransmitters* in certain areas of the lower portions of the brain. Levels of these chemicals vary at different times of the day and as emotions change. The lower portions of the brain communicate with the higher portion of the brain, the cortex*, signaling it to increase its activity or to slow down. Slow-down signals come just before bedtime or at certain other times of the day; many students can relate to feeling sleepy during an early afternoon lecture. However, the cortex has some say in the matter as well. People who are nervous or under stress often have trouble falling asleep because they cannot stop thinking about what is bothering them. And the tired student can force herself to stay awake if she knows the lecture is important. In other words, the brain’s cortex can override the signal to fall asleep.
- * neurotransmitters
- (NUR-o-tranzmit-erz) are brain chemicals that let brain cells communicate with each other and therefore allow the brain to function normally.
- * cortex
- (KOR-teks) is the part of the brain that controls conscious thought. It is where people experience conscious, subjective feelings.
Another chemical substance, a hormone* called melatonin (mel-uh-TOE-nin), also seems to play a role in producing sleep. Melatonin is released into the bloodstream by a tiny gland located in the center of the brain. The amount of melatonin in the blood fluctuates widely over the course of a twenty-four-hour period, with levels being up to ten times greater at night than during the day.
- * hormone
- are chemicals that are produced by different glands in the body. A hormone is like the body’s ambassador: it is created in one place but is sent through the body to have specific effects on other parts of the body.
People also have a main “biological clock” that is located within the area in the center of the brain known as the hypothalamus*, This area receives direct input from the eye, responding to the light changes of day and night and keeping the body on a twenty-four-hour cycle. This cycle, also referred to as the circadian (sir-KAYdee-un) rhythm, involves predictable changes in body temperature, heart rate and blood pressure, sleepiness and wakefulness, and other functions. For example, body temperature drops to its lowest point between 2 A.M. and 5 P.M. and reaches its highest point in the afternoon or evening.
- * hypothalamus
- (hy-po-THALuh-mus ) is a brain structure located deep within the brain that plays a part in regulating automatic body functions such as heart rate, blood pressure, temperature, respiration, and the release of hormones.
Light and dark
The cues of light and dark help to keep a person’s sleep patterns and other bodily rhythms on track. However, studies have shown that when a person is deprived of light, his or her body resets its internal clock to a twenty-five-hour schedule. People who have taken part in such studies—living in darkness for weeks at a time—tend to underestimate the amount of time they spent living without the normal cues of day and night. Clearly, then, our environments have some effect on the patterns on sleep and wakefulness that structure our lives.
There are many types of disorders that can make sleeping difficult, including:
Researchers believe that the body’s daily “clock,” also called its circadian rhythm, is linked to the pineal gland and to the suprachiasmatic nucleus region of the hypothalamus. These structures within the brain receive information from the eye’s retina about daylight and darkness, and send signals about regulating body responses to the spinal cord and elsewhere in the nervous system.
- Insomnia (in-SOM-nee-uh) is the best-known sleep disorder. It means simply that a person has trouble falling asleep. Just about everyone experiences insomnia at some point in their lives, but for some people this is a condition that persists night after night.
- Parasomnias (par-uh-SOM-nee-uhs) are a wide range of disruptive sleep-related events, such as sleep walking, sleep talking, eating while asleep, or night terrors*. Often the person is completely unaware of what he or she is doing during these episodes.
- * night terrors
- occur during deep (stage 4) sleep, usually within an hour after a person goes to bed. People experiencing night terrors may sit up in bed, scream, cry, sweat, and appear to be extremely frightened, but they are still asleep and are unaware of their environment. Night terrors most commonly affect young children, although anyone can experience them.
- Periodic limb movement disorder is a condition in which people repeatedly twitch or jerk their limbs in their sleep.
- Restless leg syndrome is a condition in which people feel uncomfortable sensations in their legs prior to falling asleep. People with this condition often feel the irresistible urge to move their legs; they may also experience cramping, burning, pain, or a creeping or crawling sensation.
- Sleep apnea (AP-nee-uh) involves having trouble breathing while asleep; the sleeper might snore loudly or sound as if he or she is gasping for air.
Physicians who specialize in sleep medicine are trained to diagnose and treat these and other sleep problems. Typically, patients need to discuss their symptoms with the doctor and undergo overnight testing in a sleep lab. A combination of medications and behavioral therapy can usually help.
Brain Chemistry (Neurochemistry)
American Academy of Sleep Medicine, 6301 Bandel Road NW, Suite 101, Rochester, MN 55901. Telephone 507-287-6006 http://www.aasmnet.org
National Sleep Foundation, 1522 K Street NW, Suite 500, Washington, DC 20005
Fax: 202-347-3472 http://www.sleepfoundation.org
Sleep and Children's Physical Health
SLEEP AND CHILDREN'S PHYSICAL HEALTH
Sleep is not a passive extravagance that people allow themselves to indulge in. On the contrary, sleep is a highly regulated, active state of being that engages many aspects of one's physiology in a complex manner. It is essential to life. While the purpose of sleep remains a complicated mystery, depriving one's self of sleep has serious consequences for one's health and waking functions. Nevertheless, sleep continues to be encroached upon by daily activities. Of particular concern are accounts of inadequate sleep and daytime sleepiness among school-age children and adolescents, and the potential impact these conditions may have on development and learning.
Biological Factors That Affect Sleep
Sleepiness refers to the tendency for a waking person to fall asleep. This tendency may be strong or weak, and is determined by both homeostatic and circadian influences. Homeostatic determinants include the amount of time since a child last slept and the amount of sleep debt (i.e., previously poor or inadequate sleep over one or more nights) that the child is carrying. Sleep debts can only be paid back with sleep, and increasing homeostatic pressure to sleep cannot, ultimately, be denied. The circadian system influences daytime sleepiness through clock-dependent alerting. Clock-dependent altering refers to the function of people's circadian system to promote wakefulness at certain times of their biological clock–namely, at the beginning and just before the end of their biological "day"–thereby helping them wake from sleep in the morning and stay awake in the latter part of the day when homeostatic pressure increases. Clock-dependent alerting is lowest in the early afternoon, which helps to explain why an adolescent or young adult may find it easier to fall asleep in the early afternoon than in the early evening.
While sleepiness is primarily determined by homeostatic and circadian influences, environmental and time-of-day factors influence the immediate effects of sleepiness on daytime functioning. Arousing elements of one's external environment and/or internal state can temporarily mask sleep tendency. Someone out late at a nightclub after working all day has an increased tendency to fall asleep, but this can be masked temporarily by arousing environmental elements (e.g., music), the physical exercise of dancing, and possibly by consuming psychostimulants, such as caffeinated beverages, nicotine, or certain illicit drugs. But sleepiness that is masked is not diminished and could quickly be unmasked after leaving the nightclub. Depending on the time of night and the amount of homeostatic pressure, the person could experience microsleeps during the drive home. Microsleeps are brief, involuntary sleep attacks of a second or more that can occur outside of awareness. They are more likely to occur when excessive sleepiness is unmasked at a time of low clock-dependent alerting, such as during one's biological "night."
Daytime sleep tendency also appears to be affected by age or, more specifically, pubertal development. Mary Carskadon and colleagues examined sleep and sleepiness in children studied annually from age ten to age sixteen or seventeen. Study participants were allowed a sleep opportunity (i.e., bedtime to risetime) of ten hours per night at each assessment, and daytime sleep tendency was measured the following day using the Multiple Sleep Latency Test (MSLT), a series of objective tests measuring the time it takes to fall asleep under optimal "nap" conditions. Results across years showed virtually no change in the average amount of sleep (9.2 hours) recorded from bedtime to risetime. Thus, the need for sleep at night did not appear to decrease across puberty. However, when children reached midpuberty their midday sleep tendency on the MSLT appeared to increase relative to their prepubertal levels, even though participants were sleeping the same amount at night. These results demonstrate that pubertal development is associated with an increase in daytime sleepiness, suggesting that postpubertal adolescents may actually need more sleep to maximize daytime alertness.
For the average middle and high school student, getting 9.2 hours of sleep or more on school nights may seem impossible and not worth the sacrifices required to maintain such a schedule. This is not surprising. The twenty-four-hour society of the United States makes ever-increasing demands on the time available for studying, working, and exercising, and offers ever-increasing opportunities for socializing and recreating. As a result, students are easily drawn into a pattern of pursuing daily activities at the expense of a good night's sleep.
In addition, role models for marginalizing the importance of sleep are plentiful. Physicians, lawyers, stockbrokers, and even political operatives are portrayed on television as heroically pushing their physical limits and rising above their lack of sleep. Closer to home, parents often fail to convince children to "do as I say not as I do" with regard to obtaining a good night's sleep, as they often allow their own commitments to encroach on sleep. Thus, from the beginning of primary school to the end of secondary school the average amount of time students spend sleeping on school nights gradually diminishes at the rate of one hour every three years, mostly through postponing bedtime. By the end of high school students average just over seven hours of sleep each school night, close to the adult work-night average of just under seven hours. These trends in school-night sleep time have been described in industrialized countries around the world.
While societal and familial factors influence these trends, at least one biological process may also be involved. As children move through puberty they often begin to prefer activities occurring later in the day. This shift toward evening preference may be expressed biologically as a shift in the timing of the body's readiness for sleep and wake, also referred to as circadian timing of sleep phase. A shift toward evening preference accompanied by a biological tendency to delay sleep phase may make it easier for adolescents to stay up later. Sleeping later in the morning would offset this tendency and allow students to be more consistent in the sleep they obtain on school nights, but this conflicts with trends for the average school day, which usually starts and ends earlier as children move from primary to middle to secondary school.
The direct consequence of these social, behavioral, and biological trends is that older children and adolescents often do not obtain enough sleep on school nights to optimize daytime alertness and, they therefore carry a burgeoning sleep debt into the weekend. The typical solution is to wake up later on weekends. In adolescence, weekend sleep amounts average approximately nine hours per night, which might allow students to "pay back" the sleep debt accumulated across the week–if that debt was not so large. Given the amount of sleep determined to optimize alertness (approximately nine hours) and the fact that school-night sleep amounts average below 7.5 hours for adolescents, the average adolescent accumulates seven or more hours of sleep debt per school week. In addition to failing to pay back the sleep debt, going to bed later and sleeping substantially later in the morning on weekends can possibly exacerbate evening preference and delay the circadian timing of sleep phase, thus making sleep less likely to occur at a student's normal bedtime on Sunday.
Effects of Insufficient Sleep
The consequences of insufficient sleep and chronic daytime sleepiness in the lives of school-age children and adolescents are difficult to characterize at this time due to the limited number of scientific studies with this age range. Available data suggests that behavior, health, learning, and mood are likely to be impaired by excessive sleepiness among pediatric groups, but causal connections have not been proven and any relation between amount of sleep loss and amount of subsequent impairment (a dose-response relationship), has yet to be described.
Behavior. Children who show increased sleepiness or who have a disorder that compromises the quality and/or quantity of sleep appear to be at greater risk for daytime behavioral problems. Decreased behavioral difficulties have been associated with successful treatment of sleep disorders.
Health. Correlations have been shown between poor quantity and/or quality of sleep and the following: increased days sick from school, increased physical complaints, risk for accidents or injuries, and adoption of health-risk behaviors such as increased consumption of alcohol, nicotine, and caffeine. Of particular note for older adolescents, drivers age twenty-five or younger were shown to be responsible for a majority of fall-asleep automobile crashes in one region of the country.
Performance and learning. Tests of cognitive performance administered to students with sleep disorders or to healthy students experimentally sleep-restricted have generally failed to produce consistent results, but data suggest that students process information and react more slowly following inadequate sleep, and may be more prone to errors with socalled higher cognitive functions that involve abstract problem solving, creativity, or rule-governed behavior. Survey studies consistently demonstrate that students with later school-night bedtimes, more irregular bedtimes, less sleep on school nights, sleep problems, and increased complaints of daytime sleepiness have lower academic achievement than children with earlier, more regular bedtimes, more sleep, no sleep problems, and fewer complaints of sleepiness. Improved performance and academic achievement have been reported following treatment for sleep disorders.
Mood. Preliminary results from experimental and correlational studies provide consistent support for an association between inadequate quantity and quality of sleep among children and diminished happiness and/or increased depressed mood.
In conclusion, there is a need to learn more about the life-enhancing benefits of increasing sleep and the high cost of failing to protect it among children and adolescents. Determining the optimal quantity and timing of nocturnal sleep is likely to vary among individuals but existing trends suggest that many students should consider expanding school-night sleep opportunities, especially in the second decade. Students need to be more consistent with bedtimes and risetimes on school and non-school nights to avoid confusing the biological clock. Students also need to avoid caffeinated beverages and nicotine, as these substances can mask sleepiness and lead to difficulty falling asleep if taken later in the day. A brief afternoon nap is a much healthier alternative. Parents need to work with their children to create sleep-friendly family routines that make it easier for children (and adults) to protect sleep. Finally, more work is needed in communities to create sleep-friendly school schedules and work guidelines for minors, and to raise awareness about the risks associated with drowsy driving.
See also: Health and Education; Out-of-School Influences and Academic Success; Parenting.
Carskadon, Mary A. 1982. "The Second Decade." In Sleeping and Waking Disorders: Indications and Techniques, ed. Christian Guilleminault. Menlo Park, CA: Addison-Wesley.
Carskadon, Mary A. 1999. "When Worlds Collide: Adolescent Need for Sleep Versus Societal Demands." Phi Delta Kappan 80:348–353.
Carskadon, Mary A., ed. 2002. Adolescent Sleep Patterns: Biological, Sociological, and Psychological Influences. Cambridge, Eng.: Cambridge University Press.
Dahl, Ronald E. "The Consequences of Insufficient Sleep for Adolescents: Links Between Sleep and Emotional Regulation." Phi Delta Kappan 80:354–359.
Graham, Mary G., ed. 2000. Sleep Needs, Patterns, and Difficulties of Adolescents: Summary of a Workshop. Forum on Adolescence, Board on Children, Youth, and Families, National Re-search Council, Institute of Medicine. Washington, DC: National Academy Press.
Sadeh, Avi; Gruber, Reut; and Raviv, Amiram. 2002. "Sleep, Neurobehavioral Functioning, and Behavior Problems in School-Age Children." Child Development 73:405–417.
Sadeh, Avi; Raviv, Amiram; and Gruber, Reut. 2000. "Sleep Patterns and Sleep Disruptions in School-Age Children." Developmental Psychology 36:291–301.
Valent, Francesca; Brusaferro, Silvio; and Barbone, Fabio. 2001. "A Case-Crossover Study of Sleep and Childhood Injury." Pediatrics 107 (2):E23.
Wolfson, Amy R., and Carskadon, Mary A. 1998. "Sleep Schedules and Daytime Functioning in Adolescents." Child Development 69:875–887.
The rich variegation of sleep phenomena can already be appreciated in its definition as a behaviour characterized by postural immobility (but with periodic changes in body position and muscle tone), by decreased response to external stimuli (but with marked fluctuations in threshold to response), by selective sensitivity to some stimuli, and by an orderly sequence of electrical and chemical changes in the brain that affect the entire body and greatly alter the mind. Clearly, sleep is an active, global, organismic state requiring central control by the brain and affording the brain and body a wide variety of functional opportunities.
Subjective experience was not the only obstacle to appreciation of the manifold complexity of sleep. Because of our modesty, we do not normally welcome the observation of our sleep. And because we all tend to sleep at the same time, there is no one to watch over those few who are willing to be observed. The development of sleep laboratories in the last half century has begun to counter these trends and to create the detailed picture we have today, but naturalistic studies of sleep are still woefully inadequate.
Sleep laboratory studiesMost sleep laboratories consist of two rooms; one with a bed for the subject, connected via a one-way window and by cables to the other, an instrument room where a technician monitors the sleeping subject (sometimes also by video). Recordings are made of electrical signs from the brain (electroencephalogram or EEG); from the eye (electro-oculogram or EOG); and from the muscles (electromyogram or EMG). A polygraph is used to keep track (graph) of the several (poly) signals simultaneously. Other important bodily functions, like body temperature, breathing, heart rate, blood pressure, and even sex organ volume, can also be recorded.
A typical night of sleep in an adult human is divided into four or five distinct cycles of body and brain activity. Each cycle begins with a relaxation phase, showing declines in brain wave (EEG) activation, muscle tone (EMG), eye movement (EOG), heart rate, breathing rate, and blood pressure, all of which typically reach a nadir after 45–60 minutes. This relaxation phase then gradually gives way to an activation phase, in which many of the brain and bodily functions resume the high levels of the awake state. In the face of this activation, sleep is maintained by the active suppression of sensory (input) signals and motor (output) commands.
Over the course of the night the length and depth of the relaxation phase (which is called quiet, NREM (non-rapid eye movement), or EEG slow-wave sleep) declines as the duration and intensity of the activation phase (called active, REM or EEG fast-wave sleep) increases. About 70–80% of an average sleep bout of 6.5–8.0 hours consists of NREM sleep, while 20–30% is REM. Other bodily functions which are associated with NREM sleep include the secretion of the hormones regulating growth and sexual maturation. REM sleep is associated with profound muscle relaxation and with sex organ distension, including full erection (and is therefore a built-in test of physiological potency), and a loss of the capacity for internal temperature regulation. The rapid eye movements that give REM sleep its name are not continuous but occur in flurries or clusters, each of which is associated with (sometimes dramatic) increases in the rate, or with irregularity, of heartbeat and breathing. Awakenings which follow these REM clusters are very likely to yield long and detailed reports of dreaming.
Variations in sleepSleep varies markedly over the life cycle as well as overnight. New-born infants lack the capacity for long, deep NREM sleep. This only develops, with brain maturation, during childhood and adolescence. But babies have an exaggerated propensity for REM sleep, often entering it directly from waking (so it can easily be observed by curious carers). Since sleep duration is about twice as long in neonates (16 vs. 8 hours) and REM is twice as common (50% vs. 25%), the new-born spends four times longer in REM than does the adult (8 vs. 2 hours). REM sleep declines dramatically as sleep depth increases with brain maturation and the emergence of the adult pattern.
But this is not the end of the dynamism of sleep development. The capacity for deep NREM sleep falls precipitately between ages 30 and 40. This leads to a normal decline in the ability to sustain sleep and to feel deeply rested by it. REM sleep remains relatively stable, but its decline may cause further deterioration of sleep quality after age 60, especially as other medical problems interfere with sleep.
Individuals also show marked differences in sleep behaviour. Most of us lie between two extreme ends of a bell-shaped curve of sleep length and efficiency. At one end are the short-sleepers, who need as few as 3–5 hours, and at the other are long-sleepers, who need 8–11 hours to feel rested and refreshed by sleep. Short-sleepers tend to be energetic, active, and productive, while long-sleepers tend to be lethargic, sedentary, and reflective. Society, with its interest in tight schedules and productivity, is kind to short but merciless to long-sleepers. Long-sleepers are ill-advised to seek professions, like medicine, which greatly curtail sleep.
Even within individuals of a given sleep need and age, sleep varies from night to night, and poor or lost sleep tends to be rapidly compensated. This reciprocal dynamic is dramatically revealed by studies in which one or another sleep phase or time is deliberately altered and the recovery process is monitored.
Much has been learned about sleep from laboratory studies of non-human animals. For example, the diversity of sleep behaviour increases as the brain becomes more and more specialized during evolution. Below the level of the reptiles (who have clear-cut NREM sleep but not REM), it is difficult to distinguish sleep from simple inactivity. REM sleep first appears in birds and then only fleetingly, because while hatchlings have it in abundance, adults have little or none. REM sleep is first clearly and enduringly seen in mammals, suggesting a relationship to the two features which distinguish that class of animal: large, highly developed brains and the capacity for strong internal temperature control.
Brain mechanisms of sleepThere is exquisite control of sleep by the brain. In mammals, sleep is one of the key bodily functions controlled by the body clock in the hypothalamus. By these means it is also tied to the rhythm of body temperature, such that sleep occurs as body temperature falls and waking occurs when body temperature is highest. For most animals, including humans, these peaks in alertness and energy availability occur during the daylight hours, but animals (like rats) that rely more on smell than on vision are active at night and sleep in the daytime. In very hot climates humans may also shift their activity into the darker, cooler night and have a siesta during the forbiddingly hot period of the early afternoon.
The body clock times the occurrence of sleep via its direct nervous connection between the hypothalamus and other subcortical structures in the lower brain. Of particular importance are those collections of brain cells in the brain stem which manufacture and liberate from their endings two brain chemicals, noradrenaline (norepinephrine) and serotonin, which appear to have energizing effects needed for the waking functions of the brain and the body. In order for sleep to occur the activity of these brain cells must be quelled by the mechanism of inhibition. As their activity is more and more completely diminished, another group of cells becomes increasingly active and liberates more and more molecules of another chemical (acetylcholine), which appears to mediate restorative functions throughout the body and the brain. It is the reciprocal interaction of the two cell groups that appears to provide the basis of the cyclic alternation of NREM and REM sleep and their functional differentiation.
Functions of sleepSleep is vitally necessary. Recent experiments on the effects of prolonged sleep deprivation give hints as to why even short-term sleep loss is so disabling and why it is so vigorously compensated by the brain. If sleep deprivation is extended beyond two weeks, rats develop a distinctive group of signs that inevitably leads to their demise. Their skin breaks down and they show an increasing craving for food but cannot maintain their body weight no matter how much they eat. At the same time they develop more and more determined heat-seeking behaviour, as they cannot control their body temperatures when exposed to normal variation in environmental temperature. Short of these extreme effects, more modest sleep deprivation has been shown to create a wide variety of difficulties. Taken together these suggest that sleep may normally play an important role in the maintenance of such important bodily functions as the immune response and metabolic balance, as well as such critical mental functions as attentiveness, learning and memory, and emotional equilibrium. Shakespeare may have been correct when he said that sleep ‘knits up the raveled sleeve of care’, but he was underestimating the more active developmental and survival functions of sleep.
J. Allan Hobson
See also dreaming; electroencephalogram; sleep disorders; snoring.
sleep / slēp/ • n. a condition of body and mind such as that which typically recurs for several hours every night, in which the nervous system is relatively inactive, the eyes closed, the postural muscles relaxed, and consciousness practically suspended: I was on the verge of sleep| [in sing.] a good night's sleep. ∎ chiefly poetic/lit. a state compared to or resembling this, such as death or complete silence or stillness: a photograph of the poet in his last sleep. ∎ a gummy or gritty secretion found in the corners of the eyes after sleep: she sat up, rubbing the sleep from her eyes.• v. (past and past part. slept / slept/ ) [intr.] rest in such a condition; be asleep: she slept for half an hour| [as adj.] (sleeping) he looked at the sleeping child. ∎ (sleep through) fail to be woken by: he was so tired he slept through the alarm. ∎ have sexual intercourse or be involved in a sexual relationship: I won't sleep with a man who doesn't respect me. ∎ [tr.] (sleep something off/away) dispel the effects of or recover from something by going to sleep: she thought it wise to let him sleep off his hangover. ∎ [tr.] provide (a specified number of people) with beds, rooms, or places to stay the night: studios sleeping two people cost $70 a night. ∎ fig. be inactive or dormant: Copenhagen likes to be known as the city that never sleeps. ∎ poetic/lit. be at peace in death; lie buried: he sleeps beneath the silver birches.PHRASES: one could do something in one's sleep inf. one regards something as so easy that it will require no effort or conscious thought to accomplish: she knew the music perfectly, could sing it in her sleep.get to sleep manage to fall asleep.go to sleep fall asleep. ∎ (of a limb) become numb as a result of prolonged pressure.lose sleepsee lose.put someone to sleep make someone unconscious by the use of drugs, alcohol, or an anesthetic. ∎ (also send someone to sleep) bore someone greatly.put something to sleep kill an animal, esp. an old, sick, or badly injured one, painlessly (used euphemistically). ∎ Comput. put a computer on standby while it is not being used, esp. in order to reduce power consumption.sleep easysee easy.sleep like a log (or top) sleep very soundly.sleep on it inf. delay making a decision on something until the following day so as to have more time to consider it.the sleep of the just a deep, untroubled sleep.sleep roughsee rough.sleep tight [usu. in imper.] sleep well (said to someone when parting from them at night).sleep with one eye open sleep very lightly, aware of what is happening around one.PHRASAL VERBS: sleep around inf. have many casual sexual partners.sleep in remain asleep or in bed later than usual in the morning. ∎ sleep by night at one's place of work.sleep out sleep outdoors.sleep over spend the night at a place other than one's own home: Katie was asked to sleep over with Jenny.ORIGIN: Old English slēp, slǣp (noun), slēpan, slǣpan (verb), of Germanic origin; related to Dutch slapen and German schlafen.