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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.

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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 waysbehavioral, subjective, physiologicalbut 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 14, 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 90120 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., Alzheimers, Parkinsons, and Huntingtons diseases) increase in both prevalence and severity with increasing age, and they can amplify sleep disruptions in older people. Alzheimers 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 Alzheimers, 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 Alzheimers 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 partners 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 insomniaan increase in sleep disruption after withdrawal of the drugwhich 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 Alzheimers Disease; Brain; Depression.


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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 10581071.

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 2642.

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 1525.

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 401415.

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): 425432.

Morin, C. M. Insomnia: Psychological Assessment and Management. New York: The Guilford Press, 1993.

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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.

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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 4555 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 2025 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

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.

Common problems

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 1530 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."

Losing sleep

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 1214 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.

Parental concerns

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 <> (accessed October 6, 2004).

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American Sleep Disorders Association. 1610 14th Street, NW, Suite 300, Rochester, MN 559012201. Web site: <>.

National Sleep Foundation. 1522 K Street, NW, Suite 500, Washington, DC, 20005. Web site: <>.


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Driver, Helen. "Parasomnias." Canadian Sleep Society. Available online at <> (accessed July 29, 2004).

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"Tips for Healthy Sleep." American Sleep Disorders Association. Available online at <> (accessed August 4, 2004).

Clare Hanrahan


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.

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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 clocknamely, 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.

Societal Factors

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 weekif 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.


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Gahan Fallone

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FALLONE, GAHAN. "Sleep and Children's Physical Health." Encyclopedia of Education. 2002. (May 28, 2016).

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sleep is a complex behaviour that is an integral part of the body's strategic adaptation to daily changes in light and temperature. Because we lose consciousness so dramatically when we fall asleep, it was erroneously assumed that brain activity ceased in sleep. The presence of vivid dreams made such a simplistic theory unlikely and, during the past fifty years, scientific research on the brain and body has shown sleep to be richly variegated, exquisitely controlled, and essential to life. It is now also clear that sleep is not always benign but has its own built-in propensity for disorder and disease.

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 studies

Most 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 sleep

Sleep 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 sleep

There 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 sleep

Sleep 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.

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COLIN BLAKEMORE and SHELIA JENNETT. "sleep." The Oxford Companion to the Body. 2001. 28 May. 2016 <>.

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A state that suspends the voluntary exercise of bodily functions and consciousness.

A healthy adult sleeps an average of 7.5 hours each night and most people (approximately 95 percent) sleep between 6.5 and 8.5 hours. Tracking brain waves with the aid of electroencephalographs (EEGs), researchers have identified six stages of sleep (including a pre-sleep stage), each characterized by distinctive brain-wave frequencies. Stage 0 is the prelude to sleep, which is characterized by low amplitude and fast frequency alpha waves in the brain. At this stage, a person becomes relaxed, drowsy, and closes their eyes. Stages 1 through 4 are sometimes characterized as NREM (non-rapid eye movement) sleep. In Stage 1, the eyes begin to roll and rhythmic alpha waves give way to irregular theta waves that are lower in amplitude and slower in frequency as the person loses responsiveness to stimuli, experiences, fleeting thoughts, and images. In Stage 2, electroencephalogram tracings show fast frequency bursts of brain activity called sleep spindles, marked by muscle tension and accompanied by a gradual decline in heart rate, respiration, and temperature. Stages 3 and 4 normally occur 30 to 45 minutes after falling asleep. In Stage 3, there are fewer sleep spindles, but high amplitude and low frequency delta waves appear. When these begin to occur more than 50 percent of the time, the fourth stage of sleep has been entered. Delta waves demarcate the deepest levels of sleep, when heart rate, respiration, temperature, and blood flow to the brain are reduced and growth hormones are secreted. A person roused from Stage 4 sleep will be groggy and confused. Altogether, it takes about a half hour to pass through these four stages of sleep.

Rapid eye movement (REM sleep), which makes up approximately 20 percent of sleep time, is interspersed with NREM sleep every 30 to 40 minutes throughout the night. It is during REM sleep that dreams are experienced. In this state, the same fast frequency, low-amplitude beta waves that characterize waking states occur, and a person's physiological signsheart rate, breathing, and blood pressurealso resemble those in a waking state. However, muscle tone decreases to the point of paralysis, with sudden twitches, especially in the face and hands. REM periods may last from 15 minutes at the beginning of a sleep cycle to one hour at the end of it. Most people complete four to six complete sleep cycles each night, with each cycle lasting about 90 minutes. These cycles vary in composition, however; early in the night most of the time is spent in Stage 3 and 4 sleep, with Stage 2 and REM sleep predominating later on. Sleep patterns also vary in the course of a person's life. On the average, an infant sleeps about 16 hours a day, in contrast to a 70-year-old who sleeps only about six hours. While REM sleep comprises about half of total sleep at birth , it eventually decreases to only 25 percent. Sleeping patterns also vary greatly among individuals, and even among different cultures (in terms of napping, for example).

Two theories of sleep, the repair and the adaptive theories, attempt to explain why sleep occurs. In the repair theory, sleep serves a biological need, replenishing key areas of the brain or body which are depleted during the day. The adaptive theory suggests that sleep as a function evolved over time because it prevented early humans from wasting energy and exposing themselves to nocturnal predators, thus aiding in survival. REM sleep in particular has been thought to serve special functions. Research subjects whose REM sleep was interrupted made up for the loss by spending extra time in the REM stage on successive nights. It has also been suggested that REM sleep aids the activity of neurons that use the neurotransmitter norepinephrine, thus maintaining waking alertness. Persons deprived of REM sleep have shown poorer retention of skills learned during the day, leading to the hypothesis that REM sleep helps in assimilating daytime learning experiences.

As with many other physiological processes, sleep is linked to a 24-hour circadian rhythm and affected by signals such as light and dark. The effects of disrupting the sleep-wake cycle can be seen in jet lag, which is characterized by fatigue, irritability, lack of alertness, and sleeping problems. A person affected by jet lag feels like sleeping at the wrong times of day. It has been found that the body maintains a circadian sleep-wake rhythm even in the absence of external cues like lightness and darkness, although research subjects deprived of such cues eventually adopt a 25-hour "day." The "internal clock" that maintains this pattern is a section of the brain called the supra chiasmatic nucleus (SCN), located in the hypothalamus .

Various disorders interfere with sleep. The most common is insomnia, the inability to fall asleep or stay asleep. Nearly one-third of all Americans are affected by some degree of insomnia. Often associated with mental distress, insomnia is treated with medication, psychotherapy , relaxation techniques, or a combination of these methods. The medications most commonly prescribed are benzodiazepines (Valium, Halcyon, Restoril) and barbiturates. While they alleviate insomnia in the short run, these drugs interfere with normal sleep patterns, and can lead to increased tolerance and dependence. Researchers and clinicians have had success treating insomnia with the hormone melatonin, a naturally occurring substance related to sleep onset and secreted by the pineal gland. Melatonin supplements first became available in American health food stores in 1993 and have become increasingly popular as a sleep aid, although their use has caused some controversy in medical circles.

Narcolepsy , a disorder characterized by sudden and uncontrollable occurrences of sleep, afflicts 100,000 people in the United States. This condition is genetically linked, and may be curable in the future. Individuals affected by narcolepsy abruptly enter REM sleep states during the daytime, collapsing and remaining immobile for a period of time after awakening. Napping and stimulants have both been used to treat this condition. Another disorder associated with sleep is sudden infant death syndrome (SIDS), in which a healthy baby stops breathing during sleep, fails to awaken, and suffocates. While the exact cause of SIDS is unknown, researchers are attempting to identify and save at-risk infants by studying the relationship between the disorder and sleeping patterns. In sleep apnea, a person repeatedly stops breathing while asleep but awakes each time. The disrupted sleep that results from these multiple awakenings leaves the sleeper fatigued and sleepy during the daytime. Night terrors are non-REM dream experiences from which the sleeper never fully awakes and which he or she does not recall upon awakening. This condition mostly occurs in children and can be treated with hypnosis or medication in severe cases.

See also Sleep Disorders.

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Sleep is a very important process and is characterized by a stereotypical posture, little movement, and a decrease in response to stimuli. These characteristics might also describe coma, but in sleep, unlike in coma, the characteristics are reversed each morning. Because creatures are not eating or mating and are also very vulnerable to attack by predators during sleep, sleep must have a very important function to make it worthwhile.

Sleep is also a very insistent drive. Whereas a person can voluntarily stop eating until he or she dies, the human body cannot force itself to stay awake indefinitely. In fact, there are situations when falling asleep might mean death (while driving a car, for example), yet the desire to sleep is so insistent that body will still succumb to it.

Stages of Sleep

Sleep is divided into two main stages: REM sleep (which stands for "rapid eye movement"), and non-REM (NREM) sleep. These stages are characterized by changes measured on instruments such as the electroencephalograph (EEG), which measures changes in electrical signals in the brain; electrooculogram (EOG), which measures eye movements; and the electromyogram (EMG), which measures muscle movements. In humans, REM and NREM sleep alternate in ninety-minute cycles approximately three to six times per night. During the first part of the sleep cycle, REM sleep takes approximately ten minutes of each cycle, but REM sleep periods become longer and closer together as the course of sleep progresses (see figure 1).

Non-REM sleep is divided into four stages. As one progresses from stage one to stage four, sleep gets deeper and EEG waves become taller and slower; stages three and four are often grouped together and called slow wave sleep (SWS). During SWS, muscle movements and eye movements are diminished in comparison to wakefulness, and the EEG is more synchronized, indicating that large portions of brain tissue are firing together.

REM sleep is characterized by a desynchronized EEG, a lack of thermoregulation , loss of tone in the skeletal muscle, erections of the penis or clitoris, rapid eye movements, and dreams. As seen by the desynchronized EEG, which is similar to the brain patterns seen during wakefulness, the brain is very active during REM sleep. However, one part of the brain that does shut off during REM sleep is the part of the hypothalamus that is responsible for temperature regulation. During REM sleep, the body does not thermoregulate and therefore does not shiver or sweat.

Skeletal muscles are also less active during REM sleep and thus lose muscle tone . This loss enables the muscles to relax during REM sleep. It also prevents people from acting out their dreams (sleep walking occurs during NREM sleep, when muscle tone is maintained but diminished). Not all muscles lose their tone during REM sleep. The diaphragm, necessary for breathing, continues to contract. Muscles are also active in the eyes: although the lids are closed, the eyes dart back and forth during REM sleep, which gives REM its name.

During REM sleep the penis and clitoris often become erect, but this is not necessarily related to dream content. Although REM sleep is associated with dreams, dreams actually occur during all stages of sleep. The dreams that occur during REM sleep have characteristics different from those dreams in NREM sleep. REM dreams are longer, more emotional, and more visual than NREM dreams, and they usually do not follow the events of the day as closely as NREM dreams.

Neurological Control

A common misconception is that the brain shuts down during sleep. In truth, parts of the brain may be even more active during NREM or REM sleep than during wakefulness. The level of consciousness depends upon the activity of the reticular activating system, a network of neurons in the brainstem that send projections throughout the thalamus, hypothalamus, and cerebral cortex . Certain areas of the brain have been found to be responsible for causing different sleep stages. Whereas NREM sleep is controlled by the basal forebrain (the anterior hypothalamus and adjacent forebrain areas), REM sleep is mostly controlled by an area in the brainstem called the pons.

Functions of Sleep

Although humans spend approximately one-third of their lives asleep, no one knows for sure what the function of sleep is. It is known that sleep is necessary for life. Constant sleep deprivation in rats leads to death. Studies suggest that constant deprivation of REM sleep alone causes metabolic changes in rats that can also lead to death.

There are a number of theories on the function of sleep. Sleep may help the body recover from an active day and give it the chance to restore substances that are lost during the day. However, since simply resting the body without sleep does not fulfill the same function as sleep, it is thought that there is more to sleep than resting.

Sleep may have developed evolutionarily as an adaptive mechanism to keep animals out of harm's way, preventing them from wandering around in the dark, vulnerable to accidents and attack by predators, during a time when food foraging may be less efficient. It is interesting to note that small animals with safe hiding places and large predators sleep a lot, whereas large prey sleep less often, suggesting sleep may be related to an animal's relative safety, although the evidence for this is far from clear. The question remains, however, why the complex process of sleep would have evolved to merely keep animals out of danger.

It is known that REM sleep is a necessary stage of sleep; however, the function of REM sleep is also unclear. Temporary REM sleep deprivation can lead people to become bad-tempered and uneasy. If REM deprivation continues, and the subject is then allowed to sleep undisturbed, he or she experiences "REM rebound," which means that REM sleep occurs more frequently and lasts longer than normal. There is evidence that REM sleep is necessary for learning and memory. Furthermore, theorists are proposing that REM sleep may be important in the development of the brain.

see also Brain; Hypothalamus; Temperature Regulation

Martha S. Rosenthal


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Myers, David G. "States of Consciousness." In Exploring Psychology, 4th ed. New York: Worth Publishers, 1999.

Rechtschaffen, A., et al. "Sleep Deprivation in the Rat. X: Integration and Discussion of the Findings." Sleep 12 (1989): 6887.

Sleep Research Society. "Basics of Sleep Behavior Syllabus." WebSciences International and Sleep Research Society, 1997. <>.

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A state of unconsciousness or partial consciousness in which, according to psychical belief, the human organism is being perpetually replenished with energy from an unseen world. Here-ward Carrington, writing in Your Psychic Powers and How to Develop Them (1920), notes: "Various theories have been advanced in the past to explain sleep, but no satisfactory theory has even been fully accepted. Thus we have the so-called 'chemical theories,' which endeavor to account for sleep by assuming that certain poisonous substances are formed in the body during waking hours and are eliminated during sleep. Others have suggested that sleep is due to peculiar conditions of the circulations of blood in the brain; still others that the action of certain glands explains sleep; others that muscular relaxation accounts for it, others that the lack of external stimuli is sufficient to induce profound slumber. All these theories have been shown insufficient to explain the facts. We shall never arrive at a satisfactory theory of sleep, doubtless, until we admit the presence of a vital force and the existence of an individual human spirit which withdraws more or less completely from the body during the hours of sleep, and derives spiritual invigoration and nourishment during its sojourn in the spiritual world."

In the paranormal phenomena observed in dreams and the hypnotic state, F. W. H. Myers found indications that "the self of sleep is a spirit freed from ordinary material limitations, and this conclusion conforms to the hypothesis that we live in two worlds; the waking personality is adapted to the needs of terrestrial life, the personality of sleep maintains the fundamental connection between the spiritual world and the organism, so as to provide the latter with energy while developing itself by the exercise of its spiritual powers."

Related to theories of sleep are theories on astral projection, also known as OBE (out-of-body experience) or soul traveling, in which the soul is said to leave its body, and travel about the astral plane. People who experience this, claim OBEs eliminate their fears of death, while convincing them of their connection to the spiritual realm. Writer Sylvan J. Muldoon, ex-plains the theory through his own experience: "the astral body discoincides during sleep for the purpose of recharging and the depth of sleep and the amount of recuperation depend upon the distance between the astral and physical bodies; i.e., the greater the distance of separation, the freer the inflow of cosmic energy, or prana, into it." Precursors to astral projection are lucid dreaming (an awareness of the self in dream state) and interrupted sleep (in which the physical body arises during the sleep state).

Considerable study is presently being conducted in the area of Rapid Eye Movement (REM) sleep. REM sleep is said to constitute between 20 and 50 percent of sleep activity. During this type of sleep, the brain seems to behave as if the body is awake: brainwave activity is high, heart rate increases, and sexual stimulation occurs. It is during this REM period that most of the night's dream activityand perhaps paranormal activity occurs.

While the increased neural activity during REM sleep may facilitate the development and maturation of the nervous system in infants, the role of REM in adults remains unclear. William C. Dement of the Stanford Sleep Disorders Clinic proposes some of the possibilities:

"There is strong evidence that REM sleep plays a role in the regulation of mood and/or drive; that it is related in some way to excitability of the central nervous system; and that its suppression may in some way jeopardize the learning and memory functions. The occurrence of large amounts of REM sleep among newborn infants remains possibly the most provocative puzzle of all; it suggests a very important role for REM in the earliest stages of life."

Another subject currently receiving significant attention is sleep deprivation. Murphy, in an article written for the Atlantic Monthly (1996), asserts Americans living a century ago could sleep 20% longer than the average American today. Ten million Americans each year seek medical, alternative medical, or therapeutic help for the treatment of sleeping disorders. Also sleep deprivation is thought to be a greater contributor to traffic fatalities than intoxication. Severe sleep deprivation is known to cause substantial detrimental alterations in both behavior and perception. Since a considerable amount of people who have been deprived of sleep report paranormal experiences, there exists the question of what role sleep deprivation plays in perceived paranormal incidents.


Angoff, Allan, ed. The Psychic Force: Essays in Modern Psychical Research from the International Journal of Parapsychology. New York: G. P. Putnam's Sons, 1970.

Bigelow, J. The Mystery of Sleep. London: Unwin; New York: Harper, 1903.

Braid, James. Neurypnology; or, The Rationale of Nervous Sleep. London, 1843. Reprint, AMS Press, 1976.

Cohen, D. Sleep and Dreaming: Origins, Nature and Functions. New York: Pergamon Press, 1981.

Crookall, Robert. During Sleep: The Possibility of "Co-operation" Between the Living and the Dead. New Hyde Park, N.Y.: University Books, 1974.

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Jyotir Maya Nanda, Swami. Waking, Dream, and Sleep. Miami, Fla.: Yoga Research Foundation, 1974.

Muldoon, Sylvan, and Hereward Carrington. The Projection of the Astral Body. London: Rider, 1929.

Murphy, Cullen. "Hello Darkness: Dealing with Yet Another Deficit." Atlantic Monthly March 1996. 22-24.

Perl, James. Sleep Right in Five Nights. New York: William Morrow and Co., Inc. 1993.

Taylor, Albert. Soul Traveler. Covina, Calif.: Verity Press, 1996.

Ullman, Montague, and Stanley Krippner. Dream Studies and Telepathy: An Experimental Approach. New York: Parapsychology Foundation, 1970.

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Sleep is a difficult topic to grapple with historically, and at present the most interesting historical episode involves relatively recent change. Child-rearing manuals in nineteenth-century America did not deal with sleep as a problem, despite or perhaps because of extensive health advice in other categories. Surely, individual parents faced children with unusual sleep difficulties, but a sense of a larger category of issues did not emerge. Snippets of advice, for example in American women's magazines, suggested that relatively short periods of formal sleep were required of childrensix to eight hours were often mentioned, which confirms the impression that sleep was not viewed as a source of problems.

Analyzing a lack of concern is a challenging task historically. Several factors help explain why nineteenth-century adults (and probably their counterparts in earlier periods as well) did not pay much attention to children's sleep in general. First, naps were common, for many adults as well as children. Historians have noted how adult sleep patterns before modern times were less rigidly defined than they are now, and children's sleep benefited from a similarly relaxed definition. Where sleep was an issue, for individual children, many parents undoubtedly used opiates or alcohol to help. The absence of much artificial light reduced nighttime stimulation and facilitated getting children off to bed.

Concern about adult sleep began to increase toward the end of the nineteenth century, as part of the attack on stress disorders such as the then-popular disease, neurasthenia, and the general tensions seen in modern life. Growing use of electric lights and the popularization of caffeine drinks added to the emergence of sleep as a problem. Some discussions began to spill over into the treatment of children, but it was not until the 1920s that child-rearing manuals picked up the question of children's sleep as a standard topic. From that point onward, sections of all the major manuals, plus publications like Parents Magazine, were devoted to sleep. Pediatricians also dispensed sleep advice, and doctors took the lead in recommending increasing amounts of sleep, from infancy onward. Getting children to bed became a major daily ritual for parents and children alike, a regular opportunity for contests between freedom and authority. For their part, child experts, headed by the behaviorists, urged that set bedtimes were a vital part of the socialization of children, as well as the protection for their health. New rituals such as daily bathing, story reading, mass-produced toys like the new teddy bears, or night lights were variously employed.

Why did sleep standards escalate, and why did sleep become a more significant issue? Experts were clearly eager to promote a variety of problems to which they had solutions, and the inclusion of sleep obviously qualified as yet another area where well-meaning parents needed outside help. Growing psychological interest in dreams, and research on the troubled dreams of children, provided an additional scientific basis for sleep concerns. New distractions, such as the radio, plus the noise of modern urban life may have made sleep, in fact, more difficult to achieve than before. Schooling requirements reduced the opportunity for naps except for the very young. Most children, like most adults, now had to be taught to sleep intensively for a period of time, rather than indulging more on the spur of the moment.

It was also true that children's sleep arrangements had been changing in the United States, from the late nineteenth century onward. Babies were increasingly placed in cribs at a fairly young age, rather than rocked in cradles as their parents worked or relaxed. Learning to sleep alone was an important modern discipline, and cribs allowed parents to put infants in a separate space. As adults had new recreations in the evening, they looked for ways to free themselves from on-the-spot care of sleeping children. Older children graduated from cribs to beds and were not as likely to sleep with other siblings. Birth rate reductions meant that there were fewer siblings, and experts urged that children were better off with their own, separate rooms. These specific changes in sleeping arrangements, aimed at a new level of individuation, may well have created the kinds of new sleep problems to which the child-rearing literature responded.

Specific recommendations about getting children to sleep oscillated during the twentieth century. The strict regimen of the behaviorist approach in the 1920s was modified in the mid-twentieth century by more permissive experts such as Dr. Benjamin Spock. But adult concern about children's sleep persisted at a fairly high level. Many children who learned of sleep as a problem in their own early years may have been encouraged to worry about sleep in new ways even as they grew to adulthood. The implications of the twentieth-century change in approach to children's sleep, at least in the United States, remain a fascinating area of study.

See also: Child-Rearing Advice Literature; Children's Spaces.


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Stearns, Peter N., Perrin Rowland, and Lori Giarnella. 1996. "Children's Sleep: Sketching Historical Change." Journal of Social History 30: 345366

Peter N. Stearns

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STEARNS, PETER N.. "Sleep." Encyclopedia of Children and Childhood in History and Society. 2004. (May 28, 2016).

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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.

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"sleep." The Oxford Pocket Dictionary of Current English. 2009. 28 May. 2016 <>.

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sleep, resting state in which an individual becomes relatively quiescent and relatively unaware of the environment. During sleep, which is in part a period of rest and relaxation, most physiological functions such as body temperature, blood pressure, and rate of breathing and heartbeat decrease. However, sleep is also a time of repair and growth, and some tissues, e.g., epithelium, proliferate more rapidly during sleep. Sleep also aids in the strengthening of memories, and researchers have discovered that an increased circulation of fluids in the brain during sleep removes waste products from the brain.

In humans, sleep occurs in cyclical patterns; in each cycle of 11/2 to 2 hr, the sleeper moves through four stages of sleep, from Stage 1 to Stage 4, and back again to Stage 1. In the first stage, low-frequency, low-amplitude theta waves characterize brain activity. The stage usually lasts only several minutes, before the individual drifts into Stage 2 sleep, and the brain moves into low-frequency, high-amplitude waves. Stage 3 signals an increase of low-frequency, high-amplitude delta waves, and at Stage 4 sleep these delta waves account for more than half of all brain wave activity (see electroencephalography). Rapid-eye-movement (REM) sleep occurs during Stage 1 sleep at the end of each cycle, and people woken up at this time usually report that they have been dreaming. Dream deprivation or sleep deprivation results in detrimental changes in personality, perceptual processes, and intellectual functioning. There is some evidence that emotional and environmental deprivation disrupts the sleep patterns of young children, which in turn inhibits the secretion of growth hormone, normally secreted maximally during sleep.

The amount of sleep needed depends on both the individual and the environment: For instance, worrying, critical individuals tend to need both more sleep and more dream sleep than easygoing ones, and stress and worry during the day result in an increase in REM sleep. It has been hypothesized that while deeper stages of sleep are physically restorative, REM sleep is psychically restorative. REM sleep is also believed to integrate new information in the brain and to reactivate the sleeping brain without waking the sleeper. There is evidence that the hypothalamus and thalamus of the brain initiate sleep and that part of the midbrain acts as an arousal system. See also dream; insomnia; narcolepsy; sleep apnea.

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"sleep." The Columbia Encyclopedia, 6th ed.. 2016. 28 May. 2016 <>.

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372. Sleep

See also 129. DREAMS ; 154. FATIGUE .

autohypnotism, autohypnosis
1. the process of hypnotizing oneself.
2. the resulting state.
the practice of hypnotism by Dr. James Braid, British physician, in the mid 19th century.
an obsession with bed rest.
an abnormal fear or dislike of going to bed.
Obsolete, the act of lulling or rocking to sleep.
the state of being dormant or inert.
a somnambulist, or sleepwalker.
somnambulism. hypnobate, n.
the science dealing with the phenomena of sleep and hypnotism. See also 215. HYPNOSIS . hypnologist, n. hypnologic, hypnological, adj.
a mania for sleep.
hypnopedia, hypnopaedia
the art or process of learning while asleep by means of lessons recorded on disk or tapes.
an abnormal fear of sleep.
the condition of sleepwalking only in the moonlight. Cf. somnambulism. lunambulist, n. lunambulistic, adj.
Medicine. a numbness often feit upon waking from sleep.
Pathology. a condition characterized by frequent and uncontrollable lapses into deep sleep. narcoleptic , adj. narcolept , n.
1. a method of treating certain mental disorders by inducing sleep through barbiturates.
2. a type of psychotherapy involving the use of hypnotic drugs. Also narcoanalysis. narcotherapist, n.
somnambulism. Also noctambulation. noctambulist, noctambule, n. noctambulous, noctambulant, noctambulisdc, adj.
the condition of sleepwalking. Also called hypnobatia, noctambulism. somnambulant , n., adj. somnambulist, n. somnambulistic, adj.
1. the tendency to talk in ones sleep. Also somniloquy .
2. the words spoken. somniloquist, n. somniloquous, adj.
a state of sleep induced by hypnosis or mesmerism.
the condition of drowsiness or sleepiness. Also somnolency, somnolism. somnolent, adj.

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"Sleep." -Ologies and -Isms. 1986. 28 May. 2016 <>.

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593. Sleep

  1. Amina in her sleep, walks on a dangerous bridge, complaining of her unhappiness. [Ital. Opera: La Sonnambula in Osborne Opera ]
  2. Cupid while sleeping, revealed by Psyches lamp as her lover. [Gk. Myth.: Benét, 822]
  3. Deiphobus while sleeping, he is betrayed by Helen and slain by Menelaus. [Rom. Lit.: Aeneid VI ]
  4. dormouse snoozes all through the mad tea-party. [Br. Lit.: Alices Adventures in Wonderland ]
  5. Endymion man kept immortally youthful through eternal sleep. [Gk. Myth.: Howe, 91]
  6. Epimenides philosopher nods off for 57 years in cave. [Gk. Legend: LLEI, I: 283]
  7. hypnale asp which kills by inducing sleep. [Medieval Animal Symbolism: White, 174]
  8. Hypnos god of slumber. [Gk. Myth.: Hall, 250]
  9. Joe (Fat Boy) Damn that boy, hes gone to sleep again. [Br. Lit.: Dickens Pickwick Papers ]
  10. Lady Macbeth while sleepwalking, discloses her terrible deeds. [Br. Drama: Shakespeare Macbeth ]
  11. land of Nod mythical land of sleep; humorous reference to biblical land in Genesis. [Am. and Br. Usage; O.T.: Genesis 4:16]
  12. Morpheus Hypnos son and god of dreams. [Gk. Myth.: Howe, 172]
  13. poppy attribute of Hypnos, Greek god of sleep. [Art: Hall, 250]
  14. Sandman induces sleep by sprinkling sand in childrens eyes. [Folklore: Brewer Dictionary, 966]
  15. Seven Sleepers youths who fled Decian persecution; slept for more than 200 years. [Christian and Muslim Tradition: Benét, 918]
  16. Sleeping Beauty enchanted heroine awakened from century of slumber by princes kiss. [Fairy Tale: Brewer Dictionary, 1011]
  17. Snow White poisoned apple induces her sleep; prince awakens her. [Childrens Lit.: Bettelheim, 213]
  18. Somnus god of sleep; son of Nox. [Rom. Myth.: Wheeler, 349]
  19. Van Winkle, Rip slept for 20 years, thereby missing war. [Am. Lit.: Rip Van Winkle in Hart, 714]
  20. Winkie, Wee Willie made sure all the children were asleep. [Nurs. Rhyme: Opie, 424]

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"Sleep." Allusions--Cultural, Literary, Biblical, and Historical: A Thematic Dictionary. 1986. 28 May. 2016 <>.

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sleep A readily reversible state of reduced awareness and metabolic activity that occurs periodically in many animals. Usually accompanied by physical relaxation, the onset of sleep in humans and other mammals is marked by a change in the electrical activity of the brain, which is recorded by an electroencephalogram as waves of low frequency and high amplitude (slow-wave sleep). This is interspersed by short bouts of high-frequency low-amplitude waves (similar to wave patterns produced when awake) associated with restlessness, dreaming, and rapid eye movement (REM); this is called REM (or paradoxical) sleep and is often accompanied by an increased pulse rate and dilation of the pupils. Several regions of the brain are involved in sleep, especially the reticular formation of the brainstem.

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"sleep." A Dictionary of Biology. 2004. 28 May. 2016 <>.

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Asleep for Twenty-Five Years

Why Do People Need to Sleep?

What Are the Stages of Sleep?

What Makes a Person Fall Asleep?

What Are Sleep Disorders?


Sleep is a state marked by a lowered level of consciousness, decreased movement of the bodys 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

Circadian rhythm


Sleep disorders

Sleep medicine

Sleep research

Asleep for Twenty-Five Years

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.

Why Do People Need to Sleep?

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 brains 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 nights 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.

What Are the Stages of Sleep?

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 Nights 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 nights sleep. A team of researchers at Harvard University found that college students who got a full nights 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.

Rapid Eye Movement (REM)

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 persons 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.

What Makes a Person Fall Asleep?

There are a variety of physical, psychological, and environmental factors involved in the sleep process.

Brain chemistry

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 brains 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 bodys ambassador: it is created in one place but is sent through the body to have specific effects on other parts of the body.

Circadian rhythm

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 persons 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 studiesliving in darkness for weeks at a timetend 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.

What Are Sleep Disorders?

There are many types of disorders that can make sleeping difficult, including:

Researchers believe that the bodys 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 eyes 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.

See also

Bedwetting (Enuresis)

Brain Chemistry (Neurochemistry)






American Academy of Sleep Medicine, 6301 Bandel Road NW, Suite 101, Rochester, MN 55901. Telephone 507-287-6006

National Sleep Foundation, 1522 K Street NW, Suite 500, Washington, DC 20005
Fax: 202-347-3472

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sleep unconscious state in which the physical powers are suspended. OE. (Angl.) slēp, (WS.) slǣp = OS. slāp (Du. slaap), OHG. slāf (G. schlaf), Goth. slēps :- Gmc. *slǣpaz, rel. to corr. vb. *slǣpan, whence OE. slǣpan (mod. sleep), OS. slāpan, etc., and by gradation, to *slap-, whence LG., Du. slap inert, sluggish, G. schlaff slack, lax.
Hence sleeper one who sleeps XIII; stout horizontal timber XVII; apartment for sleeping (orig. U.S.) XIX.

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T. F. HOAD. "sleep." The Concise Oxford Dictionary of English Etymology. 1996. 28 May. 2016 <>.

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sleep Periodic state of unconsciousness from which a person or animal can be roused. During an ordinary night's sleep there are intervals of deep sleep associated with rapid eye movement (REM) sleep. It is during this REM sleep that dreaming occurs. Studies have shown that people deprived of sleep become grossly disturbed. Sleep requirement falls sharply in old age. Difficulty in sleeping is called insomnia.

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"sleep." World Encyclopedia. 2005. 28 May. 2016 <>.

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sleep (sleep) n. a state of natural unconsciousness, during which the brain's activity is not apparent (apart from the continued maintenance of basic bodily functions, such as breathing) but can be detected by means of an electroencephalogram (EEG). See also REM.

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"sleep." A Dictionary of Nursing. 2008. 28 May. 2016 <>.

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sleepasleep, beep, bleep, cheap, cheep, creep, deep, heap, Jeep, keep, leap, neap, neep, peep, reap, seep, sheep, skin-deep, sleep, steep, Streep, sweep, veep, weep •slagheap • scrapheap • antheap •housekeep • upkeep • chimney sweep

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