Biological rhythms

Biological rhythms are often referred to as biological clocks, since they operate on time schedules on a daily, monthly, seasonal, or annual basis. Some biological rhythms even occur on the basis of fractions of seconds. These internal clocks operate independent of the environment, but they are controlled by environmental conditions in changing situations. During times of change, such as seasonal decreases of light , or those caused by travel in an east-west or west-east direction, human beings are able to reset their biological clocks to become synchronized with the environment.

History

Daily rhythms in plants and animals have been noticed since early times. As early as the fourth century b.c., Alexander the Great's scribe Androsthenes noted that the leaves of certain trees opened during the day and closed at night. Two centuries before modern gardeners noticed their day lilies closed at night, the famous taxonomist Carolus Linnaeus discovered the petals of many flower species opened and closed at regular times. He even created a garden with flowers that opened at various times so that he could tell the time of day by looking in his garden.

Karl von Frisch observed that bees visited flowers only at specific times. He and Ingeborg Beling trained bees to visit a nectar feeding station between 4 and 6 P.M. The bees did not visit at other times, and they still visited even when the nectar was removed. When outside cues such as light were removed in laboratory trials, the bees still fed at prescribed times. Although von Frisch did not know it, the bees were operating on an internal clock. It wasn't until the 1950s that Gustav Kramer and Klaus Hoffmann proved the existence of a biological clock. With an ingenious apparatus, Kramer demonstrated that starlings used the sun as a compass to migrate even though the sun itself moves throughout the day. That is, the bird's internal clock reorients it in the direction of the moving sun. Hoffmann showed that the clock persisted in dim light and thus is endogenous to the animal . He showed that the animal's clock was synchronized to local time by the influence of the local environment.

Research in the area of biological clocks did not begin with humans. Early research was done on a variety of animals including rats , hamsters , sparrows, lizards, marine snails , and fruit flies . The choice of an animal depended on many considerations—size, rate of reproduction, expense to maintain, and availability—as well as behavioral issues, such as whether it entrained easily and what environmental factors seemed to affect its behavior .

In 1920, a landmark paper was written by W.W. Garner and H.A. Allard in which they showed that tobacco plants would flower only if exposed to a certain number of hours of light. The term "photoperiodism" was used to designate the response of organisms to relative length of day and night. The ability to sense day length is an important ability for plants so that they grow, reproduce, and develop during favorable times of the year. The changing times of dawn and dusk contain seasonal information as well as time of day information so that the organisms have, in effect, an internal clock and calendar. In plants, a photoperiodic clock not only controls flowering, but also induction and termination of dormancy in buds and bulbs, seed germination , and daily rhythms such as leaf movements, petal movements, and nectar secretion.

The study of biological rhythms is called chronobiology, a relatively new scientific specialty that began in the 1950s when researchers used new heart and lung monitors to answer some of the basic questions that chronobiologists ask. Some of these questions deal with sleep/wake cycles, time of day energy and productivity levels, and mood changes. Other important areas of study in chronobiology deal with growth patterns, hormone secretions, and menstruation.

Types of internal clocks

Some biological rhythms occur more than once a day and are called ultradian rhythms. The release of hormones from the male pituitary gland of mammals occurs about every one to two hours during the day. Sleep cycles, that is, the cycle from drowsiness to REM (rapid eye movement, dream sleep) to dozing, then light and deep sleep, and finally slow-wave sleep, is a 90-minute sleep cycle that repeats itself during a night's sleep. Constant breathing and the beating of our hearts are also ultradian rhythms, but these activities are also affected by the daily sleep/wake cycle, which is a circadian rhythm. Heart rate and breathing both slow during sleep.

Circadian rhythms are those that occur once a day and relate to the sun. The one that dominates our activities is the sleep/wake cycle people experience every 24 hours. Body temperature , response to medications, alcohol blood level, alertness, and fatigue all have a daily up and down cycle. Circadian rhythms also control such daily activities as eating. Chronobiologists explain the synchronizing of circadian rhythms to sunlight. This external factor helps the body regulate its daily activities. When people travel in an east or west direction through many time zones, they may suffer from jet lag and will need several days to adjust their sleep/wake cycle to the daylight and darkness in their new environment.

The clock in humans is located in the suprachiasmatic nucleus (SCN), a distinct group of cells found within the hypothalamus. The SCN is only one part of the mechanism by which "time" is kept. There are light receptors found in the retina that have a pathway, called the retinohypothalamic tract, leading to the SCN. The pineal gland is a pea-like structure found behind the hypothalamus in humans. The pineal gland receives information indirectly from the SCN. It appears that the SCN takes the information on day length from the retina, interprets it, and passes it on to the pineal gland, which secretes the hormone melatonin in response to this message. Nighttime causes melatonin secretion to rise, while daylight inhibits it. Even when light cues are absent, melatonin is still released in a cyclic manner; yet if the SCN is destroyed, circadian rhythms disappear entirely.

Infradian cycles are monthly cycles, the most common of which is the monthly menstruation of females. Illness and death have been correlated to certain times within infradian cycles, with more deaths occurring in the second half of the menstrual cycle . Research with men has shown that weight fluctuations, hormone levels, growth of beards, body temperature, pain threshold, lung capacity, and physical strength all demonstrate monthly cyclical patterns. More men experience symptoms of prostate enlargement during the new moon period, while more deaths and accidents occur around the time of a full moon.

Lunar cycles are somewhat longer than circadian ones. They last about 24 hours and 50 minutes. Some marine invertebrates , such as the fiddler crab, synchronize daily activity to the tides , which are affected by the moon. The change in water levels during the tides influences the activity of many marine invertebrates (animals without a spinal column).

The longest cycle is of course the life cycle, which has the distinct stages of growth, maturation, decline, and death in all forms of life.

The circannual cycle is a yearly occurring one. Some people and animals periodically gain or lose weight at certain times of the year. Transplant patients respond better to certain drug treatments at certain times of the year than at others.

A disorder called SAD (seasonal affective disorder) afflicts many people during the winter months of the year when the days are shorter. The hormone called melatonin is secreted from the pineal gland, which is located just behind the hypothalamus in the brain . Melatonin affects our moods. Its greatest production is during the night and when there is more darkness during a 24 hour period, as it is in winter, there is a tendency for some people to become depressed. An effective treatment is to expose the person to intense bright light.

Adaptations to time

The basic time adaptation in biological rhythms is entrained, which means it is influenced by external cues. Other aspects of our biological rhythms are so influential that most of us adapt our activities to the time of day during which we function best. Some people are morning people, while others are night people. Morning people (sometimes called larks) wake up with lots of energy and perform their best work in the early hours of the day. Night people are sluggish in the morning and do their best work late in the day or in the evening. They are often called owls . Work environments in our society are not adjusted to these differences, though, and many people have to work at times when they are least likely to be productive.

An experiment that took place in New Mexico in 1989, when a woman spent 130 days in isolation in a cave that had no natural light, demonstrated how external cues affect our biological clocks. After six weeks, she was functioning within a 44 hour cycle of sleeping and wakefulness. Her perception of time was also compressed to a considerable degree. In other experiments, volunteers drifted into a 25-hour day, while others have experienced 50-hour days or irregular cyclical patterns.

Problems due to circadian desynchrony

Diminished performance is not the only problem caused by being out of phase with one's environment. Worker and public safety also can be compromised. Researchers have found that the neural processes controlling alertness and sleep produce an increased sleep tendency and diminished capacity to function during certain early morning hours (circa 2-7 a.m.) and, to a lesser degree, during a period in the mid-afternoon (circa 2-5 p.m.), whether or not one has slept. Several studies of single-vehicle car accidents that have been judged to be fatigue-related have shown two peak times for accidents—a major one between midnight and 7 a.m., and a secondary peak between 1 and 4 p.m.

Shift work is a necessity in an industrially developed country. Manufacturing, transportation, and health care rely on shift workers to maintain operations around the clock. A shift worker can be defined as someone who works evenings, nights, rotating shifts, or extended shifts. Unlike jet lag, which is usually a temporary disruption, shift work schedules and their disruptions can last for years. Humans are diurnal creatures (active during the day), and shift workers are often out of sync with environmental and social cues. Circadian adjustment to a new shift is gradual (taking a week or more), and changing to a new shift before adjustment is complete can cause perpetual desynchronization. Research shows that shift workers suffer from sleep disruption and fatigue, domestic disturbances, and health problems such as gastrointestinal disorders and increased risk of cardiovascular disease . Task performance is also poorer at night. In order to minimize circadian disruptions, many large companies employ consulting firms to advise them on how to manage or prevent problems caused by shift work. Other than adjusting schedules, implementing educational and support programs regarding biological rhythms, sleep, and family counseling may be a good way to improve the coping ability of shift workers.

Compromised performance among health care workers can jeopardize patients' lives. The medical profession is notorious for having its doctors-in-training work very long hours. In the case of a young woman who died eight hours after being admitted to the emergency room, the intern and resident had each been on duty for 18 hours before treating her. The grand jury examining the case determined that the number of hours the residents and interns were required to work contributed to her death. Some improvements may be on the way in the medical profession as well. The Accreditation Council for Graduate Medical Education (ACGME), which sets standards for medical residency training programs, has begun within the past few years to introduce new standards for residents' work hours.

Medical uses

The work of chronobiologists in the area of biological rhythms has been useful to medical science in helping them diagnose illness more accurately. Twenty-four hour monitoring of heart rate and blood pressure gives the treating physician a better picture of health problems. Newborn infants of families with histories of heart disease can be monitored and abnormalities can be seen. Early detection of breast cancer can be made through the recording of skin temperature fluctuations over breasts. Noncancerous temperatures of the skin of breasts has a greater fluctuation cycle than temperatures recorded of the skin of cancerous breasts.

Research on biological clocks is shedding new light on standard prescribing practices for medication. For example, cortisone injections are given in the morning for the treatment of adrenal gland malfunctions. Hormones in various glands throughout the human body are released when they are needed by major organs and the chemical changes that occur manage our biological rhythms. As new ways of monitoring them are discovered, early warning signs of disease will become more apparent to diagnosing physicians. Evidence shows that certain medical illnesses whose symptoms show a circadian rhythm respond better when drugs are coordinated with that rhythm. Medications for asthma , epilepsy , cancer, cardiovascular disease, and allergies all have shown better results with minimum side effects when given at particular times.

Researchers are currently trying to re-educate the medical profession about the limitations of traditional prescribing practices and the greater potential benefits of administering medication at the most appropriate time. In the future, other research will no doubt provide even stronger arguments for coordinating medical tests and procedures more closely with temporal information and with influences of the biological clock that have not yet been discovered.

See also Depression.

Resources

books

Campbell, Jeremy. Winston Churchill's Afternoon Nap. New York: Simon and Schuster, 1986.

Dotto, Lydia. Losing Sleep. New York: William Morrow, 1990.

Hughes, Martin. Body Clock. New York: Facts on File, 1989.

Orlock, Carol. Inner Time. New York: Birch Lane Press, 1993.

Perry, Susan L. The Secrets Our Body Clocks Reveal. New York: Rawson Associates, 1988.

Shafii, Mohammad and Sharon Lee Shafii. Biological Rhythms,Mood Disorders, Light Therapy, and the Pineal Gland. Washington, DC: American Psychiatric Press, 1990.

periodicals

Hardin, P.E. "From Biological Clock to Biological Rhythms." Genome Biology 1 (2000): 1023.1-1023.5.

Vita Richman

KEY TERMS

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Chronobiology

—The study of biological clocks.

Circadian rhythm

—The rhythmical biological cycle of sleep and waking which, in humans, usually occurs every 24 hours.

Entrainment

—Regulation of the biological cycle to the environment.

Free-running clock

—Response to internal clocks without any influence from the external world.

Infradian cycle

—The monthly biological cycle.

Lunar rhythm

—The regulation of the biological cycle to the movement of the moon.

Synchrony

—The adjustment of biological rhythms to the environment.

Ultradian rhythm

—Biological cycles of less than a day.