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Biological Rhythms



People frequently talk about body clocks, a term that refers to the patterns of energy and exhaustion, functioning and resting, and wakefulness and sleep that characterize everyday life. In fact, the concept of the body clock, or circadian rhythm, is part of a larger picture of biological cycles, such as menstruation in mammalian females. Such cycles, which assume a variety of forms in a wide range of organisms, are known as biological rhythms. These rhythms may be defined as processes that occur periodically in an organism in conjunction with and often in response to periodic changes in environmental conditionsfor example, a change in the amount of available light. Not all aspects of the body clock are part of day-to-day experience, and this is fortunate, since these interruptions in the healthy flow of biological rhythms can threaten the well-being of the human organism. Among these challenges to the ordered working of bodily "clocks" are jet lag, seasonal affective disorder (SAD), and other disorders linked to a range of causes, including drug use.


Understanding Biological Rhythms

Among the many varieties of biological rhythm, the most well known are those relating to sleep and wakefulness, which are part of the circadian rhythm that we discuss later in this essay. Circadian, or daily, cycles are only one type of biological rhythm. Some rhythms take place on a cycle shorter than the length of a day, while others are based on a monthly or even an annual pattern.

Nor do all cycles involve sleep and wakefulness: menstruation, for instance, is a monthly cycle related to the sloughing off of the lining of the uterus, a reproductive organ found in most female mammals. Another biological rhythm is the beating of the heart, which, of course, takes place at very short intervals. Nonetheless, the circadian rhythm is the most universal of biological cycles, and it is the focus of our attention in this essay.


In discussing the operation of biological rhythms, the term biological clock often is used. A biological clock is any sort of mechanism internal to an organism that governs its biological rhythms. One such mechanism, which we examine in the next section, is the pineal gland. Internal clocks operate independently of the environment but also are affected by changes in environmental conditions.

Examples of such alterations of conditions include a decrease (or increase) in the hours of available light due to a change of seasons or a change in time alteration due to rapid travel from west to east or north to south. In the latter instance, a condition known as jet lagincreasingly familiar to humans since the advent of regular air travel in the mid-twentieth centurymay ensue.

The Pineal Gland

Governing human biological cyclesthe "computer" that operates our biological clocksis the pineal gland, a cone-shaped structure about the size of a pea located deep inside the brain. At one time, the great French philosopher and mathematician René Descartes (1596-1650) held that the pineal gland was actually the seat of the soul. Though it might seem absurd now that a respected thinker would seriously attempt to locate the soul in space, as though it were a physical object, Descartes's claim resulted from hours of painstaking dissection conducted on animals.

In searching for the human soul, Descartes sought that ineffable quality described some fifteen centuries earlier by the Roman emperor and philosopher Marcus Aurelius (121-180), who wrote, "This being of mine, whatever it really is, consists of a little flesh, a little breath, and the part which governs." As it turns out, the pineal gland is, in a sense, "the part which governs": it may not be the home of the soul (which, in any case, is not a question for science), but it does govern human circadian rhythms and thus has a powerful effect on the manner in which we experience the world.


The pineal gland secretes two hormones (molecules that send signals to the body), melatonin and serotonin. During the late 1990s, melatonin became a popular over-the-counter treatment for persons afflicted with sleep disorders, because it is believed that the hormone is associated with healthful sleep. Scientists do not fully understand the role that melatonin plays in the body, although it appears that it regulates a number of diurnal, or daily, events.

In addition, melatonin seems to serve the function of controlling fat production, which is one reason why good sleep is associated not only with a healthy lifestyle but also with a healthy physique. Many health specialists maintain that for adults there is a close link between a "spare tire" (that is, fat accumulation around the waist) and stress, lack of sleep, and low melatonin levels.

Among the many roles melatonin plays in the body is its job of regulating glucose levels in the blood, which, in turn, serve to govern the production of growth hormone, or somatotropin. Growth hormone is associated with the development of lean body mass, as opposed to fat, which is why athletes involved in the Olympics and other major sporting competitions sometimes have illegally "doped" with it as a means of increasing strength. It is not surprising, then, to learn that childrenwho clearly need and use more growth hormone and who also need more hours of sleep than adultsalso have higher melatonin levels.


Melatonin is not the only important hormone that is both secreted by the pineal gland and critical to the regulation of the body clock. Complementary to melatonin is serotonin, which is as important to waking functions as melatonin is to sleepiness. Like melatonin, serotonin serves several functions, including the regulation of attention.

Serotonin is among the substances responsible for the ability of a human with a healthily functioning brain to filter out background noise and sensory data. Thanks in part to serotonin, you are able to read this book without having your attention diverted by other sensory data around you: the voice of someone talking nearby, the sunlight or a bird singing outside, the hum of a light or a fan in the room.

By contrast, a person under the influence of the drug LSD (lysergic acid diethylamide) is not able to make those automatic filtering adjustments facilitated by serotonin. Instead, he or she is at the mercy of seemingly random intrusions of outside stimuli, such as the color of paint on a wall or the sound of music playing in the background. The secret of LSD's powerful hallucinatory effect can be attributed in part to the fact that it apparently mimics the chemistry of serotonin in the brain, "tricking" the brain into accepting the LSD itself as serotonin.

With regard to body clocks and biological rhythms, serotonin plays an even more vital governing role than does melatonin, since melatonin, in fact, is created by the chemical conversion of serotonin. On regular daily cycles the body converts serotonin to melatonin, thus influencing the organism to undergo a period of sleep. Then, as the sleeping period approaches its end, the body converts melatonin back into serotonin.


Circadian Rhythms

The term circadian derives from the Latin circa ("about") and dies ("day"), and, indeed, it takes "about" a day for the body to undergo its entire cycle of serotonin-melatonin conversions. In fact, the cycle takes almost exactly 25 hours. Why 25 hours and not 24? This is a fascinating and perplexing question.

It would be reasonable to assume that natural selection favors those organisms whose body clocks correspond to the regular cycles of Earth's rotation on its axis, which governs the length of a dayor, more specifically, a solar day. Yet the length of the human daily cycle has been confirmed in countless experiments, for instance, with subjects in an environment such as a cave, where levels of illumination are kept constant for weeks on end. In each such case, the subject's body clock adopts a 25-hour cycle.


One might suggest that the length of the cycle has something to do with the fact that Earth's rate of rotation has changed, as indeed it has. But the speed of the planet's rotation has slowed, becauselike everything else in the universeit is gradually losing energy. (This is a result of the second law of thermodynamics.)

About 650 million years ago, long before humans or even dinosaurs appeared on the scene, Earth revolved on its axis about 400 times in the interval required to revolve around the Sun. This means that there were 400 days in a year. By the time Homo sapiens emerged as a species about two million years ago, days were considerably longer, though still shorter than they are now. This only means that the 25-hour human body clock would have been even less compatible with the length of a day in the distant past of our species.

One possible explanation of the 25-hour body clock is the length of the lunar day, or the amount of time it takes for the Moon to reappear in a given spot over the sky of Earth. In contrast to the 24-hour solar day, the lunar day lasts for 24 hours and 50 minutesvery close in length to the natural human cycle. Still, the exact relationships between the Moon's cycles and those of the human body have not been established fully: the idea that lunar cycles have an effect on menstruation, for instance, appears to be more rumor than fact.


On the other hand, circadian rhythms do mirror the patterns of the Moon's gravitational pull on Earth, which results in a high and low tide each day. Likewise, the human circadian rhythm has its highs and lows, or peaks and troughs. In the circadian trough, which occurs about 4:00 a.m., body temperature is at its lowest, whereas at the peak, around 4:00 p.m., it reaches a high. A person may experience a lag in energy after lunchtime, but usually by about 4:00 in the afternoon, energy picks upa result of the fact that the body has entered a peak time in its cycle.

This fact, by the way, points up the great wisdom of a practice common in Spanish-speaking countries and some other parts of the world: siestas. The siesta devotes one of the least productive parts of the day, the post-lunch lag, to rest, so that a person is equipped with energy for the rest of the afternoon and early eveningat precisely the time when energy is at a high. To compensate for the time "lost" on napping, many such societies maintain a later schedule, with offices closing in the early evening rather than late afternoon and with evening meals served at about 9:00 p.m.

Note that even though our body clocks run on a 25-hour day, they readily adjust to the 24-hour world in which we live. As long as a person is exposed to regular cycles of day and night, the pineal gland automatically adapts to the length of a 24-hour solar day. If a person has been living in a sunless cave, with no exposure to daylight for a length of time, it would take about three weeks for the pineal gland to reset itself, but thereafter it would track with Earth time consistently.

The adjustment of the body clock is not simply a matter of sending signals for sleep and wakefulness. In fact, the pineal gland is at the center of a complex information network that controls sleep cycles, body temperature, and stress-fighting hormones. Hence the link that we noted earlier between body temperature and circadian rhythms: just as the body reaches its lowest temperature in the circadian trough, it also enters a period of extremely deep sleep.


Tied in with these sleep patterns are many other bodily functions. For example, bodybuilders and others who work out with weights experience their greatest benefits not when lifting (which, in fact, tears muscles down rather than building them up) but when restingand particularly when sleepingafter having worked out earlier in the day. Likewise, deep sleep is associated with growth, as we have noted. Furthermore, it appears that dreaming may be essential to the well-being of the psyche, providing an opportunity for the brain to "clean out" the signals and data it has been receiving for the preceding 16 hours of wakefulness.

Given these and other important functions associated with deep sleep, it follows that the maintenance of the body clock is of great importance to the health of the human organism. Fortunately, animals' brains are programmed to make adjustments of the body clock so as to accommodate the daily cycles of light and dark. We have discussed the means by which the human brain achieves this accommodation, but it is not the only animal brain thus equipped. "Bird brains" (quite literally) are similarly able to make an adjustment: whereas humans have a natural 25-hour clock, birds run on a 23-hour circadian cycle, but their pineal glands likewise assist them in adapting to the 24-hour solar day.

The brains of birds, humans, and other animals respond to environmental features known collectively as zeitgebers (German for "time givers"), which aid in the adjustment to the solar schedule. The most obvious example is the change from day to night, but there are other zeitgebers of which we are less aware in our ordinary experience. For example, Earth's magnetic field goes through its own 24-hour cycle, which subtly influences our biological rhythms.

Interfering with the Body Clock

In modern life humans often interfere with their own body clocks, either deliberately and directly or indirectly and by accident. On the one hand, a person may drink coffee to stay awake at night, but he or she also may experience a sleep disorder as a result of some other situation, which may or may not be the result of purposeful action. Examples of sleep disorders that are the by-product of other activities include jet lag as well as the malfunctioning of the body clock that often stems from recreational drug use.

The causes for interference with a person's body clock may be outside that person's control to one degree or another. Working at night, for instance, is a condition that almost never suits a human being, no matter how much a person may insist that he or she is a "night person." Nevertheless, a person may be required by circumstances, such as schedule, economic necessity, or job availability, to take a night job. Another example of interference with the body clock would be narcolepsy (a condition characterized by brief attacks of deep sleep) or some other condition that is either congenital (something with which a person is born) or symptomatic (a symptom of some other condition rather than a condition in and of itself).


At least one example of human experience involving interference with the body clock relates to conditions completely outside people's control. This is the situation of the "white nights" or "midnight sun," whereby regions in the extreme northRussia, Alaska, and Scandinaviaundergo periods of almost constant daylight from mid-May to late July. (These are matched by a much less pleasant phenomenon: near constant darkness from mid-November to late January.)

During those times people often line their windows with dark material to make it easier to go to sleep in a world where the Sun is nearly as bright at 3:00 a.m. as it is at 3:00 p.m. The situation is even more pronounced in Antarctica, where researchers and adventurers may find themselves much closer to the South Pole than people in Saint Petersburg, Anchorage, or Oslo are to the North Pole.

In Antarctica the human population is much higher in the summer, a period that coincides with the depth of winter in the Northern Hemisphere, and scientists or mountaineers trekking through remote regions may be forced to sleep in tents that keep out the cold but let in the light. Usually, however, the rugged conditions of life near the South Pole involve such exertions that by nighttime people are ready to sleep, light or no light.


Few people ever get to experience the white nights, but almost everyone has suffered through a temporary bout of insomniaa condition known specifically as transient insomnia. An unfortunate few suffer from chronic insomnia or some other sleep disorder. Insomnia, the inability to go to sleep or to stay asleep, is one of the two most common sleep disorders, the other being hypersomnia, or excessive daytime sleepiness.

Transient forms of insomnia are usually treatable with short-term prescription drugs, but more serious conditions qualify as actual disorders and may require long-term treatment. These disorders may have as their cause drug use (either prescription or illegal) as well as medical or psychological problems. Among the most common of these more specialized disorders is apnea, the regular cessation of breathing whose most noticeable symptom is snoring.

Apnea, which affects a large portion of the United States population, is a potentially very serious condition that can bring about suffocation or even death. More often its effects are less dramatic, however, and manifest in hypersomnia, which is a result of lost sleep due to the fact that the sufferer actually is waking up numerous times throughout the night.

At the other extreme from apnea, in terms of prevalence among the population, is Kleine-Levin syndrome, which typically affects males in their late teens or twenties. The syndrome may bring about dramatic symptoms that range from excessive sleepiness, overeating, and irritability to abnormal behavior, hallucinations, and even loss of sexual inhibitions. Added to this strange mix is the fact that Kleine-Levin syndrome typically disappears after the person reaches the age of 40.


There are numerous classes of sleep disorders, among them circadian rhythm disordersthose related to jet lag or work schedules. As we have seen, the pineal gland can adjust easily from a natural 25-hour cycle to a 24-hour one, but it can do so only gradually, and it cannot readily adapt to sudden changes of schedule, such as those brought about by air travel.

Jet lag is a physiological and psychological condition in humans that typically includes fatigue and irritability; it usually follows a long flight through several time zones and probably results from disruption of circadian rhythms. The name is fitting, since jet lag is associated almost exclusively with jets: traveling great distances by ship, even at the speeds of modern craft, allows the body at least some time to adjust.

Older modes of travel were too slow to involve jet lag; for this reason, the phenomenon is a relatively recent one. The only people who manage to experience jet lag without riding in a jet are those traveling in even faster craftthat is, astronauts. An astronaut orbiting Earth in a space shuttle experiences rapid shifts from day to night; if manned vessels ever go out into deep space, scientists will face a new problem: assisting the adjustment of circadian cycles to that sunless realm.

On a much more ordinary level, there is the jet lag of people who travel from the United States East Coast to Europe or between the East Coast and West Coast of the United States. The worst kinds of jet lag occur when a person flies from west to east across six or more time zones: anyone who flies to Europe from the East Coast is likely to spend much of the first day after arrival sleeping rather than sightseeing. Thereafter, it may take up to ten days (usually as long as or longer than most European vacations) for the body to adjust fully.

By contrast, someone who has flown from the East Coast to the West Coast feels unexpected energy. The reason is that when it is 6:00 a.m. in the Pacific time zone, it is 9:00 a.m. in the eastern time zone, to which a person's body clock (in this particular scenario) is still adapted. Therefore, at 6:00 in the morning, the newly arrived traveler will feel as good as he or she would normally feel at 9:00 a.m. back east. Conversely, at 9:00 p.m. in the west, it is midnight in the east. This means that the traveler is likely to feel tired long before his or her ordinary bedtime.

There are steps one can take to avoid, or at least minimize the effects of, jet lag. One is to ensure a regular sleep schedule prior to traveling, so as to minimize the effects of sleep deprivation, if the latter does occur. It is even better if one can, in the days prior to leaving, adopt a schedule adjusted to the new time zone. For example, if one were traveling from the East Coast to California, one would start going to bed three hours earlier, and rising three hours earlier as well. Changing eating habits in the days prior to departure may also help. Some experts on the subject recommend a four-day period in which one alternates heavy eating (days one and three) and very light eating (days two and four.) It is believed that high-protein breakfasts stimulate the active, waking cycle, while high-carbohydrate evening meals stimulate the resting cycle; conversely, depriving the liver of carbohydrates may prepare the body clock to reset itself.


At least the body does adjust to jet lag; on the other hand, it may never become accustomed to working a night shift. If you stay up all night studying for a test, you will find that around 4:00 a.m. you hit a "lull" when you feel sleepyand because of the lowered temperature at the circadian trough, you also feel cold. You might assume that this situation would improve if you worked regularly at night, but the evidence suggests that it does not.

As long as a person lives in a sunlit world of 24-hour solar days, the body clock remains adapted to that schedule, and this will be true whether the person is at home and in bed or at work behind a desk or counter during the hours of night. In other words, the person always will hit the circadian trough about 4:00 a.m. This is one of the reasons why most people find the idea of working at night so unattractive, even though it is clear that in our modern society some night-shift positions are essential.

People who have offices in their homes may find it beneficial to work at late hours, when the phone is not ringing and the world is quiet, but the "extra time" gained by working at night ultimately is counterbalanced by the body's reaction to changes in its biological rhythms. Such is also the case with night-shift workers, who never really adjust to their schedules even after years on the job.

There is such a thing as a "night person," or someone with a chronic condition known as delayed sleep phase syndrome. A person with this syndrome is apt to feel most alert in the late evening and night, with a corresponding lag of energy in the late mornings and afternoons. Even so, given the role of sunlight in governing the body clock, the condition does not really lend itself to regular night work but rather merely causes a person to experience problems adapting to the schedule maintained by most of society. One possible means of dealing with this problem is to go to bed three hours later than would be normal for an ordinary 9-to-5 schedule, and wake up three hours later as well; unfortunately, that is not practical for most people. Another treatment applied with success is exposure of a person to artificial, high-intensity, full-spectrum light, which augments the effect of sunlight, between the hours of 7:00 and 9:00 a.m.


In this vein it is interesting to note that some of the optimistic predictions made in 1987 by Murray Melbin in his fascinating book Night as Frontier: Colonizing the World After Dark have not come to pass. Melbin, who explains circadian rhythms and the body clock in a highly readable and understandable fashion, makes a brilliant analysis of the means by which industrialized societies have extended their daily schedules into the nighttime hours. Thus, to use his analogy, such societies have "colonized" the night.

Until the invention in 1879 of the first successful incandescent lamp by the American inventor Thomas Edison (1847-1931), activity at night was limited. Torches, crude lamps, and candles in ancient times; metal lamps in the Middle Ages; and the various oil-burning lamps that applied the glass lantern chimney devised in 1490 by the Italian scientist and artist Leonardo da Vinci (1452-1519) all made it possible for a person to read at night and to perform other limited functions. After their introduction in the nineteenth century, street lamps in London, the first of their kind, made the streets safe for walking at late hours, but travel, large gatherings, and outdoor work after dark remained difficult before the advent of electric light.

Since 1879 the Western world has indeed "colonized" the night with all-night eateries, roads that are never free of traffic, and round-the-clock entertainment on radio, TV, and now the Internet. There are even hardware stores open all night in some major cities. Certainly today there are more gas stations, restaurants, television programs, and customer-service telephone lines that operate 24 hours than there were in 1987, when Melbin wrote his book, but it is unlikely that Americans will ever fully "colonize" the night in the thoroughgoing fashion that their ancestors colonized the New World. An example of the limits to night colonization is in air travel.

Before the events of September 11, 2001, when terrorists crashed hijacked planes into the World Trade Center in New York City and the Pentagon in Washington, D.C., the burden on America's airports had become almost unbearable. The concourses of Hartsfield International in Atlanta, Georgia, the world's busiest airport, were a nonstop melee of people, luggage, and noise, as travelers fought to change flights or pick up their bags. One obvious solution to the problem would have been to adopt a round-the-clock airport schedule, with flights regularly leaving at 3:00 or 4:00 in the morning.

No airport rushed to enact such a measure, however, and after September 11 heightened security concerns made it unlikely that any facility would adopt a 24-hour schedule, with the additional security threats it entailed. For a time at least, the volume of air traffic decreased dramatically, but even as it climbed back up in the months after the terrorist attacks, airports continued to operate on their ordinary schedules. The reason appears to be the difficulty of persuading people to adjust to a late-night schedulethat is, finding enough people willing to fly in the middle of the night and enough baggage handlers and ticket agents willing to service them. There are, it seems, limits to the extent to which nighttime can be colonized.

Other Examples of Biological Rhythms

Although the circadian rhythms of sleep and wakefulness are particularly important examples of biological cycles, they are far from the only ones. Not all rhythms, in fact, are circadian. Some are ultradian, meaning that they occur more than once a day. Examples include the cycles of taking in fluid and forming urine as well as cell-division cycles and cycles related to hormones and the endocrine glands that release them. For instance, the pituitary gland in the brain of a normal male mammal secretes hormones about every one to two hours during the day.

The overall cycle of sleeping and waking is circadian, but there is an ultradian cycle within sleep as the brain moves from drowsiness to REM (rapid eye movement, or dream, sleep) to dozing, then to light and deep sleep, and finally to slow-wave sleep. Over the course of the night, this cycle, which lasts about 90 minutes, repeats itself several times. Among the functions affected by this cycle are heart rate and breathing, which slow down in deep sleep. Additionally, heartbeat and respiration are themselves ultradian cycles of very short duration.


In contrast to the ultra-quick ultradian cycles of the beating heart and the lungs' intake and outflow of oxygen, there are much longer infradian, or monthly, cycles. By far the most common is menstruation, which begins when a female mammal reaches a state of physical maturity and continues on a monthly basis until she is no longer able to conceive offspring.

When she becomes pregnant, the menstrual cycle shuts down and, in some cases, does not resume until several months after delivery of the offspring. Assuming she is in good health, the human female will experience fairly regular menstrual periods at intervals of 28 days. Among human females, it has long been known that the menstrual cycles of women who live or work in close proximity to one another tend to come into alignment. For example, college girls on the same floor in a dormitory are likely to share menstrual cycles.

The reasons for this alignment of menstrual cycles are not completely understood. Nor is the cause of the 28-day cycle evident. If it were the result of the Moon's cycles, all women on Earth would have menstrual cycles that last 29.5 days, which is how long it takes the Moon to travel around Earth. Furthermore, if there were a clear connection between the Moon and menstruation, the periods of all menstruating females on Earth would be aligned with the Moon's phases. Neither of these, of course, is the case.


Longer still than infradian cycles, circannual cycles, as their name suggests, take a year to complete. Among them is the cycle of dormancy and activity marked by the hibernation of certain species in the winter. There are also certain times of the year when animals shed thingsfur, skin, antlers, or simply pounds. Likewise, at some points in the year animals gain weight.

People are affected strongly by the seasonal changes associated with the circannual cycle. There is almost no person who lives in a temperate zone (that is, one with four seasons) who is not capable of calling strong emotions to mind when imagining the sensations associated with winter, spring, summer, or fall. Some sensations, however, are better than others, and though there can be negative associations with spring or sum mer, by far the season most likely to induce ill effects in humans is winter.

The thirteen weeks between the winter solstice in late December and the vernal equinox in late March have such a powerful impact on the human psyche that scientists have identified a mental condition associated with it. It is SAD, or seasonal affective disorder, which seems to be related to the shortened days (and thus, ultimately, to the altered circadian rhythm) in winter-time.

As we have noted, the body responds to the onset of night and sleep by the release of melatonin, but when darkness lasts longer than normal, melatonin secretions become much more pronounced than they would be under ordinary conditions. The result of this hormone imbalance can be depression, which may be compounded by other conditions associated with winter. Among these conditions is "cabin fever," or restlessness brought about by lengthy confinement indoors. An effective treatment for SAD is exposure to intense bright light.

Studying Biological Rhythms

Treatment of SAD is just one example of the issues confronted by scientists working in the realm of chronobiology, a subdiscipline devoted to the study of biological rhythms. Naturally, a particularly significant area of chronobiological study is devoted to sleep research. The latter is a relatively new field of medicine stimulated by the discovery of REM sleep in 1953. In addition to studying such disorders as sleep apnea, sleep researchers are concerned with such issues as the effects of sleep deprivation and the impact on circadian rhythms brought about by isolation from sunlight.

Note that the scientific study of biological rhythms has nothing to do with "biorhythms," a fad that peaked in the 1970s but still has its adherents today. Biorhythms are akin to astrology in their emphasis on the moment of a person's birth, and though biorhythms have a bit more scientific basis than astrology, that in itself is not saying much. As we have seen, biological rhythms do govern much of human life, but the study of these rhythms does not offer special insight into the fate or future of a personone of the principal claims made by adherents of biorhythms. As with all pseudosciences, belief in biorhythms is maintained by emphasizing those examples that seem to correlate with the theory and ignoring or explaining away the many facts that contradict it.

An example of scientific research in chrono-biology and related fields is the work of the psychologist Stephany Biello at Glasgow University in Scotland, who in June 2000 announced findings linking the drug, ecstasy, to long-term damage to the body clock. As with LSD and many another drug, ecstasy plays havoc with serotonin and may exert such a negative impact on the pathways of serotonin release in the pineal gland that it permanently alters the brain's ability to manufacture that vital hormone. Thus the drug, which induces a sense of euphoria in users, can induce serious sleep and mood disorders as well as severe depression.


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Melbin, Murray. Night as Frontier: Colonizing the World After Dark. New York: Free Press, 1987.

Orlock, Carol. Inner Time: The Science of Body Clocks and What Makes Us Tick. Secaucus, NJ: Carol Publishing Group, 1993.

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Waterhouse, J. M., D. S. Waters, and M. E. Waterhouse. Your Body Clock. New York: Oxford University Press, 1990.

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A mechanism within an organism (for example, the pineal gland in the human brain) that governs biological rhythms.


Processes that occur periodically in an organism in conjunction with and often in response to periodic changes in environmental conditions.


A subdiscipline of biology devoted to the study of biological rhythms.


A biological cycle that takes place over the course of approximately a day. In humans circadian rhythms run on a cycle of approximately 25 hours and govern states of sleep and wakefulness as well as core body tempera ture and other biological functions.


Molecules produced by living cells, which send signals to spots remote from their point of origin and which induce specific effects on the activities of other cells.


A biological cycle that takes place over the course of a month.


A physiological and psycho logical condition in humans that typically includes fatigue and irritability; it usually follows from a long flight through several time zones and probably results from disruption of circadian rhythms.


The point at which menstrual cycles cease, a time that typical ly corresponds to the cessation of the female's reproductive abilities.


Sloughing off of the lining of the uterus, which occurs monthly in non pregnant females who have not reached menopause (the point at which menstrual cycles cease) and which manifests as a discharge of blood.


A small, usually cone-shaped portion of the brain, often located between the two lobes, that plays a principal role in governing the release of certain hormones, including those associated with human circadian rhythms.


A biological cycle that takes place over the course of less than a day. Compare with circadian rhythm.

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biological rhythms

biological rhythms Life has evolved in a rhythmic environment. For example, the daily rising and setting of the sun, and the seasonal variations in day length, temperature, and rainfall are all major factors to which the physiology and behaviour of different species must adapt in order to survive. The most obvious manifestation of human rhythmicity is the cycle of sleeping and waking. Humans are diurnal creatures; that is to say we are active during the light phase of the day and sleep at night. Anyone who has kept a pet hamster will know that these and many other species are nocturnal, i.e. active at night. But whilst the sleep–wake rhythm is obvious, virtually all the rest of our functions have their own, less evident rhythms. In fact it would be reasonable to say that everything that happens in our bodies is rhythmic until proved otherwise.

Many different frequencies are present in biological rhythms apart from the sleep wake (a 24-hour rhythm) and seasonal variations. The human pulse rate, at around 72 beats per minute, and the firing of nerve fibres (usually at rates up to hundred per second) are examples of rhythms with high frequencies. Population variations (most evident in sub-human species) are examples of low-frequency rhythms. The menstrual cycle of 28 days has been associated with the lunar cycle, but there is no proof of a definite link.

We know most about our 24-hour rhythms. It would be reasonable to assume that the setting of the sun or the extinction of artificial lights at night, perhaps combined with social conditioning to go to bed in the evening, makes us sleep. These certainly play a role, but the most important factors determining the timing (and structure) of sleep are an internal drive to sleepiness (the biological clock) together with accumulated tiredness since the last sleep. Many years ago, experiments in deep caves and in ‘temporal isolation’ units showed that, in the complete absence of any known time cues such as changes in the light level and ambient temperature, with no knowledge of clock time, radio, TV, newspapers, telephone, or contact with other people, humans still continue to live on an approximately 24-hour day. This observation is taken as evidence for the existence of an internal rhythm generating system of approximately 24 hours, which has come to be known as the biological clock.

Most people in such a time-free environment get up a little later and go to bed a little later each day: their personal in-built periodicity is slightly longer than 24 hours. The average period of the human body clock was thought to be 24.9 hours, but has recently been revised to be about 24.2 hours, on average. This natural rhythm, close to but not exactly 24 hours long, is called a circadian (meaning ‘about a day’) rhythm. A very few people have a periodicity shorter than 24 hours. Periodicity appears to be an inherited characteristic. Together with the sleep rhythm, many other major body functions exhibit circadian rhythms, including: secretion of hormones (e.g. the ‘darkness’ hormone melatonin, usually high at night, and the stress hormone cortisol, usually high in the morning); the production of urine and the variation in deep body temperature (usually low at night); the biochemical composition of the blood; alertness; and the ability to perform cognitive and dextrous tasks. Examples of rhythms that are not internally generated include the salivation and insulin responses to periodic meals.

Endogenous rhythms predict and prepare our bodies for forthcoming events: increased sleepiness in the evening prepares us for sleep, increased deep body temperature in the morning heralds wake-up. Some of the most striking examples of predictive rhythms are seasonal breeding patterns in long-lived animals. Sheep mate in the autumn and give birth in the spring — a time of year most propitious for survival of the lambs. These events are dictated by an endogenous seasonal rhythm of reproductive competence.

Since endogenous rhythms do not have exactly the same periodicity as the corresponding environmental cycle, they have to be reset by external time cues. The most potent of the signals for circadian synchronization, in the vast majority of species, including humans, is the daily light–dark cycle. The annual change in day length is the primary cue for timing seasonal cycles in most species. An inherent rhythm that is delayed a little each day (such as the natural human circadian clock of some 24.2 hours) must obviously be advanced each day (by 0.2 hours) to stay locked to the outside world. Exposure to bright light in the early morning, shortly after the deep-body temperature reaches its minimum value, will advance the circadian rhythm, while similar exposure to light in the late evening, before the temperature minimum, will delay it. Social interactions, mealtimes, exercise, and knowledge of clock time all help to keep us synchronized.

The major internal rhythm-generating system of mammals is found within the brain, in a pair of tiny structures known as the suprachiasmatic nuclei (SCN). Each SCN is a group of a few thousand nerve cells, sited in the hypothalamus, just above the optic chiasma (the crossing of the optic nerves). Destruction of this small area in rodents abolishes nearly all circadian rhythms, although there is a supplementary ‘clock’ in the retina itself (at least in hamsters). Amazingly, transplantation of the SCN from one hamster to another in which it has been damaged restores rhythmicity (of activity–rest) to the host animal, and confers on it the natural periodicity of the donor animal. It is assumed that the same would be true of humans. A small number of nerve fibres branch off from the optic nerve into the SCN. The information they carry enables the mammalian circadian rhythm generated in the SCN to be reset by light.

In most lower vertebrates, both the retina of the eye and the pineal gland in the brain are also capable of generating circadian rhythms. All rhythm-generating tissues (SCN, retina, pineal gland) show circadian rhythms of physiological activity even if they are removed and maintained, alive but isolated, in a cell culture chamber (a clock in a dish). Nerve cells of the SCN fire impulses in extremely regular patterns, like metronomes, and the frequency of the pattern varies with the time of day; cells of the retina and the pineal gland secrete the hormone melatonin in amounts that vary with the time of day. Each rhythm-generating cell contains a self-sustaining ‘oscillator’. In the case of the retina and the pineal of lower vertebrates, the cells involved are direct photoreceptors. In some non-mammalian vertebrates the pineal gland appears to be the master clock. In the sparrow, Passer domesticus, for example, removal of the pineal gland leads to loss of the activity–rest rhythm, and transplantation of a pineal from another bird not only restores rhythmicity but confers the phase of the donor to the host.

The hormone melatonin is the main output of the ‘clock’ of most vertebrates so far investigated. In mammals, where the SCN is the major clock, melatonin is synthesized mostly in the pineal gland under the control of the SCN. It is normally made at night (or during the dark phase). Light serves both to synchronize the rhythm to 24 hours and to regulate the duration of night-time secretion: in winter (long nights) the secretion profile is longer than in summer (short nights). It is this changing duration of melatonin (the darkness hormone) that acts as a time cue for the organization of seasonal physiology in photoperiodic mammals (those that depend on daylength to time their seasonal functions). The role of melatonin in circadian organization is very important in those species that use the pineal and/or the retina as a clock. In mammals (humans and rodents) it has only modulatory effects on circadian rhythms. It probably serves to reinforce and elicit physiology and behaviour associated with darkness. And it can also act on the SCN itself, producing ‘feedback’ resetting of the clock.

The real importance of the biological clock is evident when things go wrong. Disturbed rhythms are found in blindness, shiftwork, jetlag, certain insomnias, some psychiatric conditions and in some elderly people. Blind people with no conscious (or even unconscious) light perception lack the external light–dark changes that normally reset the body clock each day, and hence many have problems living on a 24-hour day. They manifest their own, endogenous periodicity, which means that, every few days or weeks, when their own clock has drifted out of phase with the world, they go through periods of feeling extremely sleepy and/or napping during the day, and being wide awake at night. Night shift workers are usually unadapted: they are therefore trying to work at the lowest ebb of their alertness and performance rhythms, and have problems sleeping, out of phase, during the day. This is the probable explanation for the increase in accidents on night shifts and for the many health problems of shift workers. A similar problem occurs for people travelling quickly over many time zones (jet lag): the clock adapts only slowly to such abrupt changes of phase. There are moreover a number of other conditions of clock dysfunction, such as delayed and advanced sleep-phase syndrome, and possibly a general lack of robustness of rhythms in some elderly people.

It is reasonable to assume that improper functioning of the circadian system is deleterious to health. Since bright natural light is more effective at synchronizing rhythms than domestic intensity light, urban populations (who generally live in a relatively light-deprived environment) are most at risk. Both bright artificial light and administration of melatonin at night time can be used to reinforce circadian organization and to hasten adaptation to phase shifts, for instance as a remedy for jet lag. At present there is considerable interest among pharmaceutical companies in the development of artificial analogues of melatonin.

Josephine Arendt


Arendt J. (1995). Melatonin and the mammalian pineal gland. Chapman Hall, London, New York.
Miller, J. D.,, Morin, I. P.,, Schwartz, W. J., and and Moore, R. Y. (1996). New insights into the mammalian circadian clock. Sleep 8 641–67.

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Theory of biochemical phasing, which claims that human beings experience three major biological cycles: (1) a 23-day cycle of physical strength, energy, and endurance, (2) a 28-day cycle of emotional sensibility, intuition, and creative ability; and (3) a 33-day cycle of mental activity, reasoning, and ambition. Charts of these cycles indicate periods of maximum or minimum potential in any of the three cycles, as well as critical dates of stress when two or three of the cycles intersect. By studying such advantageous or disadvantageous points of the cycles, it is claimed that an individual can be aware of the best and worst dates to maximize effort for success and confidence and avoid over-stress at dates of minimal confidence and energy. The theory has some attraction as it relates to other natural cycles such as the ebb and flow of the tides and the menstrual cycle in women.

Since body cycles relate to birth dates, the system of biorhythms is analagous with medical astrology. During the 1970s the system became a popular fad. Some physicians attempted to use biorhythm diagnostically, and some used biorhythms to predict football games. Billie Jean King is said to have won her famous match against Bobby Riggs when at a "high" in two of her cycles. Practitioners claimed that Judy Garland and Marilyn Monroe committed suicide on their critical days. The Omi Railway in Japan credited biorhythms with their accident-free record of safety. Other Japanese firms and several European airlines tested the use of biorhythms.

The concept of biorhythms was first proposed by William Fliess, a German friend of Sigmund Freud. Fliess proposed two basic cycles, and Austrian engineer Alfed Teltscher added the idea of a third cycle. Herman Swoboda tied the cycles to the birth date. Other writers also explored the idea through the twentieth century, but in the early 1960s the writings of George S. Thommen succeeded in popularizing the idea. Thommen found a leading supporter in Bernard Gittelson. Apparatus designed to simplify charting of biorhythm cycles have been developed and include the biomate (a manual computer), and the biolater (a small electronic calculator with mathematical functions).

During the 1970s various trials attempted to verify the claims about biorhythm. The most notable were in the field of sports where, it was predicted, outstanding performances would tend to appear on days of biorhythmic highs. In fact, no such patterns emerged. No empirical data exists to support the biorhythm theory.


Bainbridge, William S. "Biorhythms: Evaluating a Pseudo-science." Skeptical Inquirer (spring/summer 1978): 41-56.

Gittelson, Bernard. Bio-Rhythms: A Personal Science. New York: Warner Books, 1977.

Luce, Gay Gaer. Body Time: Physiological Rhythms and Social Stress. New York: Pantheon Books, 1971.

. Biological Rhythms in Psychiatry and Medicine. Washington, D.C.: National Institute of Mental Health, 1970. Reprinted as Biological Rhythms in Human and Animal Physiology. New York: Dover Books, 1971.

Schadewald, Robert. "Biorhythms: A Critical Look at Critical Days." Fate (February 1979): 75-80.

Thommen, George. Is This Your Day? New York: Award, 1964.

Wernli, Hans J. Biorhythm: A Scientific Exploration into the Life Cycles of the Individual. New York: Crown, 1961.

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biorhythm or biological rhythm, cyclic pattern of changes in physiology or in activity of living organisms, often synchronized with daily, monthly, or yearly environmental changes. Rhythms that vary according to the time of day (circadian rhythms), in part a response to daylight or dark, include the opening and closing of flowers and the nighttime increase in activity of nocturnal animals. Circadian rhythms also include activities that occur often during a 24-hour period, such as blood pressure changes and urine production. Annual cycles, called cirannual rhythms, respond to changes in the relative length of periods of daylight and include such activities as migration and animal mating. Marine organisms are affected by tide cycles. Although the exact nature of the internal mechanism is not known, various external stimuli—including light, temperature, and gravity—influence the organism's internal clock; in the absence of external cues, the internal rhythms gradually drift out of phase with the environment.

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bi·o·rhythm / ˈbīōˌri[voicedth]əm/ • n. a recurring cycle in the physiology or functioning of an organism, such as the daily cycle of sleeping and waking. ∎  a cyclic pattern of physical, emotional, or mental activity said to occur in the life of a person. DERIVATIVES: bi·o·rhyth·mic / ˌbīōˈri[voicedth]mik/ adj.

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biorhythm (biological rhythm) A roughly periodic change in the behaviour or physiology of an organism that is generated and maintained by a biological clock. Well-known examples are the annual and circadian rhythms occurring in many animals and plants. See also infradian rhythm; ultradian rhythm.

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biological rhythm

biological rhythm See biorhythm.

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