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 day—or, 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, because—like everything else in the universe—it 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 minutes—very 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 up—a 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 evening—at 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 resting—and particularly when sleeping—after 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,
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.
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 north—Russia, Alaska, and Scandinavia—undergo 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 insomnia—a 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 disorders—those 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 craft—that 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 sleepy—and 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
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 schedule—that 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.
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
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 things—fur, 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.
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 person—one 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.
Biological Rhythms (Web site). <http://faculty.washington.edu/chudler/clock.html> .
Center for Biological Timing (Web site). <http://www.cbt.virginia.edu/> .
Circadian Rhythms (Web site). <http://www.bio.warwick.ac.uk/millar/circad.html> .
"Ecstasy 'Ruins Body Clock.'" British Broadcasting Corporation (Web site). <http://news.bbc.co.uk/hi/english/health/newsid_803000/803633.stm> .
Hughes, Martin. Bodyclock: The Effects of Time on Human Health. New York: Facts on File, 1989.
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.
Rose, Kenneth Jon. The Body in Time. New York: John Wiley and Sons, 1988.
Sleep Disorders Information (Web site). <http://www.stanford.edu/~dement/sleepinfo.html> .
Waterhouse, J. M., D. S. Waters, and M. E. Waterhouse. Your Body Clock. New York: Oxford University Press, 1990.
Winfree, Arthur T. The Timing of Biological Clocks. New York: Scientific American Library, 1987.