Biological Rhythms

by Food and Drug Administration (FDA)

The year is 1729; the place, Paris, France. Astronomer Jean Jacques d'Ortous de Mairan has just locked a heliotrope in his closet.

Investigating this flower's well-known inclination to respond to the sun, de Mairan's expectation is that the plant will stay folded up in the dark. But to his surprise he discovers that, instead, the heliotrope, whose name means "turning towards the sun," continues to open its leaves by day and close them by night even without the sunlight.

A few years later, another researcher, Henri-Louis Duhamel, dubious about de Marian's results, repeats the experiment, removing not only light but heat cues as well. He finds that, as before, the plant continues opening and closing in a daily rhythmical pattern.

In the centuries since these early experiments, research has shown that not only plants, but also animals — including humans — are governed by biological rhythms. The study of biological rhythms is technically known as chronobiology. Researcher Franz Halberg, M.D., of the University of Minnesota coined the term "circadian" (from circa — about — and dian — day) to describe body rhythms that approximate a 24-hour cycle.

Like the lovely heliotrope, the human body has observable daily rhythms that seem to be governed by what some have called a "biological clock." Heart rate, blood pressure, respiration rate, body temperature, and urine excretion all have higher levels (called "peaks") that occur during the day, and lower levels (called "troughs") that occur at night. But, like the opening and closing of the heliotrope, it is not merely the presence or absence of sunlight that triggers these rhythms.

While these rhythms may be set off to a certain extent by environmental cues (called "zeitgebers") such as the dark/light cycle and food consumption, many rhythms persist even when environmental cues are removed. In experiments in which people live for weeks with no social or geophysical cues such as set meal times and the light-dark cycle, the basic rhythms continue, but they shift from a 24-hour cycle to one of approximately 25 hours. One theory to explain this phenomenon is that the 25-hour cycle is somehow genetically predetermined in all humans.

The most well-known circadian rhythm is that of body temperature, which varies daily a degree or two in a healthy person, peaking in late afternoon and troughing in the early morning hours. This rhythm persists even when the individual is confined to bed for the entire 24 hours, when the time of meals is varied, or if the person fasts. Pulse rate and blood pressure also peak around the same time as body temperature.

Levels of glycogen, a carbohydrate essential to fuel the body, start decreasing about noon. By 3 to 6 a.m., the body has used up much of its supply. This rhythm may help explain why people with diabetes experience "low blood sugar" in the morning. (Glycogen is metabolized into glucose, a sugar.)

Though human body rhythms have been systematically studied for about 30 years, practical medical application of the knowledge gained has been slow. One reason for this is that the new information challenges the long-held theory of "homeostasis," which sees the internal conditions of the human body as more or less constant. Although this concept allows for minor body fluctuations, it maintains that the fluctuations are meaningless when it comes to the treatment of disease. Another reason is the difficulty of devising human studies that can control all environmental factors.

Says Ritchie J. Feuers, Ph.D., a researcher at the Food and Drug Administration's National Center for Toxicological Research who has published on the subject, "Though it's an old field, for a long time most researchers have chosen to ignore it. But now scientists are beginning to recognize the medical implications of our findings."

Jet Lag, Shift Work, and Depression

Three areas that are beginning to benefit from knowledge about body rhythms are jet lag, shift work, and some types of depression. The biological rhythm of the hormone melatonin appears to affect all of these somewhat, but much controversy still exists about its precise role.

Melatonin is secreted at night by the pineal gland, located in front of the brain in about the middle of the forehead. Some researchers believe that hormones secreted by this gland somehow transmit information concerning the light-dark cycle, which helps to regulate the "biological clock."

"Jet lag" is more than an excuse for sleeping until noon the day after a plane trip across time zones. Research in biological rhythms has led to the realization that jet lag is a legitimate medical condition. When people jet across time zones, their bodies continue to operate on the rhythms established at their homes, and it can take several days for body rhythms to adjust to local time. Symptoms of jet lag include daytime sleepiness and nighttime insomnia, confusion, poor concentration, slowed reflexes, indigestion, hunger at odd hours, irritability, and sometimes mild depression. For people whose careers required frequent travel between time zones, these problems can be crucial.

Symptoms of jet lag are commonly more severe when traveling from west to east. This may be due, at least in part, to the body's tendency when separated from time cues to gravitate to a day that is closer to 25 hours than 24. For this reason, body rhythms may have an easier time adapting to westward travel — which lengthens the day. Because melatonin secretion is thought by some to play a part in regulating the biological clock, researchers are currently investigating whether administering the hormone in pill form will help alleviate jet lag symptoms. Preliminary studies involving 25 people traveling from New Zealand to London and back gives some evidence that the medication is effective. However, it will take years of additional studies to determine what the long-term side effects of this drug use of a hormone might be and whether it is best taken before, during and after a flight, or just after it.

In the meantime, some experts recommend that jet setters might try the following:

If staying in the new location more than two days, adopt local time for routines immediately upon arrival. If staying less than two days, maintain home schedule if possible.

If staying more than two days, several days before departure, try to gradually shift sleeping and eating routines to coincide with time at destination.

Before flight, avoid overeating and alcohol.

In flight, drink water and juices, not alcohol. Don't smoke.

If possible, break up long flights in one direction with layovers of at least a day.

Allow plenty of time for sleep in the new location.

After flying east, take walks outside in the morning to get used to earlier appearance of light. After traveling west, take walks outside in the afternoon to acclimate to later waning of light.

Melatonin is also being investigated for use in helping people adapt to shift work and to treat a particular type of depression known as winter depression or seasonal affective disorder (SAD). This particular type of depression, according to one theory, may, in some cases, be related to desynchronization of body rhythms. One symptom of this, some researchers think, is the early morning awakening symptomatic of this illness. It is not clear, however, whether such observed desynchronization is a cause or an effect of the depression.

About the Author

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