Controlling Circadian Rhythm : Study Sheds New Light on Sleeping Patterns
The human circadian rhythm, a tyrant that dominates human existence, may be reset far more easily than previously believed, according to a new study in today’s issue of Science magazine.
The study comes at a time of increasing research into the mysterious, periodic cycles that govern life, and such studies are yielding new knowledge that is of potential benefit to millions of people who work non-traditional hours.
Already, chronobiologists--scientists who study periodic changes in biological patterns--are slowly learning how to ease the problems of rotating-shift work or alleviate the symptoms of jet lag.
A study earlier this summer reported that both heart attacks and strokes occur most frequently around 9 a.m., raising the possibility that altering biorhythms could minimize the risks of such events.
Reset Biological Clock
The latest study suggests that it is possible to reset the body’s biological clock by exposing a person to periods of artificial light.
Neuroendocrinologist Charles Czeisler and his colleagues at Brigham and Women’s Hospital in Boston report that they have reset the biological clock of a 66-year-old woman whose biological rhythms were out of synchronization with her sleeping patterns.
Normally, body temperature starts falling as a person prepares for sleep, and reaches a minimum about 6:30 the following morning before rising again. The concentration of hormones in the blood follow a similar pattern.
Czeisler’s patient’s temperature, however, reached a minimum about midnight, six hours before it should have. This condition did not interfere with the woman’s sleep, but many other individuals with a similar condition have great difficulty sleeping normal hours.
Exposed to Bright Light
Czeisler exposed the woman to four hours of light--about 48 times the intensity of normal room light--between 8 p.m. and midnight for one week.
“Within two days of the beginning of the therapy, the woman’s circadian rhythm had shifted six hours to coincide with her sleep patterns,” Czeisler said in a telephone interview. “In effect, we tricked her body into thinking it was dusk at midnight.”
His research paper reported on the results with only one patient, an unusual occurrence in scientific literature. But Czeisler pointed out that he had to study 40 other individuals for 14 days each to find the woman, and he then studied her for a total of 119 days.
Besides, he added, one patient was enough to show that the technique actually worked. “Once you’ve seen a frog jump, you know you are not dealing with a turtle.”
Every cell in the body has its own rhythm that governs when the cell reproduces, when it produces critical enzymes or hormones, or when it goes into a rest phase. Without centralized coordination, according to biologist Charles F. Ehret of the Argonne National Laboratory, these cells would flounder around aimlessly, just like a group of oarsmen without a coxswain.
The human’s equivalent of a coxswain is a small group of cells in the brain’s hypothalamus. Located just behind the eyes, these cells send out hormonal and perhaps electrical signals that tell all the other cells in the body “when to stroke and how fast,” he said.
Left to its own devices, the human body prefers a natural rhythm somewhat longer than the Earth’s 24-hour period. Research has shown that individuals isolated from outside influences such as sunlight, television and social contacts tend to adopt a “day” that is about 24 1/2 to 25 1/2 hours long.
Consequently, our biological clocks must be reset every day.
External Forces
Until the early 1980s, most scientists believed that human circadian rhythms were reset by social contacts and the scheduling of sleeping periods. They have since found, however, that there are many external factors that can reset the biological clock. The most important of these is sunlight.
Researchers have long known that the behavior of animals is controlled by the lengths of day and night, and that this control is associated with the secretion of the hormone melatonin--whose exact function is still somewhat of a mystery. Melatonin is secreted by the pineal gland, a small appendage to the brain whose structure is similar to that of the eye and which is sometimes thought of as a vestigial third eye.
Melatonin secretion in animals is suppressed by exposure to light. In rodents, exposure to light from the moon or a candle suppresses melatonin secretion. Humans, however, were thought to have evolved away from this susceptibility to light.
A Sedative Effect
But in 1980, psychologist Alfred Lewy, who was then at the National Institutes of Health and is now at the Oregon Health Sciences Center in Portland, discovered that melatonin secretion in humans, like that in animals, is suppressed by light, but that the light must be very bright--at least 10 times as intense as normal room light.
Some scientists have shown that melatonin has a sedative effect in humans.
Subsequently, psychiatrists have recognized a syndrome known as winter depression that is characterized by intense depression and inability to function efficiently during the short days of winter.
Scientists at the NIH and elsewhere have had great success treating about 150 winter depressives by exposing them to bright lights for two to four hours per day, “tricking” the body into believing that it is summer.
Lewy was also the first to suggest, in 1983, that biological clocks could be reset with light therapy. He has since treated seven patients whose bodies wanted to sleep at inappropriate times. Five of the patients went to sleep too early; these he treated with two hours of light in the evening for a week.
Minimize Jet Lag
Two others went to sleep late and slept too long in the morning; these he treated with two hours of bright light early in the morning. In all cases, the patients’ circadian rhythms were normalized.
Both Lewy and Czeisler speculate that light also can be used to minimize jet lag by shifting body rhythms. “If you are traveling east to west across six or less time zones,” Lewy said, “you want to get at least an hour of sunlight exposure in the evening. If you are traveling in the opposite direction, you want to get it in the morning.”
Czeisler also thinks bright lights could help shift workers adjust to a rotating-shift schedule. He is now planning a study in which exposure to light would be used to reset the workers’ biological clocks every time they change shift schedules. Such a study would be valuable, he said, because 33% of U.S. men and 26% of women are exposed to shift work each year.
Shift Rotation Studied
Czeisler, Lewy and others have previously also shown that the direction in which shifts are rotated is very important. The least stressful schedule is one that would, for example, rotate a worker’s shift so that he works daytimes one week, evenings the next and then nights the following.
In a classic study in 1982 at the Great Salt Lake Minerals and Chemicals Corp. in Ogden, Utah, Czeisler showed that employee morale and productivity increased and the number of injuries decreased after the shift rotation schedule was changed this way.
The researchers have subsequently found that such a schedule is even more effective if the workers remain on each shift for three or four weeks, and if they are taught how to adapt to these changes by altering their dietary and sleep patterns.
