Mention circadian rhythms, and most of us think of jet lag, sleep disorders or how tired we were after that all-night party.
But researchers say that circadian clocks – which control a 24-hour biological cycle first documented by a French scientist in a darkened closet in 1729 – have profound effects on just about everything that walks, crawls or grows.
Circadian clocks control leaf and petal movements in plants, migratory patterns of birds, the life cycles of insects and biochemical reactions in the tiniest of bacteria. They also have major effects on us.
“We’re finding there’s no limit to the role these rhythms play,” said Jay Dunlap, chairman of genetics at the Dartmouth Medical School, who studies circadian rhythms in fungi. “There’s enormously rich biology behind this phenomenon.”
Researchers study rats, mice, sea slugs and fruit flies to try to solve mysteries about the role circadian rhythms play – not only in our sleep cycles, but in how we learn, our tolerance for pain and how we respond to alcohol and cancer therapy.
Circadian rhythms are found in some of the earliest known forms of bacteria. Scientists say the rhythms probably evolved as a survival mechanism, allowing animals to maximize activity at the time of the day or night when they can evade predators best, or find food and mates.
In humans, circadian rhythms generally produce peak alertness about 9 a.m. and 9 p.m. daily. They also explain why we tend to feel sleepy in mid-
afternoon, with the drowsiest point usually about 3 p.m., researchers say.
“It can vary an hour or so from person to person, and it can depend on the time you got to bed and wake up, but you generally see the same patterns,” said Charles DeRoshia, a research psychologist who spent decades studying circadian rhythms before his recent retirement from the National Aeronautics and Space Administration Ames Research Center.
Other physical and emotional functions follow circadian rhythms.
Pain tolerance, for example, is highest in the afternoon, while blood pressure rises in the morning and stays elevated throughout the day.
Circadian rhythms can be triggered by environmental cues, such as the cycles of day and night, but not all organisms are dependent on them. In fact, circadian rhythms have been found in cave-dwelling fish and insects that never see the sun.
Experiments also have shown that the rhythms continue in people living for weeks with no environmental cues, such as set meal times or a light-dark cycle.
Understanding how certain genes work along with the body’s master clock to dictate circadian rhythms has far-reaching implications.
Researchers hope to pinpoint the best times of day to administer pain medication. They also want to know why certain health patterns are linked to a time of day. Why, for instance, does blood pressure rise in the morning and stay elevated until late afternoon? Researchers also are evaluating how circadian rhythms affect cancer therapies, their role in the learning process and the possible contribution of abnormal circadian rhythms to alcoholism.
