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 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 likely evolved as a survival mechanism, allowing animals to maximize activity at the time of the day or night when they can best evade predators, or find food and mates.
In humans, circadian rhythms generally produce peak alertness about 9 a.m. and 9 p.m. each day. They also explain why we tend to feel sleepy in midafternoon, 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 NASA 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. Body temperature can vary throughout the day by as much as 4 degrees, reaching its lowest point right before we wake up and generally peaking about 5 p.m.
"If you're trying to determine if someone has a fever, the time of day does matter," said Robert Refinetti, a psychologist at the University of South Carolina and the editor of the peer-reviewed Journal of Circadian Rhythms.
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 who never see the sun.
Experiments have also shown that the rhythms continue in people living for weeks with no environmental cues, such as set meal times or a light-dark cycle.
The term circadian comes from the Latin words circa, which means "around," and dias, which means "day." It was coined in the 1950s by Dr. Franz Halberg, a Minnesota physician and a pioneer in the field who discovered variations in blood cells based on the time of day.
"These rhythms are built into you and me like the color of our eyes," said Halberg, 86.
But the effects were first documented in 1729, when French astronomer Jacques d'Ortours de Mairan placed heliotrope plants in a dark closet. Observing that they continued to open their leaves in the day and close them at night, he concluded that the plant's daily cycles were triggered by innate properties and not by sunlight or other external influences.
Almost 270 years later, scientists discovered that cryptochrome, a type of light reactive pigment found in the human eye and in some plants, plays an important role in driving molecular machinery that generates circadian rhythms.
Researchers also have discovered that a master clock in a portion of our brains known as the suprachiasmatic nucleus controls circadian rhythm by receiving signals from light receptors in the retina of the eye and passing them to other organs.
But even if the brain is not receiving light, the master will run by itself - which is why people and plants denied light will still operate by a 24-hour clock.
Biologists also have since identified several genes and proteins associated with circadian rhythms in different types of cells. Understanding how the genes work along with the master clock to dictate circadian rhythms has far-reaching implications.
For example, researchers hope to pinpoint the best times of day for 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, and why do people seem more prone to heart attacks in the early morning?
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.