circadian rhythm

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rhythm, biological

rhythm, biological, or biorhythm, cyclic pattern of physiological changes or changes in activity in living organisms, most often synchronized with daily, monthly, or annual cyclical changes in the environment. The exact nature of the internal mechanism, or “biological clock,” that controls such rhythms is not fully understood, but studies of fruit flies since the 1980s have revealed how genes and proteins that they produce control the circadian rhythm at the molecular level in cells, establishing key principles for understanding its mechanisms, and these studies have been extended to other species. Rhythms that vary according to the time of day, called circadian rhythms, include such phenomena as the opening and closing of flowers and, in humans, changes in body temperature, blood pressure, and urine production. In diurnal animals, activity increases in daylight; in nocturnal animals nighttime activity predominates. Activity of many marine organisms varies according to the tide. Monthly rhythms include weight changes in men and the menstrual period in women. Annual cycles, or circannual rhythms, include bird migrations, reproductive activity, and mammalian hibernation. Daily cycles, or circadian rhythms, are in part a response to daylight or dark, and annual cycles in part responses to changes in the relative length of periods of daylight. However, environmentally determined cyclical changes, such as changes in daylight, temperature, and availability of food, serve primarily to refine and adjust physiologically determined circadian or circannual rhythms: in the absence of external cues, the internal rhythms gradually drift out of phase with the environment. Physiological rhythms are also present in the activity of individual organs, e.g., the beating of heart muscle and the activity of electrical waves of the brain.


See G. G. Luce, Biological Rhythms in Human and Animal Physiology (1971); J. Brady, ed., Biological Timekeeping (1982); L. Glass and M. C. Mackey, From Clocks to Chaos: The Rhythms of Life (1988).

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circadian rhythm

[sər′kād·ē·ən ′rith·əm]
A self-sustained cycle of physiological changes that occurs over an approximately 24-hour cycle, generally synchronized to light-dark cycles in an organism's environment.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
References in periodicals archive ?
Importantly, it is thought that eating at unusual times, as often occurs during shift work and jet lag, is a major cause of body clock disruption.
This bright light will boost your mood and alertness, and will also provide the signal the body needs to push the body clock earlier in time.
Ironically, he pointed out that 'people who keep faith with their body clock cycle (known as the circadian cycle) by sleeping when they should, eat at the right time and work when they should live longer and are at a lesser risk of diseases.'
"Young people's body clocks alter as they enter the teenage years, meaning they often have a tendency to stay up late but then struggle to get up in the morning.
In a study published in the journal Science Translational Medicine on Wednesday, the scientists showed for the first time how our internal body clocks regulate wound healing by skin cells, and optimise healing during the day.
The findings may help explain why shift workers, whose body clocks are routinely disrupted, are more prone to health problems, including infections and chronic disease.
The company said HETLIOZ is a melatonin receptor agonist and acts as a circadian regulator that resets the master body clock in the suprachiasmatic nucleus in the brain.
Researchers at the University of Warwick in Coventry, England, and University Hospitals Coventry and Warwickshire (UHCW) NHS Trust have discovered how body clock genes could affect women's ability to have children.
Try going to bed a little earlier on the days leading up to the 30th and on Sunday morning get out and take in some sun and fresh air to speed up the resetting of the body clock! But, don't dwell too much on the lost hour!
Sleep experts, however, suggest that people often get their internal body clock set to the wrong time, which can make them feel as though they are suffering from a condition commonly known as jet lag.
THE MARTIAN DAY, WHICH IS ABOUT 40 MINUTES LONGER THAN AN EARTH DAY, can cause havoc with the internal 24-hour body clock. To help future space travelers control Martian jet lag, researchers at Brigham and Women's Hospital have developed a fatigue management program specifically set to the Martian day.