Sehgal, Amita Department of Neuroscience, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania.
Last reviewed:April 2019
Show previous versions
- Circadian rhythms in humans
- Localization of circadian clocks
- Genetic basis
- Molecular basis
- Synchronization to light
- Output signals
- Links to Primary Literature
- Additional Readings
Any of the self-sustained circadian (approximately 24-hour) rhythms that regulate daily activities, including sleep and wakefulness. A biological clock is any physiologic factor that functions in regulating innate organismal rhythms. Biological clocks were described as early as 1729 by the French scientist Jean Jacques d'Ortous de Mairan. He placed a plant in constant darkness and noticed that leaf movements continued to occur at specific times of day, despite the absence of the day-night cycle. The notion that this rhythmicity was still driven in some fashion by the Earth's rotation was resolved in the midtwentieth century, when it became clear that the period of self-sustained (free-running) oscillations usually does not match that of the environmental cycle (that is, the Earth's rotation); therefore, the expression "approximately 24 hours" is used. Moreover, the free-running period varies among species and also somewhat from one individual to another. Circadian rhythmicity is often referred to as the biological clock (Fig. 1). See also: Circadian clock (plants); Photoperiodism; Plant movements
The content above is only an excerpt.
for your institution. Subscribe
To learn more about subscribing to AccessScience, or to request a no-risk trial of this award-winning scientific reference for your institution, fill in your information and a member of our Sales Team will contact you as soon as possible.
to your librarian. Recommend
Let your librarian know about the award-winning gateway to the most trustworthy and accurate scientific information.
AccessScience provides the most accurate and trustworthy scientific information available.
Recognized as an award-winning gateway to scientific knowledge, AccessScience is an amazing online resource that contains high-quality reference material written specifically for students. Contributors include more than 9000 highly qualified scientists and 43 Nobel Prize winners.
MORE THAN 8500 articles and Research Reviews covering all major scientific disciplines and encompassing the McGraw-Hill Encyclopedia of Science & Technology and McGraw-Hill Yearbook of Science & Technology
115,000-PLUS definitions from the McGraw-Hill Dictionary of Scientific and Technical Terms
3000 biographies of notable scientific figures
MORE THAN 19,000 downloadable images and animations illustrating key topics
ENGAGING VIDEOS highlighting the life and work of award-winning scientists
SUGGESTIONS FOR FURTHER STUDY and additional readings to guide students to deeper understanding and research
LINKS TO CITABLE LITERATURE help students expand their knowledge using primary sources of information