Sehgal, Amita Department of Neuroscience, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania.
Last reviewed:February 2019
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- Circadian rhythms in humans
- Localization of circadian clocks
- Genetic basis
- Molecular basis
- Synchronization to light
- Output signals
- Links to Primary Literature
- Additional Readings
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
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