Circadian rhythms are cycles of changes in aspects of the body's physiology, including temperature and blood glucose and subsequent behavior, occurring over a period of approximately 24 hours.
Circadian rhythms, which are found in most animals, are driven by a biological clock in the brain that responds to environmental cues, the chief of which is light levels. The most obvious manifestation of circadian rhythms in humans are the daily patterns of waking and sleeping, which tend to occur during daylight and night time, respectively. Circadian rhythms have been extensively studied at the behavioral, cellular, and molecular levels.
The suprachiasmatic nuclei (SCN) are two groups of neurons found in the hypothalamus that are the location of the biological clock. These neurons are sensitive to light. When levels of light fall, they send signals to the nearby pineal gland, which stimulates increased production of the hormone melatonin, which induces sleepiness.
However, the biological clock still can function in the absence of external cues such as light levels, as has been shown by extensive research. In this situation, the length of the circadian rhythm either shortens or lengthens from 24 hours. In mice, it decreases to 23 hours, and in humans it increases to 24.5 to 25.5 hours.
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National Sleep Foundation. “Sleep Drive and Your Body Clock.” http://sleepfoundation.org/sleep-topics/sleepdrive-and-your-body-clock (accessed July 15, 2015).
American Sleep Association, 1002 Lititz Pike, Ste. 229, Lititz, PA, 17543, https://www.sleepassociation.org .