A daily cycle of biological activity based on a 24-hour period and influenced by regular variations in the environment, such as the alternation of night and day. Circadian rhythms include sleeping and waking in animals, flower closing and opening in angiosperms, and tissue growth and differentiation in fungi.
See also biological clock
A Closer Look The circadian rhythm, present in humans and most other animals, is generated by an internal clock that is synchronized to light-dark cycles and other cues in an organism's environment. This internal clock accounts for waking up at the same time every day even without an alarm clock. It also causes nocturnal animals to function at night when diurnal creatures are at rest. Circadian rhythms can be disrupted by changes in daily schedule. Biologists have observed that birds exposed to artificial light for a long time sometimes build nests in the fall instead of the spring. While the process underlying circadian rhythm is still being investigated, it is known to be controlled mainly by the release of hormones. In humans, the internal clock is located within the brain's hypothalamus and pineal gland, which releases melatonin in response to the information it receives from photoreceptors in the retina. Nighttime causes melatonin secretion to rise, while daylight inhibits it. Even when light cues are absent, melatonin is still released in a cyclical manner.