circadian rhythm (ser-kA´dê-en
A daily rhythmic activity cycle, based on 24-hour intervals, that is exhibited by many organisms.
Biological Clocks, body systems that enable living things to follow the rhythms of nature, such as the cycles of day and night and of the seasons. Most of what is known about these systems comes from the study of circadian (daily) rhythms. Circadian rhythms cue daily behavior patterns even in the absence of external cues such as sunrise or sunset, evidence that such patterns depend on internal timers. However, when living things are deprived of normal cues, they display a characteristic "free-running" period of not quite 24 hours and drift slowly out of phase with the natural world.
Light, particularly bright light, is believed to be the most powerful synchronizer of circadian rhythms. The amount of artificial indoor light to which people are exposed each day can resynchronize the body's cycle of sleep and waking. Many living things also use rhythmic variations in temperature or other sensory inputs to readjust their internal timers.
A master clock appears to
exist in the brains of most animals, communicating its timing signals chemically
to the rest of the body. Scientists believe that the human biological clock
is located in the hypothalamus, the part of the brain that regulates such
basic drives as hunger, thirst, and sexual desire. The biological clock
itself is believed to be a cluster of nerve cells called the suprachiasmatic
nucleus. Melatonin, a hormone produced in response to darkness, is also
thought to play a primary role in controlling the body's circadian rhythm.