An internal temporal rhythm, corresponding to an approximate 24-hour period, that regulates the biological and/or behavioral processes of living beings. Can also be influenced by external cues (known as Zeitgebers): daylight, for example. Some people think that a circadian rhythm is known as the biological clock, but though the are related, they are different things (please click the link below to learn more).

Example: Body temperature drops and melatonin secretion increases during the evening in humans; both are processes by which the body signals its need to rest and prepares itself for sleep.

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