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.
Do you have more questions about circadian rhythms? We recommend this Frequently Asked Questions about Circadian Rhythm from the National Institute of General Medical Sciences.