A circadian rhythm is a rhythm that repeats about every 24 hours. Plants and animals have these built-in cycles which allow them to flower at the right time, sleep at the right time, etc.^[1]

These cycles are also called circadian clocks.^[2] The word "circadian" comes from the Latin circa, "around", and diem or dies, "day", meaning "about a day". Biological time-keeping rhythms include daily, tidal, weekly, seasonal, and annual rhythms. However, most of these rhythms repeat once a day, so they are called circadian from the Latin circa (about) dies (a day).

Circadian clocks are the internal clocks which make sure that the organism is doing the right thing at the right time. With more complex animals there is the matter of sleep, which seems to be universal amongst animals which have a brain.

Circadian rhythms do some amazing things, especially in the oceanic animals whose eggs are fertilised externally. This means the eggs and sperm have to be released at the same time by both sexes of the same species. Little is known about the physiology of how this coordination is achieved.

Animal clocks have three main parts:

• a central biochemical oscillator with a period of about 24 hours that keeps time;
• a series of input pathways to this central oscillator to allow the clock to adjust itself;
• a series of output pathways that regulate rhythms in biochemistry, physiology, and behaviour throughout an organism.

In 2017, the Nobel Prize in Physiology or Medicine was awarded to Jeffrey C. Hall, Michael Rosbash and Michael W. Young "for their discoveries of molecular mechanisms controlling the circadian rhythm" in fruit flies (Drosophila).^[3]

The circadian rhythms, also called the "biological clock" or the "body clock", of humans and other animals, regulate many bodily functions including feeding, sleeping, body temperature and hormone production.

The actual machinery in plants, fungi and bacteria are different from those in the animal clock. However, they do the same kind of thing in practice.^[4]