periodic biological fluctuation in an organism that corresponds to, and is in response to, periodic environmental change. Examples of such change include cyclical variations in the relative position of the Earth to the Sun and to the Moon and in the immediate effects of such variations, e.g., day alternating with night, high tide alternating with low tide.

Behaviour, Animal.

The internal mechanism by which such a rhythmic phenomenon occurs and is maintained even in the absence of the apparent environmental stimulus is termed a biological clock. When an animal that functions according to such a clock is rapidly translocated to a geographic point where the environmental cycle is no longer synchronous with the animal's cycle, the clock continues for a time to function synchronously with the original environmental cycle. Humans similarly transported over great distances often experience fatigue and lowered efficiency for several days, a phenomenon known as “jet lag,” or jet syndrome (jet lag).

A rhythm with a 24-hour cycle is called a circadian (circadian rhythm) (from Latin circa, “about”; di, “day”—i.e., “about a day”), solar day, diel, daily, diurnal, or nychthemeral rhythm. A lunar tidal rhythm—the regular ebb and flow of oceans (ocean) and very large inland bodies of water—subjects seashore plants and animals to a rhythmic change; typically two high and two low tides occur each day (about 24.8 hours). Many species of shorebirds exhibit this rhythm by seeking food only when beaches are exposed at low tide. Monthly rhythms, averaging approximately 29.5 days, are reflected in reproductive cycles of many marine plants and in those of many animals. Annual rhythms are reflected in the reproduction and growth of most terrestrial plants and animals in the temperate zones.