Most organisms that live on or near the surface of the Earth or its oceans have evolved a circadian clock– a daily timer of all biochemical, physiological and behavioral functions.

Daily cycle of light and darkness in the environment is a selective factor – having an internal clock is an adaptation that allows organisms to predict and prepare for instead of passively react to cyclical changes in the environment. The regularity of the light-dark cycle is usually a good predictor for other (perhaps not as precise) cycles of temperature, availability of food, or activity of predators.

written by written by Biochemistry and Physiology