Basic mechanisms of circadian rhythms and their relation to the sleep/wake cycle

Organisms exhibit cyclic variations in a variety of essential functions, including the sleep-wake cycle, hormonal regulation, and reproduction. A primary environmental signal regulating these functions is the daily alternation of darkness and light exerted by the rotation of the earth. Superimposed upon the daily light-dark cycle is a seasonal influence that modifies the relative durations of day and night over the course of a year. These environmental changes make it necessary for organisms to be able to modify their behavior so that they are active during times when the opportunity to acquire nutritional resources exceeds the risk of predation, and resting during times when the need for vigilance is minimized. Be they day-active or night-active, all organisms need a means of keeping time in a 24-hour world and adjusting to changes in day length or transition times that may occur