A concept of physiological time: rhythms in behavior and reproductive physiology.

Biological clocks are self-sustained and endogenous. Animals have biological clocks in a variety of frequencies. Biological clocks provide temporal coordination among physiological, behavioral, and environmental events. One important function of steroid hormones may be to alter the temporal coordination among those events. Biological clocks provide a referent mechanism for the timing of both current and future endogenous and exogenous events. Physiological time is a unifying principle in biology.