ABSTRACT
Most organisms must contend with environments that exhibit marked daily fluctuations in a variety of biotic and abiotic factors. The response of natural selection has been the evolution of regulatory systems whose primary function is to match these cyclic environmental variations with appropriate, periodic alterations in physiology, biochemistry, and behavior. These regulatory systems function as “biological clocks” generating a precise temporal program within the organism that coordinates its activities with the periodic, and thus predictable, environment in which it lives.
