Physiology of the circadian system in animals and humans

Virtually all organisms have developed an internal timing system capable of reacting to and anticipating environmental stimuli with a program of appropriately timed metabolic, physiologic, and behavioral events. The predominant biological rhythms coincide with the geophysical cycle of day and night-the circadian rhythms. The suprachiasmatic nuclei comprise the primary pace-maker in mammals, exhibiting the properties fundamental to a rhythm-generating structure. This article summarizes recent research that has elucidated mechanisms of signal transduction within the circadian system. The roles of various neurochemicals and hormones in transmitting the circadian timing signal are described. Properties of the circadian system, including photic and nonphotic entrainment, phase response curves, masking, and the intrinsic variability in the system are discussed.