ABSTRACT
The term “circadian” refers to a circuit or period that is about 24 hours long. Many physiologic, biochemical, and behavioral activities have a circadian rhythm with the sleep-wake cycle being the most easily recognized. The sleep-wake cycle is related to other circadian rhythms, such as core body temperature and concentrations of melatonin or cortisol, through a complex interaction of afferent and efferent signals (Fig. 13.1).
