Human Circadian Physiology and Sleep-Wake Regulation

Periodic daily cycles have been observed in a wide range of physiologic and behavioral variables in humans. These include cyclic variations in sleep propensity, neurobehavioral performance, motor performance, as well as thermoregulatory, endocrine, cardiac, gastrointestinal, pulmonary, renal, and neurologic functions, including sensory sensitivity. The rhythms of all of these variables exhibit a specific temporal relationship with each other and with the environmental light—dark cycle. The mechanisms underlying these rhythms, their temporal relationships, and their functional significance have been the subject of numerous investigations (Conroy & Mills, 1970).