Physiology, Circadian Rhythm

Excerpt

Sleep regulation is governed by the homeostatic physiology of the circadian rhythm and the sleep-wake cycle. "Circadian rhythm" refers to a 24-hour internal timing system within the brain that regulates cycles of alertness and sleepiness in response to environmental light cues. Human circadian physiology and behavior evolved in concert with the Earth's rotation around its axis. The biological circadian system has expanded to facilitate adaptation to environmental changes and anticipation of fluctuations in solar radiation, temperature, and food availability. The absence of an endogenous circadian clock would impair the ability of Homo sapiens to optimize energy expenditure and internal physiologic processes.