Characteristics and determinants of normal sleep.

Although sleep appears to simply be a body and mind at rest, it is actually a dynamic and complex physiologic state that is necessary for survival. Normal sleep is characterized by behavioral and physiologic changes as well as 2 distinct sleep states, rapid eye movement (REM) and non-REM (NREM). Throughout the course of a night, people cycle between NREM and REM sleep via an ultradian rhythm, with most of sleep spent in NREM. Determinants involved in the regulation of sleep are the homeostatic and circadian processes. Despite being highly regulated, sleep is fragile, and its stages and duration may be affected by multiple factors, such as age, drugs, temperature, and medical and psychiatric disease. Variations in nighttime sleep impact subsequent sleep periods as well as daytime function.