Normal human sleep.

Human sleep is an integral part of the basic rest-activity cycle that is found in all life forms. Comprised of cycling episodes of non-REM (NREM) and REM sleep, sleep can be measured and quantified by electrophysiologic, physiologic, and behavioral indices. Physiologic homeostasis changes markedly between wakefulness, NREM sleep, and REM sleep; moreover, many interacting factors determine sleep organization and sleep quality in a given individual. For the health practitioner, basic knowledge of sleep physiology and sleep modifiers is essential for the treatment of many clinical problems that are directly or indirectly related to phenomena occurring during sleep.