Sleep can be defined as a complex reversible state characterized by behavioral quiescence, diminished responsiveness to external stimuli, and a stereotypical species-specific posture. Both components of sleep, non–rapid eye movement and rapid eye movement, are generated and maintained by central nervous system networks that use specific neurotransmitters located in specific areas of the brain. Widespread changes in physiologic processes occur during sleep, and these changes may influence the presentation and severity of specific medical disorders.