Final answer:
Sleep is a state of reduced activity and awareness, regulated by circadian and homeostatic mechanisms. It can be characterized by different brain wave patterns in NREM and REM sleep.
Step-by-step explanation:
Sleep is a state of relatively low levels of physical activity and reduced sensory awareness. It is regulated by the interplay of the circadian and homeostatic mechanisms. Homeostatic regulation of sleep is evidenced by sleep rebound following sleep deprivation, where a sleep-deprived individual falls asleep more quickly during subsequent opportunities for sleep.
Sleep can be characterized using electroencephalography (EEG), which shows different patterns of brain activity in different stages of sleep. Non-rapid eye movement (NREM) sleep has three stages: Stage 1, which is a transitional phase between wakefulness and sleep; Stage 2, where sleep spindles and K-complexes emerge; and Stage 3/4, which is slow-wave sleep characterized by delta waves. Rapid eye movement (REM) sleep is marked by rapid eye movements, paralysis of voluntary muscles, and dreaming.