- Sleep is essential, with people typically spending one-third of their lives asleep. The circadian rhythm regulates the sleep-wake cycle over approximately 24 hours, controlled by the suprachiasmatic nucleus in the brain. - There are two main types of sleep - non-REM and REM sleep. Non-REM sleep has four stages that progress from light to deep sleep. REM sleep is when most vivid dreaming occurs and the body is paralyzed.