What are the different stages of sleep?

Sleep is divided into non-REM sleep and REM sleep. Non-REM sleep contains three stages. Stage 1 is a brief transition from wakefulness into light sleep. Stage 2 is deeper than Stage 1 and makes up a large portion of the night. Heart rate and body temperature decrease, and awareness of the surroundings fades. Stage 3 is deep, slow-wave sleep, during which the body performs important restorative processes. REM sleep is associated with vivid dreaming, memory processing and increased brain activity, while most muscles become temporarily inactive. These stages repeat in cycles throughout the night. Early cycles contain more deep sleep, while later cycles usually include longer REM periods. All stages contribute to healthy functioning.