https://www.sleepfoundation.org/stages-of-sleep Sleep is the periodic, natural, reversible loss of consciousness ; as distinct from unconsciousness resulting from a coma, general anesthesia, or hibernation. There are 4 stages of sleep and each contains REM and NREM. REM( Rapid Eye Movement) describes sleep in which vivid dreams typically occur; this type of sleep increases as the night progresses.

REM sleep is when internal function is more active, and an individual’s heart rate is faster and more irregular, blood pressure rises, and breathing is quicker and more irregular. Also known as paradoxical sleep, where vivid dreams occur.NREM( Non-Rapid Eye Movement), is a recurring sleep state during which rapid eye movements and dreaming do not occur; this accounts for 75% of sleep. Stage 1 NREM, this is the brief transitional stage that lasts between 5 to 10 minutes. It is shown by a decline in breathing, pulse rates, body temperature and muscle tension.

Stage 2 NREM sleep lasts 10 to 20 minutes and is characterized by brief bursts of higher-frequency brain waves called spindles. Dreams occasionally occur in this stage however are usually very brief and meaningless.

Stage 3 is NREM Sleep which starts the deep sleep period and lasts for 10 minutes.

NREM is characterized by the appearance of slower waves with much higher  amplitudes known as delta waves. Heart rate, body temperature and blood pressure continue to drop during this period and individuals become less responsive.

The last stage is NREM Sleep and it is the deepest stage of sleep, delta waves are very dominant in this stage and it usually occurs an hour after an individual falls asleep. During this stage muscles and organs are relaxed and movement rarely happens.