The following are the five different stages of sleep. A healthy sleeper will cycle through the stages several times a night.
Stage 1. You sleep lightly and can wake up easily. Sudden jerky movements of your legs and muscles may occur as your eye movement and body movement slows down. Stage 1 sleep only lasts a short time, and people who wake up during it may think they haven’t been asleep at all.
Stage 2. Eye movement stops. Brain waves begin to slow down, although occasionally there are bursts of rapid brain waves. An adult is in Stage 2 about one half of the time that he or she is asleep.
Stage 3. Stage 3 begins the period of deep sleep. Slow brain waves are interspersed with smaller, faster waves. It is hard to wake up during Stage 3 sleep, and if awoken you may feel groggy and disoriented.
Stage 4. Deep sleep continues in Stage 4, and slow brain waves predominate. In this stage it is very hard for someone to wake you up. Stages 3 and 4 are important for feeling refreshed in the morning.
Stage 5. The last stage is Rapid Eye Movement, or REM, sleep and is characterized by darting eye movements. During REM sleep, your breathing becomes fast, shallow, and irregular. In addition, your blood pressure and heart rate rise, and your muscles become immobile to prevent you from acting out on the vivid dreams that occur during REM sleep. Over the course of a night’s sleep, the amount of time spent in REM sleep increases with each sleep cycle.