Sleep stages are divided into non-REM (NREM) sleep and REM Sleep (R).
There are 3 main stages of sleep before reaching REM Sleep. Your brain is still active during all of the stages of sleep.
Stage 1 (N1)
Stage N1 occurs right after you fall asleep and is usually less than 10 minutes long. In stage 1 of sleep, heartrate and breath slow down while your muscles relax and sometimes even twitch. This is a very light stage of sleep and you can be awakened quite easily.
Stage 2 (N2)
Stage N2 usually lasts from 30 to 60 minutes. In Stage 2 sleep your body temperature drops, while your heartrate and breath slow even more than they slowed in Stage 1. Your muscles become relaxed, and you may begin to have slow wave (Delta) brain activity. Eye movement stops during Stage 2 sleep.
Stage 3 (N3)
Stage N3 represents deep sleep and it lasts between 20 to 40 minutes. During this stage Delta brain activity increases and you may have some body movements It is the deep sleep that is needed to feel refreshed. In this stage your heart rate and breath are at their lowest levels of the night. This is the stage in which it can be hard to wake you from your slumber.
REM (R) Sleep
Rapid Eye Movement (REM) sleep is the deepest stage of sleep. In REM sleep, your eyes move rapidly and breathing is fast and irregular. In this period you can have short episodes when breathing stops (apnea). Your heart rate and blood pressure are close to what they are while you are awake. This is the stage of sleep when we dream and it usually takes 1 to 2 hours to reach REM sleep after falling asleep. This cycle of progressing through the 3 stages of sleep to reach REM sleep is repeated 3 to 4 times each night.