Your body cycles through four stages of sleep. The pattern typically repeats every 90 minutes. This means the stages will repeat about 4 to 6 times during a 7- to 9- hour sleep period.
The pattern includes three phases of non-rapid eye movement (NREM) sleep and one phase of REM sleep.
The NREM sleep stages used to be divided into stages 1, 2, 3, and 4, followed by REM sleep. The National Sleep Foundation now classifies them as follows:
N1 non-REM sleep (formerly stage 1)
Stage 1 occurs when you first fall asleep. As your body enters light sleep, your brain waves, heart rate, and eye movements slow down.
This phase lasts for about 7 minutes.
N2 non-REM sleep (formerly stage 2)
This stage involves the light sleep just before deep sleep.
Your body temperature decreases, your eye movements stop, and your heart rate and muscles continue to relax. Your brain waves briefly spike then slow down.
During a night of sleep, you spend the most time in stage 2.
N3 non-REM sleep (formerly stages 3 and 4)
In stages 3 and 4, deep sleep begins. Your eyes and muscles don’t move, and your brain waves slow down even further.
Deep sleep is restorative. Your body replenishes its energy and repairs cells, tissues, and muscles. You need this phase to feel awake and refreshed the next day.
This stage first happens about 90 minutes after you fall asleep. It can last for about an hour.
In REM sleep, your brain waves and eye movements increase. Your heart rate and breathing also speed up.
Dreaming often happens during REM sleep. Your brain also processes information during this stage, making it important for learning and memory.