// sleep cycles
Sleep Cycles: About 90 Minutes Each
Sleep is not one even state. Through the night you move through repeating cycles of non-REM sleep and REM (rapid eye movement) sleep. The National Institute of Neurological Disorders and Stroke (NINDS) describes the stages like this:
- Stage 1 (N1): a few minutes of light sleep as you drift off. Breathing and heartbeat begin to slow.
- Stage 2 (N2): light sleep in which body temperature drops and eye movements stop. You spend more of the night here than in any other stage.
- Stage 3 (N3): deep sleep. It is hardest to wake from and is longest in the first half of the night.
- REM sleep: the stage with most dreaming. NINDS notes that it first occurs about 90 minutes after you fall asleep, and REM periods get longer toward morning.
The National Heart, Lung, and Blood Institute (NHLBI) says the cycle starts over every 80 to 100 minutes, and that there are usually four to six cycles per night. So 90 minutes is a useful average, not a fixed length: cycle length differs from person to person and from one cycle to the next.
Many people feel less groggy when the alarm goes off near the end of a cycle, in lighter sleep, than in deep sleep. That is why the calculator works in whole cycles. But the total amount of sleep matters more, so choose an option that also meets the hours for your age.
