Stages of Sleep - Sleep Foundation

There are four sleep stages; one for rapid eye movement (REM) sleep and three that form non-REM (NREM) sleep. These stages are determined based on an analysis of brain activity during sleep, which shows distinct patterns that characterize each stage.

The classification of sleep stages was update in 2007 by the American Academy of Sleep Medicine (AASM). Before that, most experts referred to five sleep stages, but today, the AASM definitions of the four stages represent the consensus understanding of the sleep cycle.

Stage 1 is essentially the “dozing off” stage, and it normally lasts just one to five minutes.

During N1 sleep, the body hasn’t fully relaxed, though the body and brain activities start to slow with periods of brief movements (twitches). There are light changes in brain activity associated with falling asleep in this stage.

It’s easy to wake someone up during this sleep stage, but if a person isn’t disturbed, they can move quickly into stage 2. 

During stage 2, the body enters a more subdued state including a drop in temperature, relaxed muscles, and slowed breathing and heart rate. At the same time, brain waves show a new pattern and eye movement stops. On the whole, brain activity slows, but there are short bursts of activity that actually help resist being woken up by external stimuli.

Stage 3 sleep is also known as deep sleep, and it is harder to wake someone up if they are in this phase. Muscle tone, pulse, and breathing rate decrease in N3 sleep as the body relaxes even further.

The brain activity during this period has an identifiable pattern of what are known as delta waves. For this reason, stage 3 may also be called delta sleep or short-wave sleep (SWS).

During REM sleep, brain activity picks up, nearing levels seen when you’re awake. At the same time, the body experiences atonia, which is a temporary paralysis of the muscles, with two exceptions: the eyes and the muscles that control breathing. Even though the eyes are closed, they can be seen moving quickly, which is how this stage gets its name.