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 ...

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 delt...

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.

Sleep is not uniform. Instead, over the course of the night, your total sleep is made up of several rounds of the sleep cycle, which is composed of four individual stages. In a typical night, a person goes through four to six sleep cycyles. Not all sleep cycles are the same length, but on average...

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 m...

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 actu...