There are five stages of sleep. Stages 1-4 are non-REM sleep, followed by REM sleep. During sleep, our brain cells work more slowly but more intensively, and this shows up on an EEG as an electrical activity that is lower in frequency but higher in voltage.
When we sleep, there are also physical changes in the body such as changes in eye movement and muscular tension. Further changes in electrical activity in the brain show when each stage of sleep begins and ends.
Sleep Stage 1
In this stage, our breathing and heartbeat become regular, our muscles relax and our body temperature falls. We become less aware of external stimuli and our perception of reality starts to withdraw.
The slightest noise is enough to wake you from this stage. You must have experienced the sensation of falling suddenly, typical of this stage. We spend about 10% of the night either awake or in stage 1, which generally lasts between 13 and 17 minutes.
Some people twitch during this stage, which, in effect, is the stage during which we fall asleep. As such, it occurs only once during a night of uninterrupted sleep.
Sleep Stage 2
Sleep becomes deeper during stage 2, and our muscles relax further. Physical sensations are significantly reduced and our eyes do not move. About half of our total sleeping time is spent in stage 2, and at this stage, electrical activity in the brain occurs at a lower frequency than when we are awake.
Both stages 1 and 2 are known as the light-sleep phase, and they last for about 20–30 minutes together. We usually fall back to stage 2 several times during the night.
Sleep Stages 3
At stage 3, we reach the first of our deep-sleep stages. This occurs after approximately 20 to 30 minutes when our body is now completely relaxed, and we are more or less completely disconnected from reality. To wake someone from this deep stage of sleep, you will need to make quite a lot of noise or shake them quite hard.
Sleep Stage 4
Waking someone from stage 4 is a bit like trying to wake a hibernating animal. This is the most restful part of the night’s sleep, where muscular activity decreases even further and our eyes do not move.
Stages 3 and 4 constitute about 20 percent of our time asleep and are known as the deep-sleep phase.
It is during these deep stages of NREM sleep, that the body repairs and re-grows tissues, builds bone and muscle, and strengthens the immune system.
As you get older, however, you sleep more lightly and get less of these deep sleep stages. Aging is also linked to shorter time spans of sleep, but, you would still need as much sleep as when you were younger.
During REM (rapid eye movement) sleep, brain waves mimic activity during the waking state, and the eyes remain closed but move rapidly from side-to-side. This is perhaps related to the intense dream and brain activity that occurs during this stage.
What Is a Sleep Cycle?
A sleep cycle refers to the progressive period of time it takes for an individual to pass through the stages of sleep outlined above. You do go straight from deep sleep to REM sleep, rather, a sleep cycle progresses through the stages of non-REM sleep from light to deep sleep, then reverse back from deep sleep to light sleep, ending with some time in REM sleep before starting over in light sleep again.