Sleep is made up of different cycles, with different forms of sleep, from light sleep to deep sleep.
One cycle has five stages. These stages are distinguished by the levels of brain activity and eye movement. In the first four stages. Eye movement is slow. These stages are therefore also called “Non Rapid Eye Movement, (non-Rem sleep Eye). Only the last stage has” Rapid Eye Movement” and is therefore REM sleep.

Light sleep

Stages 1 and 2 are called light sleep. Stage 1 is a transition phase between waking and sleeping and it last about 7,5 minutes. Your eye movement slows. You have difficulty keeping your eyes open and ultimately fall asleep.
Stages 2 is where real sleep begins. You no longer wake with every sound but if you do wake you don’t yet have the feeling that that you were deeply asleep. This stages last 10 to 25 minutes.

Deep sleep

Stages 3 and 4 are deep sleep. Stages 3 begins about 30 minutes after you fall asleep. This is the transition phase to deep sleep. Your breathing becomes completely regular, your hart rate lowers and your muscles relax completely.

Then after about 45 minutes stages 4 begins. Your breathing and heart rates are at their slowest. If you are awakened from sleep in stages 4, you are disoriented and need time to realize where you are.

REM sleep

The last stage, stage 5, is the REM-sleep stages. Rapid eye movements characterize this part of the sleep cycle. Stage 5 is also called the dream stages. If you are awoken from this stages, you can often give a reasonably coherent account of the dream you were having.

After the REM sleep stage, you generally wake briefly (usually unconsciously) and the sleep cycle begins again from the beginning.

This usually happens about 7 to 10 times each night. The relative amount of time spent in REM sleep changes with each sleep cycle.

In the first cycle it is small, but in the third and fourth cycles it increases. Non-REM sleep takes comparatively more time in the first and second cycles.