Sleep cycles
The sleep cycle can be divided into 5 stages. Each night, the sleep cycle is completed 4 to 5 times. The distinction between the phases in each cycle is based on brain activity and eye movement, called 'Eye Movement'. In the first 4 phases, eye movement is slow, called Non Rapid Eye Movement (NREM) phases. In the 5th and final phase, there is rapid eye movement, hence this phase is called Rapid Eye Movement (REM) sleep.

Phase 1 (NREM1)
This is the transition phase between wakefulness and sleep. The eyes become heavy and eye movement slows down. In addition, brain activity slowly decreases so that a person eventually falls asleep. Most people with normal sleep stay in this phase for about five minutes.

Phase 2 (NREM2)
This phase involves light sleep and is the beginning of real sleep. The body relaxes even more. The sleeper can still be woken, but less easily as in phase 1. Phase 2 lasts about 45 minutes.

Phase 3 (NREM3)
This is the transition phase from light sleep to deep sleep. Breathing and heart rate become regular and muscles are completely relaxed. This phase lasts 3 to 8 minutes.

Phase 4 (NREM4)
In the 4th stage of NREM, true deep sleep occurs. The body has reduced breathing and heart rate. Physical recovery occurs during this phase that lasts about 15 to 18 minutes.

Stage 5 (REM sleep)
In REM sleep, dreaming takes place and the information absorbed during the day is processed. There are rapid eye movements and high brain activity. Physically, a lot happens. As a result, this phase consumes energy. REM sleep lasts about 18 to 24 minutes. After REM sleep, the sleeper wakes up consciously or unconsciously and the sleep cycle begins again.