Basically, the sleep that we get every night can be divided into two categories: non-REM sleep, and REM sleep

REM stands for Rapid Eye Movement sleep, named after the characteristic back and forth eye movements that occur during this phase

A complete sleep cycle takes an average of 90 to 110 minutes, with each stage lasting between 5 to 15 minutes. The first sleep cycles each night have relatively short REM sleeps and long periods of deep sleep but later in the night, REM periods lengthen and deep sleep time decreases.

Non-REM stage

Level 1: -

This non-REM standard lasts for a few minutes before we wake up from sleep. At this stage.
The pace of the heartbeat, the movement of the iris, etc.
Body muscles are immobilized over long periods of time.
Brain activity slows down and goes back to full sleep.

Level 2: -

Approximately 50% of sleep is in this condition.
Slowing down the rest of the body parts and slowing down.
The iris motility is completely stopped.
Brain function is completely dysfunctional, although the minor movements continue to recover.

Level  3 and 4; -

Achieve a deep sleep state.The heart rate and the speed of breathing slows down as the body moves.
The brain is completely exhausted. It is difficult to dissolve sleep even when loud noises arise.
This deep sleep is called slow wave sleep.

REM stage

Level 5: -

Irregular oscillation of the biceps.
Dreams can sense the movement of the brain.
During sleep, heart rate increases significantly.
The speed of breathing is higher than normal.
The arm and leg joints are oriented in the same direction.

REM sleep appears to be highly involved in processing learned information - mapping it on to experiences that you have had previously and assessing the emotional content that you have encountered in your day. This is why dreams tend to be emotionally charged.

REM sleep seems to be crucial in building and reinforcing neural connections during learning - newborn babies spend 50% of their sleep in REM, compared to only 20% in an adult. And a fetus spends 80% or more of sleep in REM.