What you need to know what happens to your body while sleeping.
Sleep consists of two radically different physiological states. There is rapid eye movement sleep (REM) and non-rapid eye movement sleep (NREM). The sleep stages seem to have different functions, but why we sleep is still not completely understood. Babies spend half of their sleep in REM, but this drops to a quarter by the age of two. It is therefore thought that REM sleep is particularly vital for the developing brain. In NREM sleep, brain activity slows and a person woken at this stage may feel groggy.