The nightly sleep cycle is divided into five different stages

The sleep can be divided into different stages depending on the depth of sleep and the pattern typically linked to it. A sleep cycle first begins with a light sleep phase (within which the sleep stages 1 and 2 would fall) followed by a deep sleep phase (sleep stages 3 and 4) and then, again, a light sleep phase which flows, finally, into an REM phase for approx. 20 minutes. REM is the abbreviation for Rapid Eye Movements and is so named because in this phase the other muscles have grown completely tired, only the eyes move quickly to and fro. Most of the dreams occur in this phase. Every person dreams in the REM sleep, even if he/she does not always remember the next day.

Such a sleep cycle lasts about 90 minutes and will go through one about four to five times in the course of a night. As the sleep cycle continues the length of the deep sleep phases become shorter, while the REM phases become longer.

The undisturbed process of this pattern is a condition for a restful sleep. With infants the REM portion amounts to approx. 50% diminishing in the course of time down to about 15 - 20% with an adult person.

Sleep, being a very complicated occurrence, can be disturbed by the many different factors.