Sleeping is not as easy as you think, there are five stages we have to run through during a sleep, 1,2,3,4 and REM (Rapid Eye Movements) sleep. Usually the brain will go through stage 1 before REM sleep, with the end of first REM sleep it starts from stage 1 again. An average sleeping cycle takes about 90 to 110 minutes. The first cycle of your sleep is begin with light sleep, then comes to short length of REM sleep, and the longer deep sleep. With each cycle, REM sleep period becomes longer with deep sleep period shorten.
Stage 1 is light sleep where you drift in and out of sleep and can be awakened easily. In this stage, the eyes move slowly and muscle activity slows. During this stage, many people experience sudden muscle contractions preceded by a sensation of falling.
In stage 2, eye movement stops and brain waves become slower with only an occasional burst of rapid brain waves. When a person enters stage 3, extremely slow brain waves called delta waves are interspersed with smaller, faster waves.
In stage 4, the brain produces delta waves almost exclusively. Stages 3 and 4 are referred to as deep sleep or delta sleep, and it is very difficult to wake someone from them. In deep sleep, there is no eye movement or muscle activity. This is when some children experience bedwetting, sleepwalking or night terrors.
In the REM period, breathing becomes more rapid, irregular and shallow, eyes jerk rapidly and limb muscles are temporarily paralyzed. Brain waves during this stage increase to levels experienced when a person is awake. Also, heart rate increases, blood pressure rises, males develop erections and the body loses some of the ability to regulate its temperature. This is the time when most dreams occur, and, if awoken during REM sleep, a person can remember the dreams. Most people experience three to five intervals of REM sleep each night.