The brain wave (amplitudes and frequencies) present at a particular time decide the stage of sleep.
Most important sleep stage (5th stage) occurs with clear dreams, called as the Rapid Eye Moment (REM) stage. Early researchers called it "paradoxical sleep.” REM stage has increased heart rate, raised blood pressure, erection in males, and the body does not regulate its temperature. Most of the vivid dreams occur during REM sleep, once awaken at this stage a person can remember the dreams. There are two basic forms of sleep: rapid eye movement (REM) sleep and non-rapid eye movement (NREM) sleep.

Adults spend nearly half of their sleep time in stage 2, about 20% in REM and other 30% has shared between the other three stages. Start first from stage1 sleep for few minutes; continue to stage2 sleep, stage3 sleep, stage4 sleep. Elderly people have fewer than 15% of their sleep time in REM sleep, balance NREM sleep.Sleep CycleOne sleep cycle takes about 90 to 100 minutes, thus for an average sleep time of eight hours, there will be a four to five complete sleep cycles. The sleep cycle begins with four-stages of sleep called by Slow Wave Sleep (SWS) or non-rapid eye movement (NREM or Non-REM) sleep. After completing the first fourth stages, instead of continue to fifth REM sleep the four stages quickly reversed and then followed by REM sleep.

The REM sleep will occur roughly after 90 minutes of falling asleep, and it will last only for about 10 minutes, giving the starting sleep cycle being around 100 minutes. The duration of sleep stages three and four (also called delta or deep sleep) slowly wanes.
On the other hand, the duration of sleep stage five (REM sleep) increases, up to about one hour in length after certain sleep cycles.

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