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There are 2 types of sleep - NREM (Non Rapid Eye Movement) and REM (Rapid Eye Movement). When we dream at night, we experience REM sleep. The NREM phase of sleep consists of 4 stages - Stage 1, 2, 3 and 4.

The process of sleep is measured using an electroencephalogram (EEG) which indicates the gross brain wave activity, a summary of  electrical activity from one area of  the brain. Brain wave patterns are of different types - alpha, beta, theta and delta waves. Beta waves are have a high frequency, are asynchronous  and of small amplitude. They are emitted by a person when awake, since a person has to do a lot of mental activity simultaneously. When a person relaxes, the brain emits alpha waves, which are larger in amplitude, of  low frequency and synchronous. These waves are also emitted during meditation. Activities which increase the emission of alpha waves are beneficial to the health of a person.

Stage 1 of NREM  sleep is drowsiness , which lasts for 5 to 10 minutes. Muscle activity slows down and eye movements reduce. The brain emits theta waves which are similar to alpha waves but larger in amplitude. In stage 2 , the eye movements stop, the heart rate drops and body temperature decreases.  Stage 3 and 4 are the deep sleep stages, when it is difficult to wake up a person. EEG records show that the brain emits delta waves of maximum amplitude during these stage of  sleep.

 After stage 4, the body goes back to stage 3 and stage 2 of  sleep. The NREM type of sleep lasts for 90 to 120 minutes, after which the body moves to REM phase of sleep. In this phase, there is a lot of  eye movement and brain activity,  but most of  the body remains immobile. . The REM sleep  is then again followed by Stage 2 , 3 , 4, 3 and 2 of  NREM sleep.

 After the first NREM sleep cycle, the REM phase lasts for 10 minutes, but the length of  the REM phase increases as the duration of sleep increases. The last phase of REM sleep, which occurs just before a person wakes up in the morning lasts for an hour.



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