Stages of Sleep
Sleep basically has two phases, NREM (Non Rapid Eye Movement) sleep and a REM (Rapid Eye Movement) sleep. Sleep occurs in cycles of these two stages, which occurs every 90min-120mins.
The NREM sleep has four stages, stages1, 2, 3&4. NREM and REM sleep are exhibited in a predictable manner. Specifically, REM and NREM alternate approximately every 90mins. In the first part of the night, there is more NREM sleep, especially deep sleep, while in the second half of the night REM sleep predominates. The yardstick to measure the sleep intensity in any sleep phase is determined by the degree of difficulty experienced to awaken the person from that stage of sleep.
It is difficult to wake a person who is in the REM phase of sleep while a person in the stage 3 and 4 (slow wave sleep) of the NREM phase of sleep, is also considered to be in deep slumber.
