When people first go to sleep, they are in NREM sleep. The first stage of NREM sleep is N1 sleep. During N1 sleep, people get very drowsy. Some people have muscle twitches during this part of sleep. People are not very conscious of, or aware of, their surroundings during this stage of sleep. Brain monitors identify small, slow, and irregular brain waves during N1 sleep. The second stage of sleep is N2 sleep. People are not at all conscious of their surroundings during N2 sleep. About 45%-55% of total adult sleep is N2 sleep. Brain monitors identify large brain waves with quick bursts of activity during N2 sleep. The third stage of sleep is N3 sleep. It is very deep sleep. Brain monitors identify very slow brain waves during N3 sleep. Therefore, N3 sleep
is called slow–wave sleep (SWS.)