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   Currently the first state of sleep is NREM sleep, and it consists of 3 stages ( onset of sleep, light sleep, and deep sleep). During these stages, the muscles of the eyes are relaxed. The second state, REM sleep, is associated with increased contraction of the eye muscles. Both types are necessary for quality sleep.

   NREM sleep was divided into four stages in the Rechtschaffen and Kales (R&K) standardization of 1968. That has been reduced to three in the 2007 update by The American Academy of Sleep Medicine.

   Stage 1 occurs mostly in the beginning of sleep, with slow eye movement. This state is sometimes referred to as relaxed wakefulness. Alpha waves disappear and the theta wave appears. People aroused from this stage often believe that they have been fully awake. During the transition into stage-1 sleep, it is common to experience hypnic jerks.]

   Stage 2 no eye movement occurs, and dreaming is very rare. The sleeper is quite easily awakened. EEG recordings tend to show characteristic "sleep spindles", which are short bursts of high frequency brain activity, and "K-complexes" during this stage.

   Stage 3 previously divided into stages 3 and 4, is deep sleep, slow-wave sleep (SWS). Stage 3 was formerly the transition between stage 2 and stage 4 where delta waves, associated with "deep" sleep, began to occur, while delta waves dominated in stage 4. In 2007, these were combined into just stage 3 for all of deep sleep. Dreaming is more common in this stage than in other stages of NREM sleep though not as common as in REM sleep. The content of SWS dreams tends to be disconnected, less vivid, and less memorable than those that occur during REM sleep. This is also the stage during which parasomnias most commonly occur.

   Note: Parasomnias are a category of sleep disorders that involve abnormal movements, behaviors, emotions, perceptions, and dreams that occur while falling asleep, sleeping, between sleep stages, or during arousal from sleep.