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The sleep stages 1 through 4 are collectively referred to as NREM (non-rapid eye movement) sleep. Rapid eye movement (REM), or stage 5, is not included. There are distinct electroencephalographic characteristics seen in each stage. Unlike REM sleep, there is usually little or no eye movement during this stage. Dreaming is rare during NREM sleep, and the muscles are not paralyzed as in REM sleep. In addition, there is a parasympathetic dominance during NREM.
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Stages of NREM sleep
NREM sleep is divided into four stages:
- Stage 1 - occurs in the beginning of sleep, with slow eye movements. 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 - is an unconscious stateverification needed, though the sleeper is easily awakened. No eye movements occur, and dreaming is very rare during this stage. EEG recordings tend to show characteristic "sleep spindles" and "K-complexes" during this stage.
- Stage 3 - is the transition between stage 2 and stage 4 where delta waves, associated with "deep" sleep, begin to occur.
- Stage 4 - is slow-wave sleep (SWS), the "deepest" stage of sleep in which there is a continuation of the delta wave. 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 in which parasomnias most commonly occur.
Polysomnography
Polysomnography (PSG) is a test used in the study of sleep; the test result is called a polysomnogram. Below are images of the NREM stages 1, 2 and 4.
The figures represent 30 second epochs (30 seconds of data). They represent data from both eyes, chin, EEG, legs, microphone, intercostal EMG, sternocleidomastoid activity, nasal/oral air flow, thoracic effort, abdominal effort, EKG, oxymetry, and body position, in that order. EEG is highlighted by the red box. Sleep spindles in the stage 2 figure are underlined in red.
Stage 1: 
Stage 2: 
Stage 4: 
Slow-wave sleep
Slow-wave sleep (SWS) is made up of the two deepest stages of NREM, and these two stages are often referred to as deep sleep.
The highest arousal thresholds (e.g. difficulty of awakening, such as by a sound of a particular volume) are observed in Stages 4 and 3, respectively. A person will typically feel more groggy when awoken from these stages, and indeed, cognitive tests administered after awakening from stages 3-4 indicate that mental performance is somewhat impaired for periods up to 30 minutes or so, relative to awakenings from other stages. This phenomenon has been called "sleep inertia."
After sleep deprivation there is usually a sharp rebound of SWS, suggesting there is a "need" for this stage. The major factor determining how much slow-wave sleep is observed in a given sleep period is the duration of preceding wakefulness.
See also
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