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. Rapid eye movement (REM) is the stage of sleep during which the most vivid (though not all) dreams occur. ... Electroencephalography is the neurophysiologic measurement of the electrical activity of the brain by recording from electrodes placed on the scalp or, in special cases, subdurally or in the cerebral cortex. ... A dream is the experience of envisioned images, sounds, or other sensations during sleep. ...

Stages of NREM sleep

NREM sleep is divided into four stages:

• Stage 1 - occurs in the beginning of sleep, with slow eye movements. People in this stage often believe that they are fully awake. During the transition into Stage 1 sleep, it is common to experience a hypnic jerk.
• Stage 2 - the person is unconscious^{[verification needed]}, though awakened easily. 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 - transition between stage 2 and stage 4. Delta waves begin to occur.
• Stage 4 - slow-wave sleep referred to the "deepest" stage of 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 and not as vivid as those that occur during REM sleep. This is also the most common stage in which parasomnias occur.

A hypnic or hypnagogic jerk is an involuntary muscle twitch (more generally known as myoclonus or a myoclonic twitch) which often occurs during the transition from wakefulness to sleep (see hypnagogia). ... Electroencephalography is the neurophysiologic measurement of the electrical activity of the brain by recording from electrodes placed on the scalp or, in special cases, subdurally or in the cerebral cortex. ... A Sleep spindle is burst of brain activity visible on an EEG that occurs during stage 2 sleep. ... K-complex A K-complex is an EEG waveform that occurs during stage 2 sleep. ... A delta wave is a large, slow brain wave associated with deep sleep. ... Polysomnogram demonstrating SWS. High amplitude EEG is highlighted in red. ... A parasomnia is any sleep disorder such as sleepwalking, sleep sex, teeth grinding, night terrors, rhythmic movement disorder, REM behaviour disorder, restless leg syndrome, and somniloquy (or sleep talking), characterized by partial arousals during sleep or during transitions between wakefulness and sleep. ...

Polysomnography

Below are images of each stage of NREM.

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: Image File history File links Download high resolution version (878x619, 205 KB) Summary EEG highlighted by red box. ...

Stage 2: Image File history File links Download high resolution version (862x618, 215 KB) Summary Stage 2 sleep. ...

Stage 4: Image File history File links Download high resolution version (837x587, 235 KB) Summary This is a screen shot of a patient during Slow Wave Sleep (stage 4). ...