How is normal sleep characterized?

Updated: Nov 05, 2019
  • Author: Pradeep C Bollu, MD; Chief Editor: Selim R Benbadis, MD  more...
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Normal sleep is divided into non–rapid eye movement (NREM) and rapid eye movement (REM) sleep. NREM sleep is further divided into progressively deeper stages of sleep: stage N1 (NREM 1), stage N2 (NREM 2), and stage N3 (NREM 3). [1]  As NREM stages progress, stronger stimuli are required to result in an awakening. Stage R sleep (REM sleep) is characterized by decreased EEG amplitude, muscle atonia, autonomic variability, and episodic rapid eye movement. REM sleep has tonic and phasic components. The phasic component is a sympathetically driven state with rapid eye movements, distal muscle twitches, cardiorespiratory variability, and middle ear muscle activity. Tonic REM is a parasympathetically driven state with no eye movements, decreased EEG amplitude, and atonia. [2]  The REM period length and density of eye movements increase throughout the sleep cycle.

Waking usually transitions into light NREM sleep. NREM sleep typically begins in the lighter stages (N1 and N2) and progressively deepens to slow-wave sleep as evidenced by higher-voltage delta waves. When delta waves account for more than 20% of the sleep EEG, the sleep stage is considered to be stage N3. [1]  REM sleep follows NREM sleep and occurs 4–5 times during a normal 8-hour sleep period. The REM period becomes progressively longer through the night where the first REM period of the night may be less than 10 minutes in duration, while the last may exceed 60 minutes. The NREM–REM cycles vary in length from 70 to 100 minutes initially to 90 to120 minutes later in the night. [2]

Typically, N3 sleep is present more in the first third of the night, whereas REM sleep predominates in the last third of the night. NREM parasomnias such as sleep walking typically occur in the first third of the night with the presence of N3 sleep. This contrasts with REM sleep behavior disorder (RBD), which typically occurs in the last half of the night.

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