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Stages and architecture of normal sleep

Douglas Kirsch, MD, FAASM
Section Editor
Susan M Harding, MD, FCCP, AGAF
Deputy Editor
April F Eichler, MD, MPH


Sleep is a rapidly reversible state of reduced responsiveness, motor activity, and metabolism [1]. It is a phenomenon observed in all animals in some form; this universality suggests that the act of sleeping likely has some evolutionary relevance. Humans spend approximately one-third of their life, or about eight hours per night, sleeping. The purpose of sleeping is poorly understood, however, and multiple theories exist. These theories include restoration, energy conservation, and memory consolidation.

The polysomnogram is the primary tool for assessing sleep in the laboratory for both clinical and research purposes. During a polysomnogram, electroencephalography (EEG) and other sensors are used to categorize sleep in discrete stages. Initial sleep staging was described in the 1930s, and formal rules for staging sleep were first propagated in 1968 [2]. Since 2007, most sleep laboratories use terminology and scoring rules from the American Academy of Sleep Medicine (AASM) manual for the Scoring of Sleep and Associated Events, which is routinely updated [3]. Sleep laboratories accredited by the AASM are required to use the AASM scoring manual, and these guidelines are being increasingly adopted worldwide [4].

This topic will review the current guidelines for adult sleep staging, the architecture of sleep, common causes of sleep stage abnormalities, and theories around the purposes for sleep. The classification of sleep disorders is reviewed separately. (See "Classification of sleep disorders".)


Sleep can be broadly segmented into rapid eye movement (REM) sleep and non-REM (NREM) sleep. Scoring of sleep stages occurs in 30-second epochs based on current American Academy of Sleep Medicine (AASM) scoring rules [3]. The current rules mandate the use of electroencephalography (EEG), electromyography (EMG) for muscle tone, and electro-oculography (EOG) for eye movements, to determine the stage of sleep. This may change in the future to better align with sleep as a continuous process, by focusing attention on shorter epochs of sleep or through spectral analysis of sleep EEG.

A typical 30-second sleep study data epoch is provided in the figure (figure 1). The EEG data is derived from electrodes placed on the head in frontal, central, and occipital locations and referenced to bone according to the international 10-20 system (figure 2). As part of this system, odd numbers refer to the left side of the head and even numbers are right sided; typically, EEGs require bilateral monitoring, as left and right cerebral hemispheres may not provide identical data. While full EEG monitoring requires placement of all 10-20 electrodes, sleep staging requires only the partial grouping mentioned above (F3 and F4, C3 and C4, O1 and O2). Brainwaves are assessed by amplitude and frequency; different frequencies are associated with different stages of sleep (table 1).

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Literature review current through: Nov 2017. | This topic last updated: Jun 16, 2017.
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