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The sleep stages 1 through 3,
previously known as stages 1 through 4, are collectively referred
to as NREM, non-rapid eye movement, sleep. Rapid eye movement (REM) is
not included. There are distinct electroencephalographic and
other 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 muscles are not paralyzed as in REM sleep. In addition, there
is a parasympathetic dominance during NREM.
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 (AASM).
- Stage 1 - occurs in the beginning of sleep,
with slow eye movement. 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" 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. These were in 2007 combined into just stage 3 for all of
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.
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 (prior to the merging of stages 3 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.
sleep (SWS) is made up of the deepest stage of NREM, and is
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 Stage 3.
A person will typically feel groggy when awoken from this stage,
and indeed, cognitive tests administered after awakening from stage
3 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
- Rechtschaffen, A; Kales, A (1968).
A Manual of Standardized Terminology, Techniques and Scoring
System For Sleep Stages of Human Subjects. US Dept of Health,
Education, and Welfare; National Institutes of Health.
- M. Massimini, G. Tononi, et al., “Breakdown of Cortical
Effective Connectivity During Sleep,” Science, vol. 309,
2005, pp. 2228–32.
- P. Cicogna, V. Natale, M. Occhionero, and M. Bosinelli, “Slow
Wave and REM Sleep Mentation,” Sleep Research Online, vol.
3, no. 2, 2000, pp. 67–72.
- D. Foulkes et al., “Ego Functions and Dreaming During Sleep
Onset,” in Charles Tart, ed., Altered States of
Consciousness, p. 75.
- Rock, Andrea (2004). The Mind
- Warren, Jeff (2007). "The Slow
Wave". The Head Trip: Adventures on the Wheel of
- Iber, C; Ancoli-Israel, S; Chesson, A; Quan, SF. for the
American Academy of Sleep Medicine. The AASM Manual for the
Scoring of Sleep and Associated Events: Rules, Terminology and
Technical Specifications. Westchester: American Academy of
Sleep Medicine; 2007.