Sleep stages

Human sleep occurs in several cycles of NREM-REM sleep across the night, each approximately 90 to 110 minutes long, which can be further classified into different sleep stages [11]. Sleep stages are identified based on three electrophysiological parameters: EEG, EOG, and EMG activity, the latter usually recorded from neck or chin muscles. For clinical and research purposes, sleep is usually scored in 30 second epochs. The criteria of sleep stage scoring, established by Rechtschaffen and Kales (R&K) [12], have been

Foundations of Psychiatric Sleep Medicine, ed. John W. Winkelman and David T. Plante. Published by Cambridge University Press. # Cambridge University Press 2010.

Table 3.1 Stages of sleep: electrophysiologicalcriteria

EEG

EOG

EMG

Wakefulness Stage W

Low-amplitude, mixed frequency; alpha rhythm (8 12 Hz) with eye closure, attenuating with eye opening

Eye movements and eye blinks (0.5 2 Hz)

High tonic activity in skeletal muscles

NREM sleep Stage N1

Low-amplitude, mixed frequency; theta rhythm (4 7 Hz), with Vertex sharp waves (V waves, <0.5 seconds)

Slow eye movements (SEM)

Slight decrease in tonic muscle activity

NREM sleep Stage N2

Low-voltage background activity with sleep spindles (11 16 Hz bursts) and K-complexes (biphasic waves lasting >0.5 seconds)

No eye movements

Further decrease in tonic muscle activity

NREM sleep Stage N3

High-amplitude (>75 |jV), slow (<2 Hz) waves lasting >20% of the epoch

No eye movements

Low tonic activity

REM sleep

Low-voltage, saw-tooth waves (2 6 Hz), predominant theta activity

Rapid eye movements (REM)

Muscle atonia with phasic twitches

recently modified by the American Academy of Sleep Medicine (AASM) [13]. In this chapter, the AASM scoring criteria are used.

During wakefulness, the EEG is characterized by low-amplitude, fast-frequency activity (>8 Hz) while the EMG shows a high-voltage, tonic activity combined with phasic increases related to voluntary movements. Also, eye blinks or eye movements are detected by the EOG when the subject is resting with the eyes open or is engaged in a specific task. When eyes are closed in preparation for sleep, EEG alpha activity (8 to 12 Hz) increases and becomes the fundamental rhythm, especially in the occipital cortex.

NREM sleep, which usually initiates each sleep cycle, can be further divided into three stages: N1, N2, and N3 (Table 3.1 and Figure 3.1). During stage N1 (stage 1 according to R&K) the alpha rhythm wanes, while the EEG is characterized by activity in the theta range (4-7 Hz) combined with sporadic high-voltage sharp waves over the central region. Slow, rolling eye movements and a generalized reduction in muscle tone are also present. Subjectively, stage N1 is accompanied by a decreased awareness of environmental stimuli and dream-like mentation. Some individuals report having hypnagogic hallucinations at sleep onset, which can range from vague and brief to vivid perceptual experiences. Sudden muscle twitches can also occur at the transition into sleep. These hypnic jerks, also known as positive myoclo-nus, are sporadic, generally benign, and are more common after sleep deprivation. Brief episodes of Stage N1, usually lasting a few seconds, tend to occur during the daytime in sleepy subjects, either due to sleep disorders or sleep deprivation, and can have dire consequences in situations that require a constant high level of awareness, such as driving.

Stage N2 (stage 2 sleep according to R&K) usually follows stage N1, and is characterized by two EEG elements: (1) sleep spindles (waxing and waning oscillations at 11-16 Hz, lasting 0.5-2 seconds); and (2) K-complexes (high-voltage, biphasic waves 0.5-1.0 seconds long with an initial, sharp, upward component followed by a slower downward peak). The transition from stage N2 to N3 is heralded by the occurrence, in the EEG, of slow waves: low-frequency (0.5-2 Hz) oscillations with a peak-to-peak amplitude >75 ^V. Indeed, when sleep epochs display at least 20% slow waves they are scored as stage N3. N3 is also described as slow-wave sleep (SWS), or deep sleep, because of the increased arousal threshold from N1 to N3. SWS was previously subdivided according to R&K criteria, based on the amount of slow waves in the sleep epoch, into NREM stage 3 (20-50% slow waves) and stage 4 (>50% slow waves). However, the biological relevance of this distinction has never been

Human sleep stages Figure 3.1 Electroencephalogram

(EEG) patterns for wakefulness

Awake (Eyes Open)

Awake (Eyes Closed)

Alpha rhythm

NREM Stage N1 Vertex waves

NREM Stage N2

K-complex Sleep spindles

NREM Stage N3 (Slow-wave sleep)

REM Sleep Saw-tooth waves and stages of sleep.

established; thus, the AASM has recently combined these sleep stages into N3. Eye movements are usually absent during both N2 and N3, and EMG activity progressively decreases from N1 to N3.

REM sleep, or stage R, usually follows a NREM sleep episode and consists of tonic (persistent) and phasic (episodic) components. REM tonic activity is characterized by low-voltage, mixed frequency EEG patterns with a predominance of theta rhythm and a marked decrease of skeletal muscle tone (atonia), except for the extraocular muscles and the diaphragm. One of these tonic EEG patterns, called saw-tooth waves because of their notched morphology, is rather common particularly before the onset of eye movements. The phasic components of REM sleep consist of irregular bursts of rapid eye movements, which occur more often and for longer periods of time in later REM episodes, along with (mostly distal) muscle twitches.

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