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FIGURE 22–1. Polysomnography in different sleep stages.The six panels show activity in the electroencephalogram (EEG), electrooculogram (EOG), and electromyogram (EMG) during wakefulness and the different sleep stages. Each tracing shows 30 seconds. A, Wakefulness is characterized by low-voltage, fast-frequency EEG, voluntary eye movements, and tonic muscle tone. The EEG segments marked "A" show alpha rhythm, which is characteristic of relaxed wakefulness with eyes closed. The arrows marked "REMs" show rapid eye movements. The channels are labeled on the left side: C3, C4, O1, and O2 are EEG leads, which are measured relative to a common, or referential (REF), lead. LOC and ROC are leads at the left and right outer canthus of the eye. B, Stage 1 non–REM (NREM) sleep is characterized by a slight increase in EEG amplitude and slowing of EEG frequencies (A), slow rolling eye movements (indicated by reciprocal "hills and valleys" in the EOG channels, [B]) and lower muscle tone (C). C, Stage 2 NREM sleep is characterized by further slowing of the EEG, together with an increase in amplitude. Phasic events include "K complexes," isolated large-amplitude slow EEG waves (K) and "sleep spindles," episodic bursts of fast EEG activity lasting approximately 0.5 seconds (S). EOG shows underlying brain electrical activity, and therefore resembles the EEG. Muscle tone is further reduced. D, Stage 3 sleep is characterized by large-amplitude, slow (0.5–4.0 Hz) EEG activity, also known as "delta" or "slow-wave sleep" activity, that constitutes 20%–50% of the 30-second epoch. EOG mirrors EEG activity, and muscle tone is low. E, Stage 4 sleep is identical to Stage 3, except that delta activity occupies greater than 50% of the epoch. F, REM sleep is characterized by the return of faster-frequency, mixed-voltage EEG, similar to Stage 1 NREM sleep. The hallmark of REM is the appearance of phasic rapid eye movements, which present as large-amplitude "spiky" waveforms that clearly differ from the slower eye movements of Stage 1 NREM. Muscle tone is essentially absent, except for the intermittent occurrence of phasic muscle twitches, which often accompany eye movements.Source. Reprinted from Buysse DJ (ed): Sleep Disorders and Psychiatry (Review of Psychiatry Series, Volume 24, Number 2; Oldham JM and Riba MB, series editors). Washington, DC, American Psychiatric Publishing, 2005, pp. 7–9. Copyright 2005, American Psychiatric Publishing. Used with permission.

FIGURE 22–2. Hypnograms of sleep stages in healthy subjects.Each 20- to 30-second epoch of sleep for an entire night is assigned a sleep stage by a human "scorer." These epoch scores can then be displayed graphically in a "hypnogram," to display the progression of sleep stages across the night. A, Hypnogram for an entire night of sleep in a healthy young adult. Sleep stages are indicated by increasing "depth" on the vertical axis, with REM sleep represented by heavy horizontal lines. Time is indicated on the horizontal axis. Note that most Stage 3–4 NREM sleep occurs in the early part of the night, and REM periods get longer toward the end of the night. B, Hypnogram for an older adult. Note the absence of Stage 3–4 NREM sleep and the greater amount of wakefulness during the sleep period.Source. Reprinted from Buysse DJ (ed): Sleep Disorders and Psychiatry (Review of Psychiatry Series, Volume 24, Number 2; Oldham JM and Riba MB, series editors). Washington, DC, American Psychiatric Publishing, 2005, p. 10. Copyright 2005, American Psychiatric Publishing. Used with permission.

FIGURE 22–3. The two-process model.Alertness level is determined by the interaction between two processes. The sleep homeostatic drive (Process S) promotes sleep and builds up during wake, reaching a maximum in the late evening (near the usual sleep time). The circadian rhythm system (Process C) promotes wakefulness during the day. It is biphasic and tends to dip in the midafternoon. Process C also reaches its peak in the evening to counterbalance the accumulation of homeostatic drive that has built up throughout the day and it begins to fall just before the usual bedtime. This system promotes wakefulness during the day and consolidates sleep at night.Source. Reprinted from Buysse DJ (ed): Sleep Disorders and Psychiatry (Review of Psychiatry Series, Vol 24, No 2; Oldham JM and Riba MB, series editors). Washington, DC, American Psychiatric Publishing, 2005, p. 191. Copyright 2005, American Psychiatric Publishing. Used with permission.
Table Reference Number
TABLE 22–1. Physiological characteristics of sleep–wake states

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