Sleep during and following critical illness: A narrative review

Table 1 Simplified polysomnographic features of the American Academy of Sleep Medicine’s phases of sleep

Sleep stage	Electroencephalogram	Electrooculogram	Chin electromyogram
Wake	Alpha activity (sinusoidal 8-13 Hz)	Rapid eye movements; Reading eye movements; Slow eye movements; Blinks	Normal or high tone
N1	< 50% alpha activity; > 50% low amplitude mixed frequency activity (4-7 Hz)	Slow eye movements	Variable, usually lower than wake
N2	Sleep spindles; K-complexes	None	Variable tone
N3	Slow (delta) wave (0.5-2 Hz) ≥ 20%; Sleep spindles may occur	None	Variable tone
REM	Low amplitude mixed frequency activity; No sleep spindles or K-complexes	Rapid eye movements	Low tone

REM: Rapid eye movement.