Table 1.

Characteristic waves¹.

Signal	Wave	Characteristics	Related stage
EEG	Alpha rhythm wave	An 8–13-Hz-frequency wave that mainly appears in occipital EEG results	W
	Arousal	An event where the frequency of EEG signals suddenly change. After this event, sleep shifts to a lighter stage, i.e., W or N1. Additionally, EMG activity related to body movement often appears at this time	W & N1
	Low-amplitude mixed-frequency activity	A low-amplitude, 4–7-Hz-frequency wave	N1 & REM
	Vertex wave	A sharp wave with a duration of less than 0.5 s. This wave can be clearly distinguished from background EEG signals	N1
	K-complex	A clear negative-sharp wave that is immediately followed by a positive wave. This wave can be clearly distinguished from background EEG signals. Its duration exceeds 0.5 s	N2
	Sleep spindle	A clear wave with a frequency of 11–16 Hz, and duration of more than 0.5 s. The maximum amplitude is located at the center of the wave. This wave can be clearly distinguished from background EEG signals	N2
	Delta rhythm wave	A wave with a frequency of 0.2–5 Hz and peak amplitude that exceeds 75 $\mu \mathrm{V}$. It often appears in frontal EEG	N3
	Saw-tooth wave	A saw-tooth wave with a frequency of 2–6 Hz	REM
EOG	Blink	Vertical eye movement with a frequency of 0.5–2 Hz	W
	Slow-eye movement	Regular and sinusoidal eye movement	W & N1
	Rapid-eye movement	Random and rapid eye movement	W & REM
EMG	Weakness	The tension becomes decreases to less than that in other sleep stages	REM
	Temporal contraction	Irregular bursts of myoelectrical activity	REM

Note that the AASM rule defines arousal as a sleep event, not a characteristic wave.

However, we have decided to treat arousal as a type of characteristic wave in this study.