睡

A phono-semantic compound:

目 (eye) — semantic component, indicates relation to eyes, seeing, eye condition;

垂 (to hang down, to droop) — phonetic component, gives the sound "su" and reinforces the image "eyes drooping downward → sleep."

Thus the meaning is visually intuitive.

수면 (睡眠) — sleep

숙수 (熟睡) — deep sleep

최면 (催眠) — hypnosis

수마 (睡魔) — sleepiness; “sleep demon”

睡 emphasizes the state of sleeping.

Related characters:

眠 — to sleep (focus on closing eyes)

寐 — to sleep (classical form)

夢 — dream

醒 — to wake