drowsy, sleep, die
睡 centers on the state of being drowsy or asleep. The meaning 'die' is a euphemistic extension, treating death as a final sleep.
睡 combines 目 (eye) with 垂, which likely contributes the sound and the idea of drooping or hanging down, suggesting eyelids drooping in sleep.
The eye 目 on the left and the drooping shape 垂 on the right picture an eyelid falling shut: drowsiness and sleep.
For スイ, imagine a sleepy person swaying and almost falling: sway -> スイ, as their eyelids droop.
sleep
sleeping pill; sleep medication
coma
deep sleep; sound sleep; profound sleep
sleeping like a log; (being) fast asleep (bombed, crashed)
sleepiness; drowsiness
water lily (any plant of genus Nymphaea)
wink of sleep; snooze; doze; nap
to doze (off)
nap; siesta
African sleeping sickness; African trypanosomiasis
non-REM sleep
microsleep
REM sleep
nap; doze
half asleep
half asleep and half awake
half awake and half asleep
National Sleep Foundation
non-REM sleep