early 15c., from sleep (n.) + -less. Old English had slæpleas but the modern word seems to be a re-formation. Similar formation in German schlaflos, Dutch slapeloos. Related: Sleeplessly; sleeplessness.
lay sleepless all night
sledgehammer
sleek
sleep
sleeper
sleeping
sleepless
sleep-over
sleepy
sleet
sleeve
sleeveless
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