sleepless adj.

Middle English slep, from Old English slæp "state of quiescence of voluntary and conscious functions; sleepiness, inactivity," from Proto-Germanic *slepaz, from the root of sleep (v.). Compare cognate Old Saxon slap, Old Frisian slep, Middle Dutch slæp, Dutch slaap, Old High German slaf, German Schlaf, Gothic sleps.

By c. 1200 as "a period of sleep." Personified in English from late 14c., on the model of Latin Somnus, Greek Hypnos. Figurative use for "repose of death" was in Old English; euphemistic put (a pet animal) to sleep "kill painlessly" is recorded from 1923. A similar imagery is in cemetery.

Sleep deprivation is attested from 1906. Sleep-walker "somnambulist" is attested from 1747; sleep-walking is from 1797. Sleep apnea is by 1976. To be able to do something in (one's) sleep "easily" is recorded as a hyperbolic phrase by 1953. Sleep apnea is by 1916.

word-forming element meaning "lacking, cannot be, does not," from Old English -leas, from leas "free (from), devoid (of), false, feigned," from Proto-Germanic *lausaz (cognates: Dutch -loos, German -los "-less," Old Norse lauss "loose, free, vacant, dissolute," Middle Dutch los, German los "loose, free," Gothic laus "empty, vain"), from PIE root *leu- "to loosen, divide, cut apart." Related to loose and lease.