Somnus n.

Proto-Indo-European root meaning "to sleep."

It forms all or part of: hypno-; hypnosis; hypnotic; hypnotism; insomnia; somni-; somnambulate; somniloquy; somnolence; somnolent; Somnus; sopor; soporific.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit svapnah, Avestan kvafna-, Greek hypnos, Latin somnus, Lithuanian sapnas, Old Church Slavonic sunu, Old Irish suan, Welsh hun "sleep;" Latin sopor "a deep sleep;" Old English swefn, Old Norse svefn "a dream."

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.