Etymology
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sleep (v.)

Old English slæpan "to be or fall asleep; be dormant or inactive" (class VII strong verb; past tense slep, past participle slæpen), from Proto-Germanic *slēpanan (source also of Old Saxon slapan, Old Frisian slepa, Middle Dutch slapen, Dutch slapen, Old High German slafen, German schlafen, Gothic slepan "to sleep"), from PIE *sleb- "to be weak, sleep," which perhaps is connected to PIE root *sleg- "be slack, be languid," the source of slack (adj.). Sleep with "do the sex act with" is in Old English:

Gif hwa fæmnan beswice unbeweddode, and hire mid slæpe ... [Laws of King Alfred, c. 900]

Related: Slept; sleeping. Sleep around is attested by 1928.

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sleep (n.)

Old English slæp "sleep, 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.

Personified in English from late 14c., on model of Latin Somnus, Greek Hypnos. Figurative use for "repose of death" was in Old English; to put (an animal) to sleep "kill painlessly" is recorded from 1923 (a similar imagery is in cemetery). Sleep deprivation attested from 1906. Sleep-walker "somnambulist" is attested from 1747; sleep-walking is from 1840. To be able to do something in (one's) sleep "easily" is recorded from 1953. Sleep apnea is by 1916.

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sleep-over (n.)
1935, from verbal phrase; see sleep (v.) + over (adv.).
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slept 
past tense and past participle of sleep (v.).
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oversleep (v.)

late 14c., "to sleep beyond the proper or desired time of waking" (intrans.), from over- + sleep (v.). Transitive sense of "to sleep beyond" is by 1520s. Related: Overslept; oversleeping. Old English had a noun oferslæp "too much sleep."

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sleepless (adj.)
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.
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asleep (adj.)
c. 1200, aslepe, o slæpe, "in or into a state of slumber," from Old English on slæpe (see a- (1) + sleep (n.)). The parallel form on sleep continued until c. 1550. In religious literature sometimes euphemistic or figurative for "dead" (late 13c.). Meaning "inattentive, off guard" is from mid-14c. Of limbs, "numb and having a prickly feeling through stoppage of circulation," from late 14c.
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sleeper (n.)
Old English slæpere "one who sleeps, one who is inclined to sleep much," agent noun from sleep (v.). Meaning "strong horizontal beam" is from c. 1600. Meaning "dormant or inoperative thing" is from 1620s. Meaning "railroad sleeping car" is from 1875. Sense of "something whose importance proves to be greater than expected" first attested 1892, originally in American English sports jargon, probably from earlier (1856) gambling slang sense of "unexpected winning card." Meaning "spy, enemy agent, terrorist etc. who remains undercover for a long time before attempting his purpose" first attested 1955, originally in reference to communist agents in the West.
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sleepy (adj.)
early 13c. from sleep (n.) + -y (2). Perhaps in Old English but not recorded. Old English had slæpor, slæpwerig in the sense "sleepy;" slæpnes "sleepiness." Similar formation in Old High German slafag. Of places, from 1851 (Irving's Sleepy Hollow is from 1820). Sleepy-head is from 1570s. Related: Sleepily; sleepiness.
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sleeping (adj.)

c. 1300, present-participle adjective from sleep (v.). Sleeping-pill is from 1660s; sleeping-bag is from 1850; sleeping sickness as a specific African tropical disease is first recorded 1875; sleeping has been used since late 14c. for diseases marked by morbid conditions. Sleeping Beauty (1729) is Perrault's La belle au bois dormant.

It is ill wakyng of a sleapyng dogge. [Heywood, 1562]
It is nought good a slepyng hound to wake. [Chaucer, c. 1385]
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