Origin and history of asleep

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 1884 (put to sleep forever). A similar imagery is in cemetery.

Sleep deprivation is attested from 1906. Sleep-walker "somnambulist" is attested from 1747; first record of 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.

prefix or inseparable particle, a conglomerate of various Germanic and Latin elements.

In words derived from Old English, it commonly represents Old English an "on, in, into" (see on (prep.)), as in alive, above, asleep, aback, abroad, afoot, ashore, ahead, abed, aside, obsolete arank "in rank and file," athree (adv.) "into three parts," etc. In this use it forms adjectives and adverbs from nouns, with the notion "in, at; engaged in," and is identical to a (2).

It also can represent Middle English of (prep.) "off, from," as in anew, afresh, akin, abreast. Or it can be a reduced form of the Old English past participle prefix ge-, as in aware.

Or it can be the Old English intensive a-, originally ar- (cognate with German er- and probably implying originally "motion away from"), as in abide, arise, awake, ashamed, marking a verb as momentary, a single event. Such words sometimes were refashioned in early modern English as though the prefix were Latin (accursed, allay, affright).

In words from Romanic languages, often it represents reduced forms of Latin ad "to, toward; for" (see ad-), or ab "from, away, off" (see ab-); both of which by about 7c. had been reduced to a in the ancestor of Old French. In a few cases it represents Latin ex.

[I]t naturally happened that all these a- prefixes were at length confusedly lumped together in idea, and the resultant a- looked upon as vaguely intensive, rhetorical, euphonic, or even archaic, and wholly otiose. [OED, 1989]