I have been googling around, searching for the origins of the phrasal verb "to fall asleep" but so far I have found no references. I was wondering specifically why we use the verb "to fall" to characterize the process of going to sleep and my conjecture is that it somehow links in with the connotations of tiredness and fatigue that the verb "to fall" bears.
The use of the phrasal verb is attested in Middle English from 1393:
1393 Langland Piers Plowman C. xxii. 5 Ich fel eft-sones a slepe.
[From "fall, v.". OED Online. September 2015. Oxford University Press. http://www.oed.com/view/Entry/67829?rskey=76QoIB&result=2&isAdvanced=false (accessed October 15, 2015).]
This use draws on a verbal sense of 'fall' attested possibly around 1225:
- a. Of persons: To pass (usually, with suddenness) †in, into, †to, upon some specified condition, bodily or mental, or some external condition or relation.
?c1225 (▸?a1200) Ancrene Riwle (Cleo. C.vi) (1972) 166 He..swa feol into unhope.
The particular meaning of "fall" is fossilized in several phrases, like "to fall in love," "to fall ill" - it's not about the feeling of falling asleep, but rather an archaic usage of "becoming (a state)."
- To fall in love is attested from 1520s; to fall asleep is late 14c. To fall down is early 13c. (a-dun follon); to fall behind is from 1856. Fall through "fail, come to nothing" is from 1781. To fall for something is from 1903.
- a 1300 Cursor M. 7428 Þe king he sal gar fall o-slepe.-