When and why did we start using zzz to refer to sleep? How long have we been catching zeds (or what Yanks call zees), and since when have we been getting our 40 winks?
The OED lists one definition of z (“usually repeated”) as “used to represent the sound of buzzing or snoring”, and it was indeed a case of onomatopoeia that first linked the letter z — or multiple zzzzzzs — to sleeping by approximating the sound of snoring. The American Dialect Society’s Dialect Notes, published in 1918, lists “z-z-z” as “the sound of whispering or snoring, and 1919’s Boy’s Life, the Boy Scouts’ yearbook, gives “Z-z-z-z-z-z-z” as the title of a joke about that most supersonic of sleep sounds. This onomatopoeic use of z’s — which later came to signify, more generally, the state of slumber — was popularized by its use in early comic strips and comic books, for example in Schulz’s “Peanuts” cartoon series. In fact, a single Z in a speech bubble is enough nowadays to indicate that a character is asleep, and this is no longer confined to just the English language: as Wikipedia explains, “Originally, the resemblance between the ‘z’ sound and that of a snore seemed exclusive to the English language, but the spread of American comics has made it a frequent feature in other countries. An exception to this is in Japanese manga, where the usual symbol for sleep is a large bubble coming out of the character’s nose.”
And why “40 winks”, meaning a short sleep or nap? It’s not clear where or when the actual expression originated, but the number 40 is known to have been used historically to signify a great or indefinite number — hence the Biblical “40 days and 40 nights” and other numerous references; as argued in The International Standard Bible Encyclopedia,” edited by James Orr in 1915, “it may have originated, partly at any rate, in the idea that 40 years constituted a generation or the period at the end of which a man attains maturity, an idea common, it would seem, to the Greeks, the Israelites, and the Arabs.” Add the informal meaning of wink as a very short period of time, especially in the context of lack of sleep (as in, not being able to “sleep a wink”), and suddenly “40 winks” makes perfect sense. The Online Etymology Dictionary attests the expression “40 winks” from 1821, and speculates that its early use might have been associated with, and perhaps coined by, the eccentric English lifestyle reformer William Kitchiner M.D. (1775-1827).
Other colloquial words and expressions for slumber are cat nap (noun), to doze or doze off (verb), to nod off (v), shut-eye (n), snooze (v & n), and going bo-bos. Cockney Rhyming Slang gives us soot (Sooty and Sweep = sleep) and Bo Peep — the latter possibly giving rise to the suggestion, cooed persuasively and desperately to British babies, of “going bo-bos”…
“Goodnight, sleep tight, and don’t let the bed-bugs bite.” Tight in this context refers not, as some contend, to ropes tied tautly across early bedsteads, but instead to the adverb tightly, defined by the OED as “soundly, properly, well; effectively”; indeed, that dictionary’s first definition of tight itself is “soundly, roundly; = TIGHTLY 1. Now chiefly in colloq. phr. (good night) sleep tight, a conventional (rhyming) formula used when parting for the night or at bedtime.” The bed-bugs can probably speak for themselves.
O soft embalmer of the still midnight,
Shutting, with careful fingers and benign,
Our gloom-pleas’d eyes, embower’d from the light,
Enshaded in forgetfulness divine:
O soothest Sleep! if so it please thee, close
In midst of this thine hymn my willing eyes,
Or wait the “Amen,” ere thy poppy throws
Around my bed its lulling charities.
Then save me, or the passed day will shine
Upon my pillow, breeding many woes,
Save me from curious Conscience, that still lords
Its strength for darkness, burrowing like a mole;
Turn the key deftly in the oiled wards,
And seal the hushed Casket of my Soul.
— by John Keats