The choice of the words that make up this little expression needs some explanation. Why, forty and why winks?
Wink is easiest to explain. On the face of it winks appears an odd word to use to denote sleep as, in order to wink, one needs to be awake – even more so to wink forty times.
That wasn’t always the case though and from at least the 14th century wink has meant a closing of the eyes for sleep. Shakespeare alluded to this usage in a scene in A Winter’s Tale, 1616:
…be-spice [drug] a Cup, To give mine Enemy a lasting Winke.
Forty is more ambiguous. It might be that forty just sounded better than thirty, fifty or some other number. It’s more likely though that forty was chosen because it had previously been used to demote a largish but indeterminate number of something. Again, the word was used by Shakespeare, in Coriolanus, 1623):
On faire ground, I could beat fortie of them
The Bible has many (146 by some accounts) references where forty is used to denote a large but indeterminate number – Jesus fasting for forty days and forty nights, the Hebrews living on manna for forty years, Ezekiel laying on his side for forty nights, and so on.
The first use of the expression forty winks that I can find comes from the UK in 1821. It’s in a book called The Art of Invigorating and Prolonging Life, by the English cook William Kitchiner. Actually the full title is:
The Art of Invigorating and Prolonging Life by food, clothes, air, exercise, wine, sleep, &c. and peptic precepts, pointing out agreeable and effectual methods to prevent and relieve indigestion, and to regulate and strengthen the action of the stomach and bowels.
He certainly leaves you in no doubt what the book is about – they don’t write them like that any more. As well as a list of recommendations as exhaustive as the title, it includes:
“A Forty Winks Nap,” in an horizontal posture, is the best preparative for any extraordinary exertion of either.