Alternative idiomatic sayings such as could not sleep a wink provide the mental picture of a wink being the shortest type of sleep available and "forty winks" therefore gives an indication of an appropriate short sleep. Indeed the saying appears to have developed in relationship to 40 being an indefinite term for a large number that has almost sacred or magical quality. For example the use of the word 40 is reflected in numerous biblical references for example Moses was on Mount Sinai for forty days and forty nights, Elijah was fed by ravens for forty days, Noah and his animals endured flooding rains for forty days, and then waited another forty days before he opened the window of the ark, and finally that Jesus Christ fasted for forty days, and then was seen forty days after his resurrection.
To emphasise that forty winks was just the right amount of sleep if a nap was to be taken F. Scott Fitzgerald in a short article titled "Gretchen’s Forty Winks", published in the Saturday Evening Post on March 15, 1924 has the main protagonist, Roger Halsey say to his wife Gretchen, Just take forty winks, and when you wake up everything'll be fine.
The idiom is used in everyday language also as a way of saying that a person has or will be refreshed by such a sleep. For example Frank De Silva, a member of the 6th Division rescued amongst 8,000 other troops from Greece in 1941 by HMAS Perth, tells of sitting next to a sailor who exhausted falls into a brief deep sleep next to his breakfast before being nudged by those around him. He immediately wakes and says, I just needed that forty winks, and then is able to return to his duties.
Finally almost so as to emphasise the link between forty winks and its biblical relationship William Ernest Henley and Robert Louis Stevenson in their play, "King’s Evidence" at Act III have the characters Smith and Moore discussing the failings of a third person, Slink Ainslie. Smith says to Moore, Give him forty winks, and he'll turn up as fresh as clean sawdust and as respectable as a new Bible.