"A wink of sleep" could also be justified as a hyperbole.  A hyperbole is an exaggeration.  It stretches the truth.  By stating that you've gotten a "wink of sleep" is stretching the truth of receiving very little sleep.

You see hyperboles in many tall tales, such as "John Henry" and "Paul Bunyan."

We use hyperboles often in our everyday language.  Anytime you are exaggerating you are using hyperbole.

- I'm so hungry I could eat a horse.

- She's the most beautiful woman in the world.

- I think of her a million times a day.

A "wink of sleep" is a metaphor.  Metaphor is an unstated comparison between two unlike things or ideas. That is, the comparison is not expressed, but is created when a figurative term is substituted for  or identified with the literal term. In this metaphor, a very small amount of sleep is compared to the quick action of winking. 

Depending upon whether the literal or figurative terms are respectively named or implied (figurative), metaphors may take one of four forms:

1. Both the terms, literal and figurative, are named.
2. The literal term is named and the figurative implied.
3. The literal term is implied and the figurative is named.
4. Both the lterms, literal and figurative are implied.

A "wink of sleep" has both terms, figurative and literal respectively, named.  This type of metaphor is rather common.