This appeared in the Washington City Paper last week. Click through for the comics-related answer:
Why is the letter Z associated with sleep?
The Straight Dope July 26, 2012
Why is the letter Z, specifically, associated with sleeping? It seems silly to have a letter correspond with sleep at all, but even sillier that we don't do this with any other action. You don't hear being awake referred to as "catching some A's."
— Ethan Reber