How did the letter Z become to be associated with sleeping/snoring?

Interesting....a friend of mine has a daughter learning Greek. She asked if Greek comics used "ΩΩΩΩΩΩΩ" (Omega is the last letter of the Greek alphabet) to depict snoring.

I wanted to know the answer, so I embarked on a journey of Greek comic searches for people snoring and eventually I found this comic [1]. It shows people using Greek to communicate, but then the person sleeping is saying "ZZZZZZZ". I find it hilarious and interesting because Greek doesn't have the letter Z.

[1] - https://scontent-dfw1-1.xx.fbcdn.net/hphotos-xta1/v/t1.0-9/1...

Greek does have the letter Ζ, which is the capital Greek zeta. It is written just like the Latin capital Z.

https://en.wikipedia.org/wiki/Zeta

Derp, you're right. I knew that.

The Straight Dope has more on this:

http://www.straightdope.com/columns/read/3063/why-is-the-let...

Isn't that more semiotics than etymology?