It's a onomatopoeia from the Greek language for snoring. It's also the sound you make when you sleep. Because it's the last letter of the alphabet, it's what resembles the end of the day, that's how I think of it.
My Dad always told us that it was the sound of sawing logs, bzzzzz...
Close to the sound of snoring, I guess.
Some ppl (expsecialy cartoons) buZZZZZ wen the dall asleep
Because that's sort of what light snoring sounds like.