It’s a well known symbol, but it must have some origin? Where did it come from? Why Z, and not X or Q?
(Yes, “catching Xs” may sound strange to you, but it wouldn’t if it was the norm, now then, would it?)
Snoring, when executed by an experienced practitioner like myself, has a sort of throaty “ZZzzzz” sound to it. If I were to start making “QQqqqq” sounds in my sleep, my gentler half would probably be reaching for the phone to call 911 (I hope).
So why Z and not S? Probably because “SSsss” was already spoken for by the Snake People. Also, it’s a little too sibilant for something that actually sounds more like “ZZNOOORRRACHBBBB”
:o z-z-z-z-z
Note that Zzzzzzzzzzzz = sleep is a Western convention; Japanese manga often indicate snoring by showing an “air balloon” (like a word balloon without words) coming out of the sleeper’s nostril.
onomatopoeia
Ace beat me to it: Zzzzz’s are an onomatopaeic (sp?) representation of the sound made by the action of snoring. Other examples of onomatopoeia include buzz, whoosh, woof, tinkle etc.
Onomatopoeic, sez the dictionary.
But I agree on the explanation.
Imho, the best & most contemporary example is “to click”.
Because Q’s would be too hard to pronounce.