I can’t a confirm this, but I think it became the standard in comics and other images because the sound of Z is *very vaguely* onomatopoetic of the sound that someone might make while breathing in their sleep. “Sleep breathing” is not a pure S sound, or the sound of a sigh/open mouthed breathing all the time (which are both called fricatives) but it can have a tiny bit of breathy vocalization depending on the positioning of the tongue and airways. S is an unvoiced fricative, and its voiced counterpart is Z; F and V form a similar unvoiced/voiced fricative pairing, as is the “th” in “the” (the unvoiced) contrasted with the “th” in “think” (the voiced).
I feel like I did read this at one point (about why we use Z) but honestly I can’t remember the source.
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