In Cecil’s article at Why is the letter Z associated with sleep? - The Straight Dope , when describing how other cultures depict sleep or snoring in print or comics, he writes:
" The Japanese use characters that transliterate as “guu guu,” "
I’ve also heard that it’s a tradition in manga, anime, and Japanese video games, to depict a sleeping character by showing a bubble of snot expanding and contracting from one of his nostrils. (Hopefully not in comics you’ll be reading at the dinner table.)