What is the origin of the bubble during sleep trope? A common trope across various anime and manga is the snot bubble trope. I've put an example below:



I can gather what it signifies from the TV Tropes description, but what's the origin of this? Does anybody know where it was first used, or where the inspiration came from?
The origin is, perhaps surprisingly, real life! This does occur among children in Japan and in other countries as well.