Informant is a Chinese-American student at USC.
“My family was just so convinced that like, nothing is ever dry until it’s bone dry. So I would go to blow dry my hair, and they’d be convinced, ‘no, it’s still wet’ and I would just keep on going, and they wouldn’t let me like go to sleep with like wet hair because I think they were just being like, ‘Oh, you’re gonna get sick’ and I was like, ‘No, it’s fine.’”
This conversation was recorded in-person.
Not sleeping with wet hair is something my parents have also told me. It’s a folk belief that is loosely tied with medicine/sickness, with the belief being propagated by the fact that it will scare children into drying their hair properly (though as my informant shows, at a certain age this wears off). There are probably scientific reasons either proving or disputing this claim, but considering that it was much easier to get sick pre-modern era, actions easy to control like drying your hair would probably be focused on the most.