I thought that, when people slept, their hearts stopped and didn't start beating again until they woke up. It made sense to me at the time.
A childhood belief about sleeping

others believed things like this
- I used to think that teachers lived in school and they slept there and just woke up to teach us.— tammy
- When I was a child, I used to believe that when we slept, we stopped breathing.Anon
- When I was five or six, I was worried that I'd lost my heart because I didn't hear it beating.Anon
- When I was a kid, I slept with my fists balled up because I thought if I didn't, E.T. would eat my fingers. — Brian
- I used to believe that the sun didn't rise until i woke up— Ashley
- I thought “guerrilla tactics” meant fighting like a gorilla— beating your chest and that sort of thing.— Kai