When I was little someone once told me that when you sleep your body became invisible. So If was asleep and their body was still there, I always thought either they were just pretending to be asleep or they weren't into a deep sleep yet.
A childhood belief about sleeping

others believed things like this
- I used to think that "extra body" shampoo meant you could use the runoff suds to clean your body.— Christy
- I used to think that the tooth fairy puts you in a deep sleep before stealing your tooth/teeth.Anon
- i used to believe there were invisible sharks in the deep end of the pool, thanks to my older brother.— Melissa
- I used to think that a clown lived under my bed and tried to eat me while i was asleepAnon
- I used to think thats rats put those eye crusties there while I were asleep— Maria
- When I was a child i used to believe that if i did not fall asleep an elf would come for me Anon