My grandmother once told my mum that every hour of sleep you get before midnight is equal to two afterwards (to get her to go to be earlier). My mum believed it until she was 40.
A childhood belief about sleeping

others believed things like this
- My Grandmother once told me she could unscrew her feet. I believed that for years.— Andy Bettison
- I used to believe that 7 times 12 was equal to 83 or 84, depending on the math problem.— Eric Swensen
- I used to believe that a quarter after the hour was 25 minutes after the hour because 25 cents = a quarter.— Marni
- I used to to believe that all adults went to bed at midnight.Anon
- I used to belive all grown ups went to bed at midnight.Anon
- I used to believe that all Street-Lamps were turned off at Midnight.Anon