My 8yr old lad had a really bad night the other day - wouldn't settle and took ages to get to sleep because he was scared.
I finally got to the bottom of it yesterday: he was freaked out by an episode of The Simpsons! It was the one with the subliminal messages getting people to join the navy (YVAN EHT NOIJ, YVAN EHT NIOJ...) - totally freaked him out.
There's something quite primal about these early fears we get, they stay with us. What things scared you as a kid?