I don't really listen to songs to fall asleep... Mainly 'cause I need my iPod every morning, so I can't run it down overnight. However, a song that I listen to quite often before I go to sleep is Vanilla Twilight (Owl City)- it's so pretty and calming. It's one of my favourite songs, and I love the video. I guess it also says all I want to say (sorry, so cliche!) but yeah.
Vanilla Twilight: http://www.youtube.com/watch?v=pIz2K3ArrWk