Hrm...the a few songs of my 'going to sleep' playlist are:
Loch Lomond - The Corries (nothing like a bit of mournful scottish folk to send you to sleep. Avoid bagpipes in this context though)
Unomathemba - Ladysmith Black Mambazo (Something about the harmonies and the deep voices...)
Utrus Horas - Orchestra Baobab (initially this sounds very slightly more upbeat than the rest, but there's a kinda deeper underlying rhythm to it)
A lot of Ali Farka Toure's pretty good as well. Just avoid some of the more manic african guitar bits
Hem - Rabbit Songs is a beautiful album for falling asleep to.
Actually it's just a beautiful album full stop.
Although I think the best music for falling asleep to is something you're very familiar with, if it's newish to you you might think about it too much.
That Ladysmith Black Mambazo song they used to have on the Heinz adverts would be well good for getting to sleep...Lighthouse Family songs...Mozart's Clarinet Concerto In A (Adagio)...Gluck's Dance Of The Blessed Spirits...a lot of classical stuff!