Honestly not any of the songs aformentioned in this thread would help me sleep, some would really relax me though.
Really, some ambiant/drones would be better suited for your needs...
Try Stars of the lid, Steve Roach, Brian eno's music for airports, Biosphere...
More ambiant would be Helios, The Album Leaf (not the newest album it's shit.) explosions in the sky was a good proposition but you need to watch out because its progressive rock and some tracks get heavier in the end. Also: If you like Sigur Ros try out Mùm.
Hope that helps you!