When I'm doing my writing I divide my playlists in six seperate ambience genres: General, Dreamy, Uplifting, Tension, Panic, and Nightmare. Tension is by far the most packed playlist I have, with Lustmord, rips from Hitman: Contracts by Jesper Kyd, rips from Twisted Metal: Black and Head-on from the Twisted Metal music team, and Mortal Kombat, amongst other miscellanious artists I've come across on Last.fm through the dark ambience group I've joined.

Depending on what I'm feeling, I'll fall asleep with whatever playing in the background. I've been trying to have nightmares lately, but the Nightmare and Tension music isn't working.

If you're looking for a very uplifting and dreamy set of ambient songs that play for about 10-20 seconds each (they're looped in the game), then rip the songs from Mirror's Edge. I was pissed off to find that the soundtrack only had remixes of the song's main theme, and didn't have the background ambience that plays in the background, so you may have to do a bit of research before downloading the "music from the game", definately worth it, though, not to mention they're low in size because of their duration.