There's quite a few. I'm always falling asleep to music. Depending on mood:
Debussy (all his solo piano works)
Ludovico Einaudi (solo piano again)
Chopin (and again)
London Philharmonic - Us and Them (yes, the Pink Floyd album, but played by a full blown orchestra)
Emiliana Torrini (all her albums)
Pat Methany
Anna Ternheim - Somebody Outside
Joan Armatrading
Azure Ray
Blackmores Night
Cara Dillon - Sweet Liberty
G. Love & Special Sauce
Tim Buckley
Typically, the tracks will be quite long. If not, the music has to be pretty unobtrusive, lyrically quite soft (hence the majority of female vocals), and the album shouldn't change pace too much (or too noticeably).
Dave