I have quite an odd assortment of pieces that I listen to when I am trying to fall asleep.
Personally, I listen to the following composers:
Chopin
Scarlatti
Kabalevsky
Ravel
Brahms
Rachmaninoff
Schubert-Liszt
Paganini-Liszt
Handel
Liszt
Mozart
The composers that I listen to probably aren't suitable for everyone, but here are a few ideas:
Chopin - Nocturnes (some have intense sections, so beware!), Etudes (Most are fast, but there are some relaxing ones), Movement II of both piano concertos and preludes... There's lots to choose from!
Liszt - Consolations (nice and relaxing), Liebestraum and Ave Maria (That's actually by Schubert, transcribed for piano by Liszt) are all nice. :3
Rachmaninoff: Vocalise is a cool piece, and his six romances are relaxing!
Ravel: I recommend the Minuet (Movement II) of his Sonatine. Very nice and calming.
Debussy: The obvious... Clair de Lune and Deux Arabesques (*yawn*)... Prelude on an Afternoon Faun, La Fille Aux Cheveux de Lin and La Mer are worth looking at as well.
Beethoven: Adagio Cantabile from Sonata Op 13 "Pathetique"
Satie: Trois Gymnopedies and Trois Gnossienes are nice :)
Bach (Yes, I do know a bit about Bach! Who knew! XD): The Largo movement from his Oboe Concerto in G minor is nice. Fantasia in A is also cool. Arioso from Cantata, No 156 is relaxing if you'd prefer chamber music. Concerto for Two violins (Largo ma non tanto) is one of my favorite Bach pieces. Then, of course, there are the obvious ones:
Air on G string from Suite No 3, Prelude No 1 in C major and Suite for Unaccompanied Cello - Prelude.
Mozart: Sonata in A Major K 331 (First movement) is relaxing to me. Divertimento in D (Andante), Sonata in G (Andante) and Clarinet Quintet in A major (Larghetto) are all nice Mozart pieces.