I came upon this song which is touted as the most relaxing, easiest to fall asleep to song ever:
Weightless by Marconi Union
Marconi Union worked with sound therapists to create this soothing tune, which also supposed to help slow breathing and reduce brain activity. Sound therapy has been used for thousands of years to help people relax and improve health and well-being.
The relaxing nature of this song is said to come from its sustaining rhythm that starts at 60 beats per minute and gradually slows to around 50.