Breathing is rhythmic process...expansion and contraction...in & out...inhale & exhale. Breathing is an example of the cycles we see in nature such as night and day, wake and sleep, seasonal growth and decay, tides going in & out. And like waves in the tides, the rhythmic nature of breathing can be very calming.
Some of my favorite sleep music has this cyclic, breathing quality to it. A chord washes in, then out. The music reaches a peak then retreats. The ebb & flow of "breathing" music puts me right out every time.
