When we go to bed, we need to transition from sympathetic (fight or flight) to para-sympathetic (rest and rejuvenate). The more efficient you are at the transition process, the greater the quality of your sleep.
Each cycle has a brain frequency (wave) that governs the portion of the brain that is repairing and the depth of the consciousness that you are in.
Delta wave is the deepest period of your sleep cycles and is responsible for the physiological component of the recovery. Ideally, a minimum of two hours a night is necessary to maintain healthy muscles protein synthesis, joints, and collagen production.
R.E.M sleep is (beta wave) responsible for emotional repair; mood, motivation, happiness. The minimum target is also 2 hours per night.
Listening to delta wave music mimics the same brain frequency as deep sleep. When you listen to it before resting, it can speed up the first sleep cycle’s effectiveness at reaching deep sleep and promote a longer stay in the most physiological restorative period of your sleep.
Delta wave music is relaxing; try listening to some before you go to bed next.
You can find an example of delta wave music here.