If you've ever wondered why you can lull yourself to sleep with music but you wake up sort of disturbedly to get rid of it anyway, it's because no matter how soft or slow what you are listening to is, your brain is constantly shifting sleep stages when unconscious and as such, the music will always end up off-beat from complementing the wave Hz at some point. Your brain needs a very particular frequency of subconscious 'sound' at different stages of sleep so at some point the music you are listening to will disturb your sleep cycle and you'll wake up feeling bad/disturbed briefly just to get rid of the music because your brain can't handle the external input disrupting the 'sound' it requires for sound subconscious thought during sleep. It can't drown it out with it's own 'music' so you are forced to get rid of it.