Why do mother natures noises allow you to fall asleep

http://rainsoundsnews9.iamarrows.com/how-come-waterfall-noises-help-you-slumber

The crash of ocean waves, the babbling of brooks, the pitter-patter of rain on shingles — Lots of people swear by these watery sounds that will help them slide asleep and stay in la-la land. Why does flowing "agua"