Scientifically, why do people fall asleep easier with noise (such as a fan, rain, white noise, etc)?

For those who have those minds that don't shut off, it gives them something to focus on without bringing in more thoughts.

We used to run a fan all night when daughter was in school. It helped drown out slamming doors, barking dogs, a neighbor's late night party, etc.

In a story for Spirit Magazine, one neurologist explains what goes on in your brain that makes white noise machines invaluable to a sound night of sleep:

Dr. Ralph Pascualy is the medical director of Northwest Hospital Sleep Center in Seattle. He explained to me that the brain naturally craves sensory input. That's why people in sensory depravation tanks hallucinate; robbed of any stimulus, the brain creates its own. During sleep in a quiet night, any random noise, whether a passing truck or a creaking floorboard, is likely to activate the restless brain, waking you up. Constant white noise, he told me, "gives the brain a tonic signal that dampens its own internal systems."

http://www.apartmenttherapy.com/sleep-better-the-science-of-sound-machines-168208