Sleep issues, tossing and turning, insomnia - these are all problems many of us deal with every night as we try and fall asleep and get some rest before another stressful day begins. The cycle is all too familiar to many, as the stress of not being able to asleep can increase the stress of not being to fall asleep! Obviously, this is a frustrating experience and is not conducive to restful sleep.
A method that some are able to use effectively as a sleep aid is to put a white noise machine in their bedrooms and "listen" to it while falling asleep. The irony of such a device is that you don't so much listen to it as use it to block out all sounds, including the sound of the white noise, with after a time, the brain ceases to consciously recognize. This happens because the brain tends to only notice what is different against a uniform background. This is why, for instance, a person who works in a barbeque restaurant doesn't smell barbeque on their clothes or skin - the smell itself has become the backdrop of the experience, insofar as the mind is concerned. However, a different smell would contrast sharply to barbeque would instantly be recognized by the brain.
The similarity with white noise is that it blocks out external sounds by creating a barrier between your ears and the outside world. This means those little sounds, such as dogs barking or cars coming by, have much less a chance at making it into your brain. At the same time though, the constant white noise provides a kind of mental room for the mind to relax in, and this is the point at which the brain uses white noise to help fall asleep.
While white noise machines are helpful as a sleep aid, the relationship is not so much a cause and effect type of setup, like that of prescription sleep drugs, but more as a "meeting in the middle". That is to say, white noise can be used for sleep induction if the user intends to use it to help him or her fall asleep.
For more information on this fascinating sleep sound technology, take a look at the White Noise Machine Sounds blog, a free resource on white noise complete with both scientific information and readable user reviews of white noise machines and other sound machines.