There is an interesting article here discussing why constant, white noise helps us to sleep. If you have even taken a child for a car ride, you have experienced this phenomenon. There is nothing better than that droning hum to put your two year old down.
The article attributes the effect to the ability of white noise to mask other sounds. My own theory is that sleep works like a thermostat. As long as the current ambient temperature, noise level, and comfort level are within a reasonable margin of the condition you fell asleep in, you will stay asleep. If, however, there is an appreciable change in any of these parameters, your body clicks into action and you wake up. This would certainly make sense from an evolutionary perspective. You shut down your senses enough to recharge, but not so much that you totally disregard you environment. Masking those changes then (at least auditory changes) would seem to make sense as a sleep strategy.
[tags]sleep,evolution[/tags]