I also sleep with a fan on low, and yes, it does work. One possible mechanism is the (well-attested) phenomenon of perceptual set. Ever buy a new refrigerator?

I also sleep with a fan on low, and yes, it does work. One possible mechanism is the (well-attested) phenomenon of perceptual set. Ever buy a new refrigerator? For a week or so, you'll notice the sound every time it turns on. After that, you'll never notice it again. A similar thing is going on when you notice someone saying your name at a crowded party. Your brain "tunes out" the background noise from the multiple conversations, but when your name is mentioned, you're suddenly listening to that specific conversation. At one time I lived near busy railroad tracks. For a week or two, I was getting blasted awake several times a night. After that? Didn't even hear 'em. What may be going on is that the fan noise raises the background sound level enough that minor, unpredictable sounds (wind, bugs hitting the window, whatever) that would normally trigger alertness are enough below the ambient sound threshold that your brain doesn't pay any attention to them.