system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do I sleep easier with a noise in the background?

I can't sleep without my fan on.
gpt
It 'masks' other noise, that is the constant droning (which is a sort of white noise) elevates the ambient noise level up a notch but still comfortable, this has the effect of softening any sudden extraneous noises say from the road traffic outside. Your instincts will wake you if any sounds are unexpected and because our hearing adapts to surroundings really quite dramatically and automatically the soft and predictable noise say of a fan keeps you hearing from going into the super sensitive mode, your basic instinctual brain will be less likely to throw up the 'alarm wake up' signals for the unexpected noise source.

tl:dr it's an instinct to help us survive midnight T-rex raids on your camp. Still useful even though we ate all the T-rex's (did that happen?) but we are safer now, it's hard wired in your brain that it you are sleeping you are a sitting duck, so wake up and run away you fool!