Why a fan or AC hum helps some people sleep (and others hate it)

A fan or air conditioner produces a mostly broadband hum with a mechanical whir layered on top, which puts it in roughly the same masking category as white noise. That covers the acoustic side of the story, but it isn't the whole story.

A huge number of people specifically grew up falling asleep with a fan running, sometimes year-round depending on climate. That means the sound has become tightly linked, through ordinary classical conditioning, to the physical context and state of falling asleep. A neutral stimulus repeatedly paired with drowsiness and a bedtime routine can eventually trigger a piece of that state on its own, the same basic mechanism behind countless other sleep and relaxation associations.

This helps explain why fan and AC sound polarizes so strongly. For someone with that specific history, it can feel uniquely effective — sometimes more effective than a studio-recorded, acoustically 'purer' white noise track. For someone without that history, it's just noise, no more or less useful than any other masking sound.

If a fan or AC hum works for you, that's a legitimate reason to keep using it, even though it isn't specially engineered the way some other sound categories are. Conditioning-based comfort is a real mechanism, not something to be talked out of in favor of a more 'scientific' option.

None of this is unique to fans, either — it's worth keeping in mind across every sound category on this site: personal history and association matter as much as the underlying acoustics.