I'm skeptical about generalizing those results to more "natural" sounds like fan noise. As others have mentioned true white noise is very unnatural due to its uniform power spectrum. Pink and brown noise though seem more natural due to the dampening at higher frequencies, and I'd bet things like fan noise, rainfall, waterfall, etc. lie somewhere along the 1/f^alpha for alpha > 1 (in between pink and brown).
I'm sure that we've evolved to handle things like noisy nights (thunder, rainfall) and living near rivers. You probably don't want to go beyond 50db though.
But to be on the safe side I'm going to turn by fan one notch down (40db -> 30db) to be on the safe side.