For starters, the moving air kicks up dust and pollen that's laying around and blows them all up in your sinuses and airways, and it's even worse if your fan has a nice layer of dust collecting on the fan. This is especially unfavorable for those prone to allergies and asthma.
The stream of air can also dry out your sinuses causing stuffiness and headaches -- both of which are no fun at all. Other potential issues are dry skin, dry eyes for anyone who happens to sleep with their eyes open, and dry mouth for mouth-breathers.
While not many people sleep with open eyes, mouth breathing at night is extremely common and many of us know all too well how awful it is to wake up with your mouth as dry as the Sahara.
Luckily, a little extra dusting, some moisturizer, eye drops, and a glass of water on the nightstand are all great solutions to these minor issues.
Those of us who enjoy the quiet hum and gentle breeze of a whirring fan at night probably won't think twice about the drawbacks and will continue to sleep with it on. Besides, just as there are cons to fans, there are also some major pros.
For instance, air circulation. While the downside is that some dust particles might float around, the upside is that the flow of air keeps the room fresh and disperses the heavy, musty smell of sleeping bodies, improving your quality of sleep.
They also cool down the room which is necessary for sound shuteye...and speaking of sound, fans also double as amazing white noise machines.
While the drawbacks of sleeping with a fan on are perfectly valid, it all boils down to what what feels good, keeps you healthy, and helps you get the best sleep.