It's possible that sleeping with a fan could mildly dry out your throat, because when air gets moved around, it makes the air feel drier, says Philip Chen , MD, assistant professor and program director in the department of otolaryngology at UT Health San Antonio. And if you're breathing in this extra-circulated air, then it could theoretically dry out your nasal passages and throat, although this is probably so minor that most people wouldn't notice the effects, he says. After all, "the mucus and anatomy of your body are also very efficient in humidifying the air you breathe," he says. But you may have also heard some people say that sleeping with a fan on is bad for allergies . Is there any truth to that?