Doctors Warn Of These Hidden Health EffectsSleeping With A Fan On? Doctors Warn Of These Hidden Health EffectsA person carries a pedestal fan in its box at a Paris branch of electronics and homeware store Darty as temperatures rise in Paris during a heatwave affecting a large part of France, June 23, 2026. REUTERS/Abdul SaboorSleeping with a fan running may seem like a summer solution, but doctors warn it comes with side effects. Cool air concentrated on one body area can also tense muscles, causing stiffness or a sore neck. Dry air also paralyzes nasal immune defenses, allowing allergens to linger longer.