When overnight lows drop and the heat comes on, dry indoor air can disrupt your sleep, and the EPA recommends keeping humidity between 30 and 50%. But a humidifier isn't the whole answer, and the American Lung Association cautions about dust mites, mold and ultrasonic humidifiers used with tap water. Find out the overnight low that signals it's time to check your bedroom humidity.

MORE: Keeping your home's humidity at this level helps fight flu