ill have to respectfully disagree with you mygirls. you are far less likely to get mold and mildew if you leave your fans running 24/7. the reason air circulation plays such a vital roll in growing is because mold and mildew love stagnant non-moving air. Not only will you reduce your chances of mold but your stems will get much stronger. so unless your trying to save the little electricity that fans use, i say run the fans all the time.