Working out actually gives you more energy. Your body makes feel-good hormones (endorphins), "and you're getting the circulation going," says Marisa Brunett, a certified athletic trainer in Orlando, Fla. It may help to work out in the morning before your day gets away from you, says kinesiologist Lynette Craft, PhD. She's an assistant professor of preventive medicine at Northwestern University. Not a morning person? No problem. Do it whenever you feel best, says Brunett, who likes to work out in the middle or at the end of the day.