After a marathon day at the office, 9 p.m. may be the only time you have left to squeeze in a workout. But a heavy sweat session too late at night will sabotage your sleep. “Your body releases hormones during exercise, including adrenaline, that remain elevated for at least a couple of hours, making it harder to fall asleep,” explains McCall. His rule: Give yourself at least two hours to cool down, plus a minimum of six to seven hours of sleep, which is necessary for rebuilding muscle. If you need to wake up at 6 a.m., don’t work out after 8 the night before. A solid night of sleep can also keep your appetite in check. A lack of sleep stimulates the production of the hormone that increases the desire for food while decreasing levels of your body's natural appetite suppressant. A good night’s sleep will make you feel rested and less hungry.