5 p.m. to 6 p.m.

"For increasing fitness, decreasing the chance of injury, and improving sleep, the best time to exercise is late afternoon or early evening," says Matthew Edlund, M.D., author of The Body Clock Advantage (Circadian Press, $15, www.amazon.com) and head of the Center for Circadian Medicine, in Sarasota, Florida. At these times, he says, your lungs use oxygen more efficiently, you're more coordinated, and your muscles are warmed up, so you're less likely to suffer a sprain or strain. Finish exercising at least three hours before bed so that when your head hits the pillow the extra adrenaline will no longer be pumping through your bloodstream (and other factors that keep you awake will also have subsided). Bonus: "If you're all wound up at the end of the day, exercise may be a great stress reliever," notes Shirley Archer of the Stanford Health Improvement Program, in Palo Alto, California.