When it’s too hot and muggy for hard workouts, take a brisk walk instead. Walk with your head held high and your arms swinging for eight minutes for an instant jolt of energy that lasts two hours or more, according to research at Stanford University. Short bursts of motion stimulate your brain to produce ATP, a co-enzyme that's the brain’s key form of energy, and flush out lactic acid, a fatigue-triggering waste product that can get clogged up in your tissues if you sit too long at a stretch. Try it: Take an after-dinner walk for eight minutes. It will cut your risk of restless sleep by as much as 70 percent, say Canadian researchers. Evening workouts ramp up production of the calming, snooze-inducing hormone serotonin.