Increased alertness, peak mental function and sustained physical power are all qualities that ancient fighters wielding bows and arrows coveted. Today's urban warriors are much the same, eager to stay alert for commutes and conference calls. "The simplest change you can make is increasing fruits and veggies," says Dee Sandquist, an American Dietetic Association spokesperson. And because fatigue can be caused by dehydration, consume fluids, advises Liz Applegate, a nutritionist at the University of California at Davis. Caffeine provides the kick in many traditional drinks from South and Central America, such as yerba mate (Ilex paraguariensis) and guarana (Paullinia cupana). How It Works Caffeine stimulates the central nervous system to speed up heart rate, raise blood pressure and rev up metabolism. Clinical trials in athletes show that it increases aerobic endurance and the ability of muscles to contract. It's good to know the amount that will satisfy your energy needs, because overuse can bring on the jitters, insomnia and other adverse reactions.