A: Both. "Caffeine is a great stimulant, but it works best prophylactically," says Scott Campbell, PhD, director of the Laboratory of Human Chronobiology at Weill Cornell Medical College. "Once you're really tired, caffeine won't do much for you." A nap, however, refreshes your brain. In a study of 32 men and women, Campbell found that people who napped performed better on complex tests, not only immediately afterward but also on the following day. And happily, napping didn't disrupt nighttime sleep. To get the most from your nap, first drink a cup of coffee, then sleep for no more than 30 minutes. You'll be waking up just as your body is processing the caffeine and will be raring to go.