In a Japanese study that examined how to make the most of a nap, people who took a "coffee nap"—consuming about 200 milligrams of caffeine (the amount in one to two cups of coffee) and then immediately taking a 20-minute rest—felt more alert and performed better on computer tests than those who only took a nap.
Why does this work? A 20-minute nap ends just as the caffeine kicks in and clears the brain of a molecule called adenosine, maximizing alertness. "Adenosine is a byproduct of wakefulness and activity," says Allen Towfigh, MD, medical director of New York Neurology & Sleep Medicine. "As adenosine levels increase, we become more fatigued. Napping clears out the adenosine and, when combined with caffeine, an adenosine-blocker, further reduces its effects and amplifies the effects of the nap."