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."