A counterintuitive combination does the trick:
[S]leep researchers in England found that drinking a cup of coffee and then immediately taking a 15-minute nap was even more effective [than either on its own]. … This method is believed to work because the short power nap helps clear the brain of the sleep-inducing compound adenosine. Caffeine, meanwhile, takes about 20 minutes to have its physiological effect — kicking in just as the napper is awakening.
The Dish 