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In a series of studies, however, sleep researchers in England found that drinking a cup of coffee and then immediately taking a 15-minute nap was even more effective. The researchers tested sleep-deprived subjects in driving simulators and found that a “caffeine nap” improved driving performance and reduced sleepiness better than other commonly employed techniques, including cold air, a short nap, a break with no nap or 200 milligrams of caffeine, roughly the amount in a 10-ounce cup of strong brewed coffee.
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.