A caffeine nap is a short nap that is preceded by the intake of caffeine. In a driving simulator and a series of studies, Horne and Reyner investigated the effects of cold air, radio, a break with no nap, a nap, caffeine pill vs. placebo and a short nap preceded by caffeine on mildly sleep-deprived subjects. The latter was by far the most effective in reducing driving "incidents" and subjective sleepiness. Caffeine in coffee takes up to a half-hour to have an alerting effect, hence "a short (<15min) nap will not be compromised if it is taken immediately after the coffee."
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