The New York Times has a short article about a study that compared caffeine with taking a short nap before doing mental and physical skills. As it turns out, the people who took caffeine felt more awake, but they didn't score any better. In fact the caffeine group performed worse than nappers on all three types of task that were studied: motor skill, perceptual task, and verbal task.
I really like my coffee, but it is no substitute for getting enough sleep, and even a short nap in the afternoon will do more good for you than just caffeine.
The research paper itself is available online. You can read the abstract for free, but to read the full paper you need to be a subscriber, or you can pay for it for $31.50.
Vital Signs - Regimens - For the Best Pick-Me-Up, Lie Down - NYTimes.com
Hat tip: Lifehacker.com