"Coffee improves alertness and reaction time in people, whether they're habitual consumers of coffee or not," says Harris Lieberman, a psychologist and caffeine investigator at the US Army Research Institute of Environmental Medicine.
But "the effect is clearly limited to the ability to maintain attention," he
"Things like memory or complex reasoning won't improve."
In the sleep-deprived, however, caffeine has a more striking impact.
"It improves almost everything you can measure," says Lieberman
"It makes you more alert, it seems like you can perform complex tasks better, and your memory is better.
"People who are falling asleep on the job can't do much of anything," says Lieberman
"There's no question that caffeine will improve aerobic physical endurance in non-athletes as well," says Lieberman
, adding that people who run, jog, swim, or cycle can last longer if they've had 200 to 600 mg. of caffeine beforehand.
"And new research suggests that caffeine can also improve anaerobic performance," he