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Caffeine's real power kicks in, though, when you're tired. That's of obvious interest to the military, which counts on servicemen and -women to make life-and-death decisions even when they have been in the field without rest for days. "When you're sleep deprived and you take caffeine," says Lieberman, who has carried out extensive tests on Navy SEALS, among others, "pretty much anything you measure will improve: reaction time, vigilance, attention, logical reasoning—most of the complex functions you associate with intelligence. And most Americans are sleep deprived most of the time." Again, caffeine doesn't make you inherently smarter; it just lets you call more effectively on the intelligence you already have.
Precisely how it all works is still being figured out by neuroscientists. What they know is that caffeine binds to receptors that normally accept adenosine, a neurotransmitter that signals brain cells to quiet down their activity. Blocking adenosine staves off sleepiness. The resulting higher level of brain activity puts the nervous system on alert, triggering the release of adrenaline—the probable cause of caffeine's tendency to focus the mind.
Caffeine also triggers the release of dopamine, mostly in the frontal areas of the brain and the anterior cingular cortex, in which the so-called executive functions like attention, task management and concentration are located. This is consistent with what the Austrian scientists reported last month at the Radiological Society of America's annual conference in Chicago. Dr. Florian Koppelstaetter and his colleagues at the Medical University in Innsbruck gave 15 male volunteers 100 mg each of caffeine—about the same amount as in two cups of coffee—and then tested their short-term memory. Not only did the caffeine drinkers perform significantly better than those on placebos (all the subjects were in both the caffeine and the control groups in different rounds of testing), but when the scientists scanned their brains with functional MRIS, the anterior cingular cortex and the frontal lobes lit up with increased activity.
Caffeine is just a single chemical, of course, whereas coffee contains scores of substances. Some of them are antioxidants, which could explain part of its protective effect against disease. Some are psychoactive. "Our research," says Martin, "has focused on some of those other elements, such as chlorogenic acids, which keep adenosine in circulation in the brain longer than normal. That might augment coffee's ability to increase concentration without increasing irritability."
And then there's tea and chocolate, both of which also have caffeine, along with their own mélanges of antioxidants and other chemicals. Teasing out the specific actions of each one and separating them from caffeine's could take years. For the patrons crowding Small World Coffee, all of that is beyond the immediate point, which seems to be nothing more than getting a morning fix of one caffeinated drink or another before setting off to conquer the intellectual challenges waiting at the university just up the street. "A mathematician," the legendary number theorist Paul Erdos used to say, "is a machine for turning coffee into theorems." Organic chemistry, neuroscience, psychology and pretty much universal experience suggest that he probably was on to something.