If you're on the shy side, it turns out you might not jive with java. In his book, "Me, Myself, and Us: The Science of Personality and the Art of Well-Being," author Brian Little says caffeine may negatively affect an introvert's productivity after their morning trip to Starbucks.
"After ingesting about two cups of coffee, extraverts carry out tasks more efficiently, whereas introverts perform less well," he writes. "This deficit is magnified if the task they are engaging in is quantitative [think: math] and if it is done under time pressure." The thinking behind the theory is that introverts, who tend to operate at an optimal level of alertness, are over stimulated by caffeine consumption, making them more distracted and less likely to perform well when tackling tasks. But, hey, introverts have still got good ol' booze when it comes to overcoming their inhibitions, so there's that.