Caffeine affects people in different ways. Scientists are not entirely sure why, though genetics has something to do with it. The average adult can consume a small cup of coffee or a Coca-Cola and not experience side effects like jitteriness or sleep disturbances, says Roland Griffiths, Ph.D., of the Johns Hopkins School of Medicine.

Children and teenagers should have even less. They might have a stronger reaction to caffeine, since they have lower body weights than adults and tend not to consume caffeine-containing drinks and foods regularly.

Gender also plays a role in how a person responds to caffeine. "Men seem to clear caffeine from their systems faster than women do, just as they process alcohol faster," says Stanley Segall, a professor of nutrition and food science at Drexel University. That's why your spouse may be able to drink a cup of coffee after dinner and fall asleep an hour or two later, while you can't even have a cup of tea at four in the afternoon without fear of a restless night.