The same cup of coffee can sharpen one mind and unsettle another. That is no paradox. Caffeine blocks adenosine receptors, reducing the signal that makes us feel sleepy, but the strength and duration of its effects vary with biology, habits and health. Alertness and discomfort can arrive in the same cup.Metabolism is an uneven bargain. The liver enzyme CYP1A2 helps break down caffeine, and genetic...More