The cup that once jolted you awake now barely registers, and that fading is not your imagination. It is caffeine tolerance, a specific and well-documented adaptation: faced with chronic blockade of its adenosine receptors, the brain builds more of them, so the familiar dose now fights a stronger opposing signal and delivers less. This piece explains the receptor-upregulation mechanism, the human tolerance and withdrawal evidence, and the uncomfortable possibility that a heavy user's daily coffee is mostly reversing overnight withdrawal rather than lifting them above baseline. It ends with what actually works: not bigger doses, but keeping intake low and resetting sensitivity with a break.