Across more than two billion cups consumed daily, caffeine meets each human body differently — shaped by a single inherited gene, the habits we carry, and the medications we take. Scientists are now articulating what many have long sensed: that the line between alertness and anxiety, between restful sleep and a racing mind, is drawn not by the cup itself but by the invisible biology each person brings to it. In understanding why caffeine affects us so unevenly, we are reminded that the most universal human rituals are still, at their core, deeply personal.