Caffeine wakes you up, but it can also leave you jittery and unfocused. L-theanine, the calming amino acid found naturally in tea, appears to smooth out those rough edges when paired with caffeine. Clinical trials show the combination improves attention, reaction time, and task switching while reducing mind-wandering, all without the anxious overshoot that caffeine often brings on its own. We dig into the science behind the synergy, what the research actually supports, and the dosing that lines up with the evidence.