Caffeine used by an introvert vs. extrovert personality:
Researchers suggest that the beneficial effects of caffeine intake on improving working memory (WM) may be linked to personality. Fifty-nine participants received caffeine (200 mg) or placebo prior to completing a N-Back WM task, similar to a memory which is used to measure working memory. Compared to placebo, caffeine improved working memory performance in extroverts but not in introverts. (35)
Caffeine and theanine:
Caffeine in combination with theanine present in black tea may improve cognitive function and mood.
Black tea, attention focus, accuracy, and alertness:
Drinking black tea was shown to improve the ability to focus attention in two double-blind, randomized, placebo-controlled, crossover studies. In both studies, black tea significantly enhanced accuracy on the switch task and self-reported alertness on the Bond-Lader questionnaire, a mood assessment which grades the degree of mental alertness, calmness and contentedness. The first study also reported better auditory and visual attention after black tea compared to placebo. The two compounds in tea most likely responsible for these effects are theanine, an amino acid found almost exclusively in tea, and caffeine. Both of these were higher in the first study, which corresponds to the stronger results reported in the first study. (15)