[To find an optimum source of caffeine] why not enjoy a cup of green tea [. . .], as the Chinese have for nearly 5000 years? It's hard to come by a better longitudinal study than that.
On the other hand, if you're worried about getting too much caffeine, then green tea is still a wise choice. Read more about green tea and caffeine, including how to decaffeinate your green tea yourself, here.
Click on the "hybrid brain" image to go to the home page of the article's author.