Fuzzy brain? Drinking caffeinated black, green or oolong tea may elicit a more alert state of mind, says a study in The Journal of Nutrition. Researchers think theanine—an amino acid present in these tea varieties—may work synergistically with caffeine to improve attention and focus. To reap the benefits, the study’s results suggest drinking 5 to 6 (8-ounce) cups of tea daily.