As you may know, tea contains theanine, an amino acid found almost exclusively in tea. It is made up of two “mirror” molecules (enantiomers). L-theanine has been associated with all kinds of good stuff like relaxing the mind, lowering blood pressure, and some anti-tumor effects. D-theanine has not. According to Tea: Bioactivity and Therapeutic Potential edited by Yong-su Zhen, D-theanine is usually only on average 1.85% of the total theanine content in tea, but storage at warmer temperatures leaves seem to increase this ratio, and that the “…relative amounts of D-theanine display inverse correlation to tea quality.” The higher quality tea, then, the less D-theanine, which means more L-theanine, which is the one that has the good health benefits.
The book also mentions some interesting information about caffeine in the same section. They cite a source showing a difference between teabag tea and loose leaf tea, as well as steeping time. At 2 minutes of steeping, a cup of teabag tea had about 48 mg of caffeine and loose leaf had about 38 mg. At 5 minutes the teabag was at 80 mg while the loose leaf was 60 mg.
So drink high-quality tea from a place that avoids storage at warmer temperatures, as it seems to be more healthful. And if you’re concerned about caffeine intake, drink loose leaf tea brewed quickly (as is done in Chinese gongfu tea preparation).