Tea contains the amino acid L-theanine and seems to mitigate the negative aspects of caffeine, such as anxiety, increased blood pressure and diminished sleep quality, while improving upon the positive aspects. L-theanine is known to be safe and effective. Its ability to enhance attention, over and above that of caffeine alone, has been repeatedly verified.
What the American Heart Association Says about Caffeine:
Caffeine has many metabolic effects. For example,