Although black tea still has a lot of caffeine, it still contains some amino acids, mainly including theanine, glutamic acid, and gamma-aminobutyric acid. When we drink tea, if we taste it carefully, we can feel a touch of sweetness. This sweetness taste comes from amino acids. These three amino acids can be transformed into each other under certain conditions. Theanine and γ-aminobutyric acid both have the functions of calming, promoting sleep, and enhancing memory. Studies have shown that theanine has the effects of promoting sleep, enhancing brain function and lowering blood pressure.