Caffeine is the main component of the "bitter" taste of tea. It is more abundant in the shoots than in the stems, which is why gyokuro, which uses many shoots, contains more caffeine.

Caffeine is effective in relieving fatigue, stimulating alertness, acting as a diuretic, and accelerating metabolism. Drinking a cup of tea while studying or working can help improve work efficiency. Tea contains theanine (an amino acid), which moderates the effects of caffeine, making it milder than the same amount of caffeine.

Caffeine dissolves more easily at higher water temperatures and less easily at lower temperatures, so experiment with brewing methods, such as using hot water if you want the caffeine to be more effective and cold water if you want it to be less effective.