The rich umami flavour found in high-grade sencha, gyokuro, and matcha can be credited to the high levels of theanine found in the tea leaves. In fact, the majority of the amino acids found in Japanese green tea are theanine.
Aside from adding to the flavour profile, theanine is known to be good for improving mental impairment, as well as possessing relaxing qualities. The consumption of theanine produces α-brain waves, which reduces stress levels and allows the drinker to tap into a calmer state. Due to this effect, tea is also sometimes used as to aid in restful sleep.
Moreover, when combined with caffeine, the effects of both the amino acid (theanine) and stimulant (caffeine) are released slowly. The consumption of tea allows the drinker to indulge in these properties at a gentler pace.