The caffeine contained in tea can excite the nerve center and produce a refreshing effect. The degree of excitement depends on the amount of it. In addition, caffeine also relieves fatigue and resists the poisonous effects of alcohol, tobacco, and morphine. After all, the side effects caused by caffeine are temporary. Caffeine does not accumulate in the human blood, and is usually excreted from the body with human metabolism within a few hours.
--China Food Technology Network