Green tea, as well as oolong, white tea, & black tea, are the leaves of the plant Camellia sinensis. Caffeine is naturally occuring in the Camellia sinensis, however the caffeine (or theine in tea) is a third to a half the amount found in a standard coffee. Black tea has more caffeine than green tea, and green tea has more than white tea varieties. White tea has minimal caffeine. Oolong has between green and black teas in caffeine content.
Unlike the rush and crash from caffeine in coffee, it is claimed that 80% of the caffeine in tea remains unabsorbed by the human body, yet it can have a number of benefits such as stimulating the nervous system & improving circulation.
Theanine in high quaity green teas can also improve mental cognition, lift the mood, and increase brain seratonin and dopamine levels.