Here's some good information on tea and caffeine -- and coffee.
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Caffeine is one the component of tea and all types of tea (except for herbal) contain caffeine. However, the amount of caffeine varies according to type. In general, Green tea contains the least caffeine, Black tea the most, and Oolong falls in the middle range.
A cup of Green tea (6 oz) contains 8 to 36 mg of caffeine. A cup of Oolong tea contains 12 to 55 mg of caffeine. A cup of Black tea ranges from 25 - 110 mg. Coffee generally contains 60 - 180 mg of caffeine.Caffeine is not all bad. In moderate quantities, caffeine stimulates the central nervous system and promotes blood circulation. It stimulates the process of elimination and acts as a diuretic promoting better kidney function. There is some evidence that regular tea drinkers have a lower incidence of kidney ailments and gallstones. Caffeine has also been shown to speed reaction time, increase alertness, and improve concentration.