The myth of green tea having less caffeine than black tea is rampant in our culture. This is even more surprising when contrasted with the Eastern attitude that drinking a fresh green tea is much more likely to raise your energy level than a delicately roasted oolong.
There is a very complex series of factors that go into the amount of caffeine in a cup. Soil, Terroir, sunlight, leaf size, tippiness of a tea, age, roast, and infusion temperature all come in to play. The only part which can be easily controlled by the consumer, however, is steep time. A longer infusion means a stronger tea. Quite clear, however, is that oxidation does not affect caffeine. This means that all types of tea (green, white, yellow, oolong, black, and puer) have roughly the same amount of caffeine by weight.
While infusing tea for a shorter time will decrease its caffeine levels, the quality of the leaf also matters greatly. A full-leaf, unbroken green tea is going to release its alkoloids much more slowly than a roughly-treated broken tea (such as you might find in a tea bag). Broken leaves or even lower-quality “dust and fannings” will usually make a cup of tea that is blunt and bitter as well as much more caffeinated than its full-leaf brothers.
Finally (and this article barely scratches the surface), there are many other compounds within the tea leaf that contribute to how it affects the body. Tea leaves are one of the only sources of the amino acid Theanine which reduces stress on the body, making even many average 30mg-of-Caffeine cups of tea quite a different physical experience than the average 150mg-of-Caffeine cup of coffee. Looking at Caffeine content alone is not sufficient to determine the physical effects of any beverage.
So the next time you’d like to decrease your caffeine intake, try a roasted oolong, an aged puer, or a tea with very few tips. Try to steep your favorite tea for much less time. Even better, as your body is unique, research and experiment with different teas and find what is true for you.