The caffeine in tea assists in its abilities to stimulate the metabolism and increase brain function and alertness, and it has the benefit of doing this gradually over a period of 10-15 minutes, providing a more subtle lift than your average caffeine beverage. Also, tea contains one-of-a-kind compounds such as theophylline, which utilize the caffeine to stimulate the respiratory system, heart, and kidneys.
The exact amount of caffeine found in teas depends on not only the variety, but through the steeping process as well. With the type of tea, it relies heavily on the length of time that the tea leaves are fermented. While decaffeinated teas are heavily-processed with chemicals, the same effect can be achieved naturally with near-equal results.
Black Teas, such as English Breakfast, Chai Spice, or my personal favorite, Earl Grey, contains the most caffeine, with an average of about 40-60 milligrams per 1 8-ounce cup of water. While the average may not be too high, the range is anywhere from 25-110 milligrams.
An Oolong, like China Oolong, Formosa Oolong, Bay Jong, or Ti Kuan Yin, averages between 30-45 milligrams per 8 ounces of water. Oolong caffeine levels can range from 12 - 55 milligrams.
Green Tea like Darjeeling generally averages about 15-20 milligrams per 8 ounces of water. Green tea can range anywhere from 8 - 30 milligrams.
The pricey White Tea - which is actually leaves and buds which have been steamed and dried, generally ranks just under Green Tea in its caffeine level, with an average 15 milligrams per 8 ounces of water. It therefore is a low caffeine alternative to those turned off by the "grassy" undertones sometimes associated with Green Teas. It ranges between 6 - 25 milligrams.
To put the above information into perspective, consider that a 5 oz coffee contains an average of 80 milligrams, with a range between 40-170, while a 12 oz cola averages at 45, with a range from 30 - 60, and 5oz of cocoa contains anywhere from 2 - 20 milligrams. Other organic products containing caffeine include Yerba Mata and Guarana.
As previously mentioned, the level of caffeine can depend on simply the length of time the beverage is steeped for, as shorter brewing equals less caffeine, but this can also be done through multi-steeping. Simply steep the tea in hot water for 45 seconds and pour off the liquid, pour off this liquid, and add more hot water to steep as usual. This brief infusion will release up to 80% of the caffeine from the tea, and little flavor and aroma will be sacrificed.
If this step seems like too much of a hassle, it is good to at least look for decaffeinated teas that have been processed with the natural method using water and CO2, as they retain about 95% of the polyphenols found in tea, compared to the 30% found in tea treated commercially with Ethyl Acetate. Polyphenol is one of the compounds in tea linked to many of the health benefits associated with tea.
It should also be noted that the smaller the tea leaf, the stronger the extraction of caffeine, and that the first and second leaves on the tea plant are believed to contain the largest amount of caffeine.