Some people experience heart palpitations and nervousness from the caffeine found in green tea. Although it generally contains less than coffee or black tea, some people are sensitive to even small levels of caffeine. There are two ways to avoid this problem. One is to drink decaffeinated green tea. The other is to buy high quality Japanese green tea such as gyokura. Better quality Japanese green tea contains higher levels of theanine - a compound which has a calming effect on the brain and reduces the stimulating effects of the caffeine.