Caffeine is a naturally occurring stimulant, which is found in over 60 plants, including coffee beans, tea leaves, and cacao. Caffeine stimulates the central nervous system and suppresses the effects of adenosine on the brain. Adenosine causes drowsiness by suppressing the arousal of certain nerve cells.
There are two types of caffeine: natural and synthetic. Natural caffeine is found in plants such as coffee beans, tea leaves, and cocoa beans. Synthetic caffeine is man-made from substances such as coal tar or from chemical reactions that break down organic substances such as cornstarch into sugars that can then be processed into a white powder.
How Much Caffeine is in Types of Tea (High to low)