What is the natural stimulant most commonly found in tea?

Caffeine is naturally found in the leaves and fruits of some plants. It is in coffee, black and green tea, cocoa, cola soft drinks and energy drinks. It may also be in chocolate bars, energy bars and some non-prescription medications, such as cough syrup and slimming tablets. Guarana (a popular additive in energy drinks) is also a natural source of caffeine. Caffeine is a stimulant, which means it increases activity in your brain and nervous system. It also increases the circulation of chemicals such as cortisol and adrenaline in the body.

Source: Better Health Channel