Caffeine is an alkaloid. Found in more than 60 plant species, caffeine acts as a natural insecticide by paralyzing and killing insects that eat them. Coffee and tea contain caffeine, and cocoa plants contain somewhat less.
Other sources of caffeine include yerba mate and guarana, plants often used in energy drinks.
Humans value caffeine for its stimulative effect on the central nervous system. It enters the bloodstream quickly, around 10 minutes after hitting the stomach, and it can have effects for up to 12 hours. Caffeine and other alkaloids have powerful effects after ingestion, including stimulating heart muscle, relaxing some smooth muscles and acting as a diuretic.