Caffeine is a naturally occurring stimulant found in coffee beans, tea leaves, and cocoa pods. It acts as a central nervous system stimulant by blocking adenosine receptors in the brain, which increases alertness. Caffeine enters the bloodstream quickly and remains there for several hours, stimulating neural activity and reducing fatigue and drowsiness. However, regular high consumption of caffeine can lead to physical dependence and withdrawal symptoms like headaches when caffeine intake is reduced.