Caffeine is a naturally occurring substance belonging to the methylxanthine group that has a stimulating effect on the central nervous system. It temporarily reduces drowsiness, increases alertness, and may improve concentration by blocking the effects of adenosine at its receptors.
Caffeine is found in coffee, tea, cocoa, chocolate, certain soft drinks, energy drinks, and some medications. Excessive consumption may cause insomnia, anxiety, irritability, tremors, and a rapid heartbeat. Sensitivity to caffeine varies from person to person.