Caffeine is a natural chemical found in coffee, tea, and chocolate that makes people feel more awake and alert. When you drink a cup of hot cocoa or your parents brew their morning coffee, caffeine enters the bloodstream and travels to the brain, where it blocks the signals that make you feel sleepy.

People have been using caffeine for thousands of years. Ancient Chinese emperors drank tea to stay sharp during long meetings. Arab scholars sipped coffee while studying late into the night. Today, caffeine appears in many drinks and foods: cola, energy drinks, coffee, black and green tea, and even some types of candy.

Caffeine affects different people in different ways. Some adults need their morning coffee to feel fully awake, while others feel jittery if they have too much. Kids are generally more sensitive to caffeine than adults, which is why parents often limit how much soda or chocolate their children consume, especially before bedtime.

The effects of caffeine wear off after a few hours as your body breaks it down. That's why someone who drinks coffee at breakfast usually feels normal again by lunchtime. Scientists consider caffeine the world's most widely used stimulant, a substance that increases alertness and energy.