What is caffeine?
Caffeine is a natural stimulant found in coffee, tea, cocoa and many soft and energy drinks. It makes you feel more alert by blocking a brain chemical that causes sleepiness.
During the day, a chemical called adenosine builds up in the brain and makes us feel tired. Caffeine has a similar shape and blocks adenosine's receptors, so we feel more awake. Its effects usually start within about 15 to 45 minutes.
Plants such as coffee, tea and cacao produce caffeine naturally, possibly to discourage insects. It is the most widely used mind-altering substance in the world.
How much is safe?
For most healthy adults, the US Food and Drug Administration says up to about 400 milligrams of caffeine a day, roughly four cups of brewed coffee, is not generally linked with harmful effects. Too much can cause jitters, a fast heartbeat, upset stomach and trouble sleeping. Pregnant people and children are advised to have less.
Did You Know?
Caffeine's effects can last for hours. Its half-life, the time for the body to clear half of it, is about five hours in many adults, which is why late-afternoon coffee can disturb sleep.