A stimulant is a substance that speeds up your body's systems, making you feel more alert, awake, and energetic. The word comes from stimulate, which means to increase activity or make something work harder and faster.

The most common stimulant is caffeine, found in coffee, tea, and many sodas. When someone drinks coffee in the morning, the caffeine acts as a stimulant, helping them feel less drowsy and more focused. Your heart might beat a little faster, and your brain might feel sharper and more ready to concentrate.

Some stimulants are medicines that doctors prescribe to help people who struggle with attention or staying awake. Other stimulants are dangerous drugs that can seriously harm the body and brain, causing addiction and other health problems.

The opposite of a stimulant is a depressant, which slows down your body's systems and can make you feel calm or sleepy. While a cup of hot cocoa before bed might help you relax, that morning cup of tea acts as a stimulant to help you wake up. Understanding how stimulants work helps explain why doctors tell people not to drink caffeinated sodas late at night: the stimulant effect can make falling asleep much harder.