What causes a yawn?

Some people yawn when they are bored, tired, or in the presence of another yawner. Yawning happens when a person is not getting enough oxygen. Oxygen, the gas needed to run body processes, and carbon dioxide, the waste gas produced by these processes, travel in your bloodstream and enter and exit your body through your lungs. When you don’t breathe deeply enough, too much carbon dioxide builds up in your body and your brain gets the message, telling you to breathe more deeply to fix the problem. A yawn is a great big breath that clears carbon dioxide from your lungs and forces you to take in fresh, oxygen-rich air.

Yawning is a reflex action that is your body’s way of telling you that you need more oxygen.