Why do we yawn?
Everybody yawns: babies, grown-ups, and even dogs and cats. Scientists still do not know every reason, but they have ruled some ideas out and found some good clues.
It is probably not about oxygen
For a long time, people said we yawn to get more oxygen. Scientists tested that. They had people breathe pure oxygen, or air with extra carbon dioxide, and even had them exercise. Breathing sped up, but yawning did not change. That suggests yawning and breathing are controlled by different parts of the body's system.
A cooling idea
One leading idea is that a big yawn helps cool the brain a little, a bit like a fan. A wide stretch of the jaw and a deep breath of air may help move warm blood away. Yawning also seems to show up when the body changes state, like waking up or getting sleepy. Scientists are still testing these ideas.
Why yawns are catching
Have you ever yawned right after someone else did? That is called a contagious yawn. In one study, very young children rarely caught yawns, but it became much more common around age 4. Scientists think catching yawns may be linked to how we copy and connect with the people around us.
Yawns before birth, and in animals
Babies can yawn before they are born. Doctors have seen it on ultrasound scans. Many animals yawn too. One study found that animals with bigger brains tend to have longer yawns. Primates, the animal group that includes people, monkeys, and apes, tend to have long yawns.
When to talk to a doctor
Yawning when you are tired or bored is normal. If someone yawns much more than usual for no clear reason, or is very sleepy during the day, it is worth talking to a doctor. For better sleep habits, see How much sleep do you really need?
This is general information. For your own health, talk to a doctor.