No single theory can explain why we yawn

Q: I get plenty of sleep. So why do I yawn all the time?

A: We all yawn frequently, more often in the early morning and late evening. Does it mean we're tired? Bored? Short on oxygen? As common as it is, we know little for certain about yawning.

We do know that yawning does not always indicate a need for sleep. It is true that people often yawn as they get ready to retire for the night. But we also yawn when we first arise in the morning and at other times during the day.

Past theories about why we yawn centered on the assumption that it was a reflex in response to low oxygen or high carbon dioxide levels. That's because breathing (which is what we do when we yawn) takes in oxygen and removes carbon dioxide. When you yawn, you take in more air than with a normal breath. So it's a reasonable theory that we yawn because we need more oxygen, or less carbon dioxide.

But reasonable theories often don't prove true. This particular theory lost favor after a study in which volunteers subjected to high oxygen levels did not yawn less, and after high carbon dioxide exposure did not yawn more.

Another theory of yawning is that it protects against a condition called atelectasis, which is the collapse of some of the lung's air sacs. The lungs are full of tiny little air sacs, but not all of them are filled with air. If an air sac remains without air, it's like a little collapsed balloon. Instead of being separated by air, the walls of the sac touch each other. If that goes on for very long, the walls can get sticky. It gets harder for the air sac to open when new air enters the lungs.

Yawning opens up tiny airways and prevents them from collapsing. So the theory that yawning is a reflex that protects against atelectasis is reasonable. This could explain why yawning seems to occur when your breathing is shallow, such as when you're tired or bored. However, we don't know if the theory is true.

Here's another theory: Yawning might be a warning system to alert you that you're getting sleepy and you had better stay awake. If you are driving a car, for example, and relaxing to the point where you might soon fall asleep, yawning might make you more conscious of the need to take a break.

Yawning is associated with stretching of the muscles and joints and an increased heart rate. That may make you more alert.

Finally, yawning may be a sign of disease. Although rarely the first sign, excessive yawning has been observed among people with multiple sclerosis, ALS (Lou Gehrig's disease) and Parkinson's disease.

To be clear, yawning is not usually a sign of disease. It's usually just a sign that you're human.

• Dr. Komaroff is a physician and professor at Harvard Medical School. For questions, go to AskDoctorK.com.