All of us yawn, and no one knows why

Q. I yawn a lot. My friend said this is a sign my brain isn't getting enough oxygen. Is that true?

A. Most of us yawn more often in the early morning and late evening. Does it mean you're tired? Bored? Not getting enough oxygen? It turns out that we actually know very little about why we yawn.

When I was in medical school, one of my teachers speculated that yawning was a response to low oxygen or high carbon-dioxide levels. That theory was fairly common. It was also plausible: When we open our mouths and take in a deep breath, we take oxygen into the body and expel carbon dioxide. Carbon dioxide is waste produced by the body's cells and needs to be eliminated.

Unfortunately, the theory that yawning reflects low oxygen or high carbon dioxide levels isn't true. Yawning occurs even when oxygen and carbon dioxide levels are normal. And research has shown that volunteers do not yawn less after being exposed to high oxygen levels, and do not yawn more after being exposed to high levels of carbon dioxide.

Another myth is that yawning always indicates a need for sleep. It is true that people often yawn as they are ready to retire for the night. But we also yawn when we get up in the morning and at other times during the day. Yawning appears to depend on a variety of factors such as arousal level, distraction, and even seeing someone else yawn.

Here are some reasonable explanations I've heard for yawning (though none has been proven):

• The lungs are full of tiny little air sacs. Not all of them are filled with air. If an air sac remains without air, it can collapse. When you yawn, you take in more air than with a normal breath. That opens up tiny airways and prevents them from collapsing. This could explain why yawning seems to occur when your breathing is shallow, such as when you're tired or bored.

• Yawning is associated with stretching of the muscles and joints and an increased heart rate. So it may serve as a preparation for an increased level of alertness, especially after a period of relaxation.

• Yawning could provide nonverbal communication to others that it is time to relax.

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. I say this with trepidation. Please don't misunderstand: Yawning is not a sign that you have one of these terrible diseases. It is just a sign that you're human.

In fact, we humans are in good company. All mammals and many other animals yawn. Why does a lion yawn? A penguin? We can't answer that question any better for them than for us.

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