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posted: 11/14/2011 6:00 AM

Why are you yawning?

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Q. I've been yawning a lot lately, and it got me thinking about what causes this phenomenon. Does anyone know why we yawn, and do you have any idea why yawns seem to be contagious?

A. Most people yawn several times per day, sometimes without even noticing it. In fact, almost all vertebrate animals yawn up to 10 times per hour, by some estimates. Yawning is most common in the early morning and late evening.

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What do these yawns mean? That you are tired? Bored?

The fact is, we don't know. It's probably a myth that yawning always means you're sleepy. People do often yawn as they are ready to go to sleep at night, but it also happens when we wake up in the morning and at other times of day. Nevertheless, I find myself yawning more often when I'm sleepy (or having trouble waking up).

Yawns do seem to pass from person to person, as if they were contagious. What causes this? It's probably the power of suggestion. We see someone yawn, and all of a sudden we also need to yawn. Maybe it's a behavior that is programmed into us. Maybe it's a way of showing someone else that we are connected and sympathetic to them.

None of the theories about why we yawn have been proven. But there are still lots of theories, anyway.

People used to think yawning meant our bodies weren't getting enough oxygen, because yawning forces us to take in more air. But a study in 1987 debunked this theory. When volunteers were exposed to high oxygen levels, they didn't yawn any less.

Some believe that yawning stretches out the lungs and nearby tissues, preventing tiny airways in the lungs from collapsing. Yawning may also help make us more alert after a period of relaxation, because it's associated with stretching of the muscles and joints and an increased heart rate. Both of those theories seem reasonable to me, particularly the first one.

My red-flag question to you, if you were my patient, would be, Has your yawning increased beyond the normal amount recently? Believe it or not, yawning may be a sign of disease.

Excessive yawning has been observed among people with multiple sclerosis, ALS (also called Lou Gehrig's disease), following radiation treatments, and among people treated for Parkinson's disease. Rarely, it may precede a migraine headache.

Don't misunderstand: If you find yourself yawning more often than usual, the chance that you have any of these diseases is still very small. I raise this point only to say that when a patient tells me that he or she is yawning more often than usual, I look harder for other symptoms.

That's why I think that noticeable changes in the amount you yawn may be worth mentioning to your doctor. Or maybe you just need to get more sleep.

Dr. Komaroff is a physician and professor at Harvard Medical School. Go to his website to send questions and get information: AskDoctorK.com.

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