Q. Why do we yawn?

There are several theories about the cause of yawning. One of them has begun to gain more advocates. This theory is that we yawn to air-condition our brains.

In a recent paper published in the Journal of Medical Hypotheses, scientists Gary Hack of the University of Maryland and Andrew Gallup of Princeton University wrote that, during yawning, the sinuses act like bellows to cool off the brain. If you suffer from sinus congestion, you might yawn more.

"Brains, like computers, operate best when they are cool," the authors said.

The findings may explain why people yawn when they are tired. When you are deprived of sleep, the temperature of the brain rises.

Yawning is catching. We mimic yawners. Human beings are highly social and suggestible; we watch each other and emulate one another. In one yawning study, scientists found that applying cold packs to the subjects' heads almost eliminated contagious yawning. Nasal breathing, which also promotes brain cooling, had a similar effect.

Most animals yawn. Dogs, cats, rodents, birds, fish and snakes yawn. Fetuses at 11 weeks yawn.

Yawns seem to be caused by chemicals in the brain serotonin, dopamine, glutamic acid and nitric oxide. The more of these compounds activated in the brain, the greater the frequency of yawns.

Walter Smitson, professor of psychiatry at the University of Cincinnati, says that yawning is a way to express emotions.

"Often, for whatever reason," he said, "people are not comfortable verbalizing anger, boredom, disagreement or rejection. Thus, the yawn states for them, 'I'm rejecting you. I'm not interested in what you have to say. I'm not interested in you as a person.' It can serve as a passive-aggressive means to express hostility, anger or rejection when an individual isn't able to articulate those verbally."

Because a yawn can express anti-social feelings, people try to cover their mouths with their hands.

Smitson said that he notices that men yawn more than women. He theorizes that women are more socially aware than men.

Excessive yawning, however, can be a symptom for a health problem.

If you yawn too much, this may be a sign of a vasovagal reaction also known as vasovagal syncope, a common cause of fainting. The vagus nerve is located in your neck, chest and intestines. It regulates your heart and blood vessels. When it is stimulated, you begin to yawn excessively. You can also feel nauseous, lightheaded and break out into a cold sweat. Your heart beats slower and your blood vessels widens. This leads to low blood pressure and the risk of fainting.

A heart attack and a rupture of a large blood vessel can cause excessive yawning. These heart conditions can stimulate the vagus nerve, and a vasovagal reaction can result as the heart pumps less and blood pressure lowers.

Narcolepsy and hypersomnia are two disorders that cause constant sleepiness. Narcolepsy produces brief sleep attacks. Hypersomnia is characterized by excessive sleepiness at inappropriate moments. Excessive yawning becomes a physical symptom of these maladies.

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