why do we yawn?

I don't know why I yawn but I am sure that when I am bored or tired, I yawn a lot. And it is very commonly noted that when you yawn in front of other people they also tend to yawn.
A definition of yawning may be that "Yawning is an involuntary action that causes us to open our mouths wide and breathe in deeply."

Everyone yawns - babies, kids, teenagers, adults. Some birds, reptiles and most mammals also yawn. However, the reason why we yawn is a bit of a mystery. There is also very little research about yawning because for most people yawning is not a problem.

Interesting Yawning Facts

- The average duration of a yawn is about 6 seconds.
- In humans, the earliest occurrence of a yawn happens at about 11 weeks after conception - that's BEFORE the baby is born!
- Yawns become contagious to people between the first and second years of life.
- A part of the brain that plays an important role in yawning is the hypothalamus.
- Research has shown that some neurotransmitters (for example, dopamine, excitatory amino acids, nitric oxide) and neuropeptides increase yawning if injected into the hypothalamus of animals.
- Your heart rate can rise as much as 30 percent during a yawn.
- 55 percent of people will yawn within five minutes of seeing someone else yawn.
- Blind people yawn more after hearing an audio tape of people yawning.
- Reading about yawning will make you yawn.
- Olympic athletes often yawn before competition.

While the dictionary tells us that yawning is caused by being fatigued, drowsy or bored, scientists are discovering that there is more to yawning than what most people think.

The Physiological Theory -- Our bodies induce yawning to drawn in more oxygen or remove a build-up of carbon dioxide. This theory helps explain why we yawn in groups. Larger groups produce more carbon dioxide, which means our bodies would act to draw in more oxygen and get rid of the excess carbon dioxide. However, if our bodies make us yawn to drawn in needed oxygen, wouldn't we yawn during exercise? Robert Provine, a psychologist at the University of Maryland, Baltimore County, and a leading expert on yawning, has tested this theory. Giving people additional oxygen didn't decrease yawning and decreasing the amount of carbon dioxide in a subject's environment also didn't prevent yawning.

The Evolution Theory -- Some think that yawning is something that began with our ancestors, who used yawning to show their teeth and intimidate others. An offshoot of this theory is the idea that yawning developed from early man as a signal for us to change activities.
The Boredom Theory -- In the dictionary, yawning is said to be caused by boredom, fatigue or drowsiness. Although we do tend to yawn when bored or tired, this theory doesn't explain why Olympic athletes yawn right before they compete in their event. It's doubtful that they are bored with the world watching them.

You know that when you are bored, you yawn. Scientists have confirmed this observation by comparing the number of yawns in 17-19 year old students who watched music videos to the number of yawns in students who watched an uninteresting color test bar pattern.
Many people assume that we yawn because our bodies are trying to get rid of extra carbon dioxide and to take in more oxygen. This may make some sense. According to this theory, when people are bored or tired, they breathe more slowly. As breathing slows down, less oxygen makes it to the lungs. As carbon dioxide builds up in the blood, a message to the brain results in signals back to the lungs saying, "Take a deep breath," and a yawn is produced.

The only problem with the excess carbon dioxide theory is that research shows that it may not be true.

In 2007, researchers proposed that yawning is used to cool the brain. They found that people yawned more often they pressed a warm or room temperature towel against their heads than when they pressed a cold towel against their heads. People who breathed through their noses (thought to reduce brain temperature) did not yawn at all.

Sources:
* What makes us yawn? at "How Stuffs Work?"

* From "Neuroscience for kids": 'Yawning'

Related posts:
* Carry on yawning - it will help keep you awake

* What does Yawning and Autism Have in Common?