why do we yawn?

Answers to why do we yawn

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. Here are a few things that are known about yawns:

  1. The average duration of a yawn is about 6 seconds.
  2. In humans, the earliest occurrence of a yawn happens at about 11 weeks after conception - that's BEFORE the baby is born!
  3. Yawns become contagious to people between the first and second years of life.
  4. 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.

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.

As you might have expected, people who watched the color test bar pattern yawned more (5.78 yawns in 30 minutes) than those who watched the "MTV-like" video (3.41 yawns in 30 minutes.) The average duration of yawns was also slightly longer in the test bar viewing group. One unexpected finding was that yawns in male students had a longer duration than those in female students.

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 1987, Dr. Robert Provine and his coworkers set up an experiment to test the theory that high carbon dioxide/low oxygen blood content causes yawning. Air is normally made up of 20.95% oxygen, 79.02% nitrogen, 0.03% carbon dioxide and a few other gases in low concentrations. The researchers gave college students the following gases to breathe for 30 minutes:

Gas #1 = 100% Oxygen
Gas #2 = 3% Carbon dioxide, 21% Oxygen
Gas #3 = 5% Carbon dioxide, 21% Oxygen
Gas #4 = Normal Air

Breathing 100% oxygen (Gas #1) or either carbon dioxide gas (Gas #2 and #3) did cause the students to breathe at a faster rate. However, neither carbon dixoide gas nor 100% oxygen caused the students to yawn more. These gases also did not change the duration of yawns when they occurred.

The researchers also looked for a relationship between breathing and yawning by having people exercise. Exercise, obviously, causes people to breathe faster. However, the number of yawns during exercise was not different from the number of yawns before or after exercise. Therefore, it appears that yawning is not due to CO2/O2 levels in the blood and that yawning and breathing are controlled by different mechanisms.

So, the question remains - why do we yawn? Dr. Provine suggests that perhaps yawning is like stretching. Yawning and stretching increase blood pressure and heart rate and also flex muscles and joints. Evidence that yawning and stretching may be related comes from the observation that if you try to stifle or prevent a yawn by clenching your jaws shut, the yawn is somewhat "unsatisfying." For some reason, the stretching of jaw and face muscles is necessary for a good yawn.

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.

It is possible that yawns are contagious because at one time in evolutionary history, the yawn served to coordinate the social behavior of a group of animals. When one member of the group yawned to signal an event, all the other members of the group also yawned. Yawns may still be contagious these days because of a leftover response (a "vestigial" response) that is not used anymore. None of this has been proven true and yawns are still one of the mysteries of the mind.

Digging deeper
THE TRUTH IS that we don’t completely understand why people, or animals for that matter, yawn.

It’s widely assumed that yawning occurs because we are tired or bored or because we see someone else doing it, but there isn’t any hard evidence to support these beliefs.

Scientists do not purport to know all of the biological mechanisms of the yawn, but tend to agree that a yawn is an involuntary respiratory reflex, which regulates the carbon dioxide and oxygen levels in the blood.

Technically, a yawn is the reflex opening of the mouth followed by the deep inhalation and slow exhalation of oxygen.

The very act of yawning is but one of a number of involuntary reflexes controlled by the spinal and nerve centers.

Scientists speculate that the onset of a yawn is triggered either by fatigue, or by sheer boredom as, at those times, breathing is shallow, and little oxygen is carried to the lungs by the oxygen-toting cardiovascular system.

When one yawns, his or her alertness is heightened, as the sudden intake of oxygen increases the heart rate, rids the lungs and the bloodstream of the carbon dioxide buildup, and forces oxygen through blood vessels in the brain, while restoring normal breathing and ventilating the lungs.

This quite plausible theory of yawning falls short of explaining many aspects of yawning. Scientists explain away the "contagious" nature of yawning, that is when one person's yawn triggers another nearby to yawn, as due to the power of suggestion, but are at a loss when attempting to explain why yawning occurs excessively in patients with lower brainstem damage or with multiple sclerosis.

Other unlocked mysteries include why fetuses in the womb yawn, when it is a well-known fact that they do not intake oxygen into their lungs until after live birth, or why individuals with high concentrations of oxygen in their blood streams yawn.

Until these questions are answered, do not assume that a person who yawns in your presence is bored with what you are saying, or suffers from exhaustion. Simply be pleased that he or she is not bored to death.

We don't know exactly why we yawn yet
No one knows why we yawn. There are lots of theories, some talk about it signalling tiredness or getting oxygen to the brain, others mention clearing out stale air from the lungs and reducing blood carbon dioxide levels. Most are baloney. But, one thing that is certain, yawns can be infectious. Catch sight of someone yawning, and nine times out of ten, you will yawn yourself within a few seconds. But, that still doesn’t really answer the question, why do we yawn? Is such an infectious yawn a message to others in the group that it’s time for bed? Probably not, otherwise why yawn first thing on getting out of bed? Either way researchers have found that people with autism spectrum disorder don’t tend to succumb to an infectious yawn.

Atsushi Senju of the Centre for Brain and Cognitive Development at Birkbeck, University of London has shown for the first time that children with some degree of autism are not susceptible to contagious yawning. Autism Spectrum Disorder is a developmental disability that severely affects social interaction and communication including empathy. Report published in the August issue of the Royal Society journal Biology Letters.

This would seem an obvious result given that contagious yawning is thought to share similar cognitive and neural mechanisms as empathy.

Senju and colleagues from the University of Tokyo showed videos of people yawning or making mouth movements to 24 children with autism spectrum disorder and to 25 non-ASD children. Both groups of children yawned the same number of times while watching the video of general mouth movements, but the non-ASD children yawned more when watching the video of people yawning.

“This is the first report that a neuropsychological or psychiatric condition can selectively impair contagious yawning, sparing spontaneous yawning,” explains Senju, “Our study confirms the prediction of ‘empathy theory’, by demonstrating that individuals with autism, who show atypical developments in empathy, also show selective impairment in contagious yawning.”

None of this answers the question of why do we yawn in the first place? Apparently, yawning becomes contagious at around one to two years of age, although unborn fetuses also yawn (presumably not contagiously though!) and can be triggered in animals by stimulating the hypothalamus in the brain with injected dopamine, excitatory amino acids, nitric oxide, and neuropeptides. None of this really explains why we yawn. The empathy angle perhaps points to an ancient benefit in group behaviour, but what that benefit is, science does not yet know.

New Meanings
The common wisdom is that people yawn because they need oxygen, but the researchers at the University of Albany in New York said their experiments showed that raising or lowering oxygen and carbon dioxide levels in the blood did not produce that reaction.

Their evidence suggested instead that drawing in air helps cool the brain and helps it work more effectively.

In a study of the 44 students, researchers found that those who breathed through the nose rather than the mouth were less likely to yawn when watching a video of other people yawning.

This was because vessels in the nasal cavity sent cool blood to the brain, they said.
The same effect was found among those who held a cool pack to their forehead, while those who held a warm or room-temperature pack yawned when watching the video.

“Since yawning occurs when brain temperature rises, sending cool blood to the brain serves to maintain optimal levels of mental efficiency,” the authors wrote.

“So the next time you are telling a story and a listener yawns there is no need to be offended - yawning, a physiological mechanism designed to maintain attention, turns out to be a compliment.”

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