why do people yawn when their tired? what causes it?

Answer 1

Here are a few links that discuss yawning and it's causes. Hope it helps. I don't think there is any definitive answer to this question at this time. But there are a couple pet theories which are discussed here and arguments for and against them. The best of these theories is this quote:
"Now, the theory goes like this: The inflation of the patient's lungs acted as a simulated yawn, which had the effect of redistributing the alveolar surfactant. Therefore, the possible purpose of yawning is help in surfactant redistribution. It may be that our bodies spontaneously inflate our lungs to refresh or speed up the surfactant redistribution. Anecdotal evidence supports this role, for every animal that uses a surfactant like substance in their respiratory systems to decrease surface tension and prevent lung edema, yawns, including fish. Now, why people tend to yawn when they are tired, or how/why yawning seems to be contagious remains a mystery, but may be related to a stimulus modifiable reflex. Hope this answers it for you!" from http://www.madsci.org/posts/archives/aug97/867090165.Me.r.html

Answer 2

Yawning is extremely contagious -- 55% of people who witness someone yawn will yawn within five minutes. If a visually impaired person hears a tape of someone yawning, he or she is likely to yawn as well. Face it, the likelihood of you making it to the end of this answer without looking like one of these gaping maws is unlikely.

Although the contagious nature of yawning is well established, we know less about why this is so. Researchers are currently giving the topic some serious attention. One theory suggests it's a holdover from a period in evolutionary history when yawning served to coordinate the social behavior of a group of animals. A recent study postulates that contagious yawning could be part of the "neural network involved in empathy."

While the mystery of contagious yawning has yet to be solved, perhaps researchers are closing in on an answer. On the other hand, given the subject matter, we wouldn't blame them for falling asleep at the wheel. In the meantime, give the "yawn challenge" a try -- it's tougher than it looks.

Answer 3

It's actually an evolutionary trait shared by many animals, including humans, and not limited to mammals either. Yawning is not exclusive to being "tired" but is more of an indicator of a need for a change in whatever state of being one is in.

"Yawning" is contagious in this sense as the members of whatever type of group is displaying this behavior will respond and tend to move on to the next set of behaviors that will change the overall state of being.

Answer 4

It is caused by lack of oxygen to the brain. Yawning is an involuntarily response to this, by taking a deep breath more oxygen is taken in.

Answer 5

We yawn because oxygen levels in our lungs are low.
But certain aspects of yawning remain even more mysterious. Fetuses, for instances, have been observed yawning in the womb, yet it’s known that they don’t take oxygen in through their lungs. And yawning seems to be a symptom of multiple sclerosis and other medical conditions, for reasons unclear.

Answer 6

Yes, and yawning forces oxygen to the brain. Just like stretching forces blood into your muscles (why it feels so good after you have slept or haven't moved in a while).

Answer 7

When people are tired, they breath less deeply (because they are tired and being lazy). So, they yawn to get more oxygen to their brains.

Answer 8

ppl yawn due to the lack of oxygen

Answer 9

wikipedia provides a good article on the hypothetical reasons for yawning...http://en.wikipedia.org/wiki/Yawning

Answer 10

Lack of oxygen