Everyone yawns on a daily basis. We yawn when we wake up, when we are bored or tired during the day, and even when we see someone else yawn. It is universal to the human race, but what causes this involuntary act?
Yawning is still a mystery in some ways. We know that it is involuntary and that it is often caused by fatigue or drowsiness, but scientists have not yet determined if it actually serves a purpose. There are various theories that attempt to explain the cause and purpose of yawning.
One theory is based on physiology. The idea behind the physiological theory is that we yawn to increase our body's oxygen and release excess carbon dioxide. In one way, this does make sense, but from a different perspective this theory can be disproved, and in fact, has been. A psychologist from the University of Maryland, Baltimore County named Robert Provine conducted a study in 1987 to test this theory. At the end of his study, he came to the conclusion that providing people with extra oxygen did not decrease the frequency at which yawning occurred. He also noted that decreasing the amount of carbon dioxide available to a person did not prevent yawning.
Another theory is based on evolution. Some people believe that many years ago, our ancestors used to open their mouths and show their teeth to others in order to intimidate them. Therefore, some people have adapted this theory and have explained it with the idea in mind that we open our mouths to express that we would prefer a change in activity.
This particular theory does make sense if you think about it in various scenarios. For example, we yawn when we are tired at night right before we want to go to sleep. Perhaps, we are not yawning simply because our bodies are tired, but because we are expressing our desire to sleep. Maybe we yawn on the job or in class because we would much rather be at the beach or at home relaxing in front of the television. We all yawn in the morning as well. Maybe this is our bodies telling us it is sick of laying around and not doing anything and would like to get up and be active again. Who knows? Quite interesting theory though, I must say!
Perhaps our ancestors yawned to express that they agreed with the people around them. Maybe this is why yawning is contagious. Maybe it is our way of telling people that we feel similarly.
Humans are not the only species that yawn. Many animals do as well. Perhaps this is evidence that yawning may be some kind of form of communication, which would support the evolution theory as well.
Lastly, there is the well-known boredom theory of yawning. This theory simply states that yawning is caused by boredom, fatigue, or drowsiness. Yet, there are plenty of times during the day when people yawn, but are clearly not tired or bored at all. Therefore, this theory does not prove much and needs better evidence to support the idea behind it.
Yawning is such a common act amongst humans and animals alike, yet there is so much left unanswered on this topic. Various studies have been conducted and theories have been created, but no one knows for sure the exact reason why we yawn. It still remains a mystery.
Sources:
http://health.howstuffworks.com/question572.htm
http://www.yesmag.bc.ca/Questions/yawn.html
译文:
至今未解开的谜团——打哈欠
基本上，我们每个人天天都会打哈欠。当我们醒来的时候，我们会打哈欠；而当我们觉得无聊或者疲惫的时候，我们也会打哈欠；甚至，当我们看到其他人在打哈欠时，我们自己也会跟着打哈欠。这对人类来说是非常普遍的现象，但是，是什么导致了这种行为呢？
在某些方面，打哈欠仍旧是个未解之谜。我们知道那是自然而然的行为，也知道那是因为疲惫和困倦而引起的，但科学家尚未确定是否真的是因为这样的原因。已经有许多不同的理论，试着去解释打哈欠的原因和目的。
其中一条理论是基于生理学的。建立在生理学理论上的观点是，我们打哈欠是为了增加我们体内的氧气，并且释放多余的二氧化碳。一方面，这个理论的确是有意义的，但是从另一个不同的观点来看，这个理论也是可以被推翻的。而事实上，的确如此。来自巴尔的摩城马里兰大学的心理学家Robert Provine，他在1987年进行了一项研究去测试这个理论。在此项研究快要结束时，他得出结论：当打哈欠时，它提供给人们额外的氧气，并且也没有减少其频率。一个人所减少的那些二氧化碳并不足以阻止人们打哈欠。
另一项理论是基于演化的基础。很多人认为，在许多年前，我们的祖先习惯张大他们的嘴表达他们的怒气，由此让其他人感到害怕。因此，一些人改编了这个理论，用这个理论去解释说，我们张开嘴巴是表示我们更喜欢在行为中有所改变。
这一特别理论对你是否以不同的想法来思考这个问题有重要意义。比如，在我们想去睡觉之前，我们觉得疲惫时便会打哈欠。也许，我们并是不单纯地打哈欠，而是我们在表达我们的身体疲惫了，我们表达了我们渴望睡觉这一感觉。也许我们在工作时或者上课时也会打哈欠，因为那时的我们其实更希望可以在海边、或者呆在家里的电视机前，放松自己。我们也会在早上打哈欠。也许这是我们的身体在告诉我们，不应该这样躺着无所事事，而是应该快点起来做事情。谁知道呢？但是我必须说，这是非常有趣的理论！
也许我们的祖先打哈欠是为了表示，他同意他周围的人。或许这也是为什么打哈欠会传染的原因之一。可能这是我们告诉人们我们也是这样认为的一个方式。
人类并不是唯一会打哈欠的物种。许多动物也会打哈欠。也许，这又可以证明，打哈欠是交流的一种形式，这同时也支持进化理论。
最后，有一条众所周知的关于打哈欠的无聊理论。这个理论简单地陈述了打哈欠不过是由于无聊、疲惫、困倦而引起的。然而，在白天，人们大多数在打哈欠的时候，明显不是因为疲惫或者无聊。因此，这个理论并没有足够多及足够好的证据，在背后支撑这条理论。
打哈欠在人类和动物之间，是一个很普遍的行为，而在这个主题中，仍旧有许多无法解答的问题。目前已经进行了许多不同的研究，并且建立了不同的理论，但是，没有人能肯定“我们为什么会打哈欠”这一确切的原因。这仍旧是一个未解之谜。
来源：http://health.howstuffworks.com/question572.htm
http://www.yesmag.bc.ca/Questions/yawn.html