Many theories have instead focussed on thestrange, contagious nature of yawning – a fact that I know only too well frommy conversation with Provine. “Around 50% of people who observe a yawn willyawn in response,” he says. “It is so contagious that anything associated withit will trigger one… seeing or hearing another person, or even reading aboutyawning.” For this reason, some researchers have wondered if yawning might be aprimitive form of communication – if so, what information is it transmitting? We often feel tired when we yawn, so one idea is that it helps set everyone’sbiological clocks to the same rhythm. “In my view the most likely signallingrole of yawning is to help to synchronize the behaviour of a social group – tomake them go to sleep more or less at the same time,”saysChristian Hess, at the University of Bern in Switzerland. With the sameroutine, a group can then work together more efficiently throughout the day.

其他很多理论都将关注点集中于打哈欠那奇怪的、具有传染性的本质特点上——这一本质在我和普罗文的谈话中我已经得到了很好的体会。“在观察哈欠行为的人之中，大约有50%的人也会同时打哈欠，”普罗文说，“打哈欠行为的传染力如此之大，以致所有与其相关的事物都会引起另一个哈欠......不论是看到还是听到另外一个人打哈欠，或甚至是阅读有关哈欠的内容。”基于这个原因，一些研究者在想打哈欠可不可能是沟通交流的一种原始形式？如果是的话，打哈欠这种形式又是在传递怎样的信息呢？我们在打哈欠的时候常常会感到疲惫，所以有人认为打哈欠是为了帮助将每个人的生物钟调整到同样的节奏。“在我看来，打哈欠最有可能的信号作用是帮助同步化一个社会群体的行为，从而使得他们在同一时间里调整睡眠时间，”瑞士伯尔尼大学的克利斯汀·赫斯这样说。有了同样的生活节奏后，一个团队就能够在一天里更有效地工作。

Yet we also yawn during times of stress:Olympic athletes often do it before a race, while musicians sometimes succumbbefore a concert. So some researchers, including Provine, believe that thestrenuous movements might have a more general role in rebooting the brain –when you are sleepy they make you more alert, or when you are distracted theymake you more focussed. Spreading through a group, contagious yawns could thenhelp everyone reach the same level of attention, making them more vigilant to athreat, for instance. The mechanism is somewhat hazy –though oneFrench researcher, Olivier Walusinski, proposes that yawning helps to pumpcerebrospinal fluid around the brain, which could trigger a shift in neuralactivity.

我们在压力大的时候也会打哈欠：参加奥运会的运动员们经常会在比赛开始前打哈欠，音乐家在音乐会前有时也会这样。因此包括普罗文在内的一些研究者相信打哈欠这一费力的行为可能有着重新激活大脑的作用——当你困倦的时候打哈欠能够让你变得更加警觉，或者当你有所分心的时候它会让你集中注意力。比如，通过在一个群体里的扩散，感染性的哈欠能够帮助每一个人获得同样程度的注意力，从而使得他们对于威胁更加警惕。这一机制仍是不确定的，尽管有一位名叫奥利维耶·瓦卢辛斯基（Olivier Walusinski）的法国研究者提出打哈欠有助于促进大脑周围脑脊液的分泌，而这种液体能够引起神经活动的变化。

With so many competing and contradictoryideas, a grand unifying theory of yawning may seem like a distant speck on thehorizon. But over the last few years, one underlying mechanism has emerged thatcould, potentially, appease all these apparent paradoxes in one fell swoop.Andrew Gallup, now at the State University of New York at Oneonta, was firstinspired with the idea during his undergraduate degree, when he realised thatyawning might help to chill the brain and stop it overheating. The violentmovement of the jaws moves blood flow around the skull, he argued, helping tocarry away excess heat, while the deep inhalation brings cool air into thesinus cavities and around the carotid artery leading back into the brain.What’s more, the strenuous movements could also flex the membranes of sinuses –fanning asoft breeze through the cavities that should cause our mucus to evaporate,which should chill the head like air conditioning.

有这么多不同的观点相互竞争，要找到一个统一的哈欠理论的可能性非常小，小得就像视野里远处的微尘。但在最近的几年里，出现了一个相关理论，它拥有一次性化解所有这些互相矛盾观点的潜力。安德鲁·盖勒普来自奥尼昂塔纽约州州立大学，他最初萌生这一想法还是在他攻读学士学位期间，在那时候安德鲁·盖勒普意识到打哈欠可能具有冷却大脑，防止大脑升温过度的作用。他认为打哈欠过程中颌的大幅度运动促进了血液在头骨周围的流动，从而带走了过度的热量，同时打哈欠过程的深吸气又将较低温的空气带入了窦穴和颈动脉周围，颈动脉又将冷却后的血液输回大脑。此外，打哈欠这一大幅度动作还能够活动鼻窦的细胞膜，从而经窦穴引起较小的气流波动，这一波动会使得我们的粘液汽化，从而达到冷却大脑的作用，效果就像空调一样。

The most obvious test was to see if peopleare more or less likely to yawn in different temperatures. In normalconditions, Gallup found that around 48% felt the urge to yawn, but when heasked them to hold a cold compress to their foreheads, just 9% succumbed.Breathing through the nose, which could also cool the brain, was even moreeffective, completely dampening his subjects’urge to yawn –potentiallysuggesting a handy trick for anyone facing embarrassment during a tediousconversation.

最明显的测试就是去看人们在不同的温度下是否会改变打哈欠的频率。在正常条件下，盖勒普发现大约有48%的人会感觉到打哈欠的冲动，但是当他让他们将一块冷的敷布压在额头上时，这一数字减到了9%。通过鼻子呼吸也可以冷却大脑，这一方式来得更为有效，可以完完全全抑制住打哈欠的冲动。对于任何置身于冗长无聊的对话，但又想避免打哈欠的尴尬的人来说，这是一个很好的应对办法。