It's easy to be offended when a colleague yawns while you're talking. But that yawn may not mean what you think.
人们很容易会被这样一种行为惹恼：当你说话的时候，你的同事却在打着哈欠。但对于这个哈欠，也可能是你想得太多了。
A growing number of researchers believe the purpose of this little-understood behavior is to cool the brain, says a research review published earlier this year in Frontiers in Neuroscience.
今年早些时候发表在《神经科学前沿》(Frontiers in Neuroscience)上的一项研究综述称，越来越多的研究人员认为，打哈欠这种人们对其还知之甚少的行为，其目的在于为大脑降温。
Changes in climate affect how often people yawn. Researchers in an earlier study conducted outdoor interviews with two groups of people in Tucson, Ariz., one in early summer and one in the winter. People were asked to look at pictures of people yawning and talk about their own yawning behavior.
气候的变化会影响到人们打哈欠的频率。研究人员在之前的一次研究中对亚利桑那州图森市(Tucson)的两组人进行了户外采访，一次是在初夏，另一次是在冬天。研究人员让被访者看了一些人们打哈欠的照片，并谈了谈他们自己的打哈欠行为。
People were nearly twice as likely to yawn when they were surveyed during the winter, when they could inhale cool air to reduce the temperature of the brain, says the study, published in 2011 in Frontiers in Evolutionary Neuroscience. Participants yawned less when surveyed in the early summer, when temperatures outdoors were about the same as the human body.
该项研究的论文2011年发表在《进化神经科学前沿》(Frontiers in Evolutionary Neuroscience)上，文章称，这些人在冬天接受调查时打哈欠的可能性是初夏时的将近两倍，在冬季，他们打哈欠能吸入冷空气来给大脑降温。而在夏初接受调查的参与者哈欠较少，因为那个时节的室外气温跟人的体温几乎是一样的。
Other studies show yawning increases after people experience heat stress or have a heat pack placed on their foreheads.
其他一些研究则表明，在人们中暑或在前额贴上发热贴以后，哈欠就会增多。
Yawning also may build empathy within groups. Yawns are seen as contagious, but 'catching' a yawn depends on a person's ability to feel empathy and closeness with the yawner, says a 2013 research review in the International Journal of Applied Basic Medical Research.
2013年发表在《应用基础医学研究国际期刊》(International Journal of Applied Basic Medical Research)上的一项研究综述称，哈欠可能还会在人群中形成共鸣。哈欠被认为是具有传染性的，但是否能“接过”一个哈欠取决于个人感同身受的能力以及与打哈欠者的亲密程度。
People observed in workplaces or restaurants yawned more often in response to others' yawns when they were kin or close friends with the yawner, the study says. People are less likely to engage in contagious yawns when they have emotional or social disorders that prevent them from feeling empathy, the study says.
该研究称，据观察，在工作场所或餐馆里若有人打了哈欠，他们的亲朋好友会更多地也打起哈欠。但那些因患有情绪或社交障碍而无法设身处地理解他人的人则不太可能会被别人的哈欠所感染。
What yawns don't accomplish, researchers say, is to re-energize the brain.
研究人员还称，打哈欠无法让大脑重新活跃起来。
For centuries, yawning was thought to remove 'bad air' from the lungs and increase oxygen circulation in the brain. Recent studies have discredited this old belief.
几个世纪以来，人们都认为打哈欠是为了清除肺里的“废气”、增加大脑中的氧循环。新近的这些研究则对这一传统观念提出了质疑。