科学家认为，打哈欠是在通过生理的方式给大脑降温。研究者发现，头上放了热水袋的参与者在看其他人打哈欠时，有41%的时候也会打哈欠，而头上放了冷水袋的只有9%。当我们看到其他人打哈欠时，大脑内的镜像神经元进入类似…
原标题：人为啥会打哈欠?
Almost all animals with a spine yawn.
几乎所有的有脊椎动物都会打哈欠。
Penguins do it as a mating ritual; snakes do it to realign their jaws after a meal and guinea pigs do it to display anger.
企鹅以此为求偶仪式，餐后蛇借此来整理下颌，荷兰猪借此表达愤怒。
So why exactly do humans yawn?
那么人为什么要打哈欠呢？
And is yawning actually contagious?
哈欠有传染性吗？
The truth is: the first time you yawned was likely as a fetus. Babies begin to yawn during the second trimester and though the reason why still unknown, it may have to do with proper brain development.
其实，你第一次打哈欠应该是在胎儿时期。婴儿会在3-6月开始打哈欠，尽管原因尚不明了，但这可能和大脑发育有关。
In adults, yawns were commonly thought to draw more oxygen into the lungs making you feel less tired, but new researches states that this may not be the case.
而成人打哈欠常被认为是要向肺部充入更多氧气，来缓解疲劳，不过，新研究却另有其词。
Scientists now believe that yawning has developed a way of physiologically cooling your brain.
科学家认为，打哈欠是在通过生理的方式给大脑降温。
Much like a computer, you brain work best at a certain temperature, and tries to avoid overheating.
和电脑一样，人脑也有特定的最佳工作温度，并且也会防止自身过热。
And it turns out, yawning increases your heart rate, blood flow and the use of muscles in your face, which are all essential to cool your brain.
事实上，打哈欠可以提升心率、血液流速、并动用脸上的肌肉。它对大脑降温来说都是必不可少的。
On top of that, deeply inhaling cold air can alter the temperature of the blood in our head.
更重要的是，深呼吸冷空气可以调整大脑内血液温度。
But, why is your brain hot in the first place?
不过，大脑原先怎么会过热呢？
Well, both exhaustion and sleep deprivation are known to increase overall brain temperature which explains why yawning occurs more in these states.
身心疲惫和睡眠缺乏都会导致大脑温度升高，这就解释了为什么这些情况下人更爱打哈欠。
Researchers have even found participants who place warm packs on their heads yawn 41% of the time while watching others yawn, as opposed to 9% with a cold pack on their head.
研究者发现，头上放了热水袋的参与者在看其他人打哈欠时，有41%的时候也会打哈欠，而头上放了冷水袋的只有9%。
So if your head is already cold, you will yawn less.
因此，大脑越“冷静“ 哈欠就越少。
But what about contagious or social yawning?
不过，受他人传染的哈欠是怎么回事？
Humans, primates and even dogs finding yawning contagious and it's most likely linked to empathy.
人类、灵长类甚至犬类都会被传染打哈欠，这很可能与“共情“有关。
Contagious yawning begins in children around the age of 4-5,and this is when empathetic behavior, along with the ability to identifying emotions, begin to develop.
儿童在4-5岁的时候开始被哈欠传染，同一时期，同情性行为，以及辨识情绪的能力开始逐渐发育。
In fact, children with empathy related disorders, such as autism, yawn less and response to videos of people yawning comparing to other children.
事实上，患有与共情相关疾病的儿童，如自闭症，在观看打哈欠视频的时候，比其他孩子打哈欠更少。
Research also suggest that you are more likely to copy the yawn of someone socially or genetically close to you.
研究表明，当身边有亲人或者朋友打哈欠时，你更容易被传染。
Even dogs are more likely to copy the yawns of their owner as opposed to the yawn of a stranger.
甚至狗相对于陌生人，更容易受主人哈欠的传染。
Finally, mirror neurons also play a role.
In our brain, mirror neuron fire when we perform a specific action view someone else doing the action, or even just hear someone talk about the action.
在我们大脑内，镜像神经元会在我们看到他人做出特定动作时产生冲动，甚至仅在听别人说到动作时也会。
They are important brain cells that are used for learning, self-awareness and relating to others.
它们是重要的脑神经细胞，作用于学习自我意识，以及于他人相处。
When we view someone else yawn, the mirror neurons in our brain become activated in a similar way and as a result we copy the yawn.
当我们看到其他人打哈欠时，大脑内的镜像神经元进入类似的活跃状态，使得我们被传染 打了哈欠。
So although yawning may occur in people who are literally hot-headed, contagious yawning allows us to be cool with the people around us.
因此，头脑发热的人可能会打哈欠，传染性的哈欠也是让我们与周围的人冷静相处。
