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.

企鵝將它視為一種求偶儀式、蛇在飽餐一頓後藉由打哈欠調整下顎，天竺鼠則藉此表現憤怒。

So, why exactly do humans yawn? And is yawning actually contagious?

那麼人類究竟為什麼打哈欠呢 ? 打哈欠真的會傳染嗎？

If you haven't already checked out our YAWN-O-METER video, click here, or use the link in the description to see how long you can last before yawning.

如果你還沒看過我們的「YAWN-O-METER」哈欠測驗影片，請點擊這裡或按說明內的連結，測試自己可以撐多久才打哈欠。

If you're anything like us, you may have even yawned at the title of this video.

如果你和我們一樣，也許一看到影片標題就哈欠連連了。

The truth is, the first time you yawned was likely as a fetus.

事實上，你第一次打哈欠時可能還在媽媽肚子裡。

Babies begin to yawn during the second trimester, and though the reason why is still unknown, it may have to do with proper brain development.

胎兒在 6 個月大的時候開始打哈欠，雖然還不清楚原因，但應該與正常的腦部發展有關。

In adults, yawns were commonly thought to draw more oxygen into the lungs making you feel less tired, but new research states that this may not be the case.

以成人來說，普遍認為打哈欠有助於肺部吸入更多氧氣而減輕疲勞，但最新研究顯示可能並非如此。

Scientists now believe that yawning has developed as a way of physiologically cooling your brain.

科學家現在相信，打哈欠是一種生理上讓大腦降溫的方法。

Much like a computer, your brain works best at a certain temperature, and tries to avoid "overheating".

與電腦運作模式相似，大腦在特定溫度下才是最佳狀態，所以會盡量避免「過熱」的情況。

And it turns out yawning increases your heart rate, blood flow and the use of muscles in your face which are all essential to cooling the brain.

而打哈欠能增加心跳速率、血流量及臉部肌肉的使用，這些都是降低大腦溫度的必要條件。

On top of that, deeply inhaling cold air can alter the temperature of the blood in our head.

除此之外，大量吸入冷空氣能改變大腦中血液的溫度。

But why is your brain hot in the first place?

但為什麼大腦一開始是熱的呢？

Well, both exhaustion and sleep deprivation are known to increase overall brain temperature, which explains why yawning occurs more in these states.

其實過度疲勞及睡眠不足都會導致大腦溫度上升，這也解釋為何在這些狀態下人們更容易打哈欠。

Researchers have even found participants who place warm packs on their head yawn 41% of the time while watching others yawn,

有研究員甚至發現，頭上放暖袋的受測者在觀看別人打哈欠時，有 41% 的時間會跟著打哈欠，

as opposed to 9% of the time with a cold pack on their head.

相較於頭上放涼袋的人，只有 9% 的時間會打哈欠。

So if your head is already cold, you'll yawn less.

因此如果頭部溫度低，打哈欠的次數就會減少。

But what about contagious or social yawning?

那麼打哈欠會傳染是怎麼一回事呢？

Humans, primates and even dogs find yawning contagious, and it's most likely linked to empathy.

人類、靈長類及狗狗的哈欠是會傳染的，原因大部分與移情作用有關。

Contagious yawning begins in children around the age of 4-5, and this is when empathetic behavior, along with the ability to identify emotions, begins to develop.

孩童從 4-5 歲之間就開始有哈欠傳染的現象，此時是同理心及情緒覺察能力開始發展的階段。

In fact, children with empathy-related disorders, such as autism, yawn less in response to videos of people yawning compared to other children.

事實上，患有自閉症等情緒覺察能力失調的孩童與其他小孩相較下，看著打哈欠影片而打哈欠的次數比較少。

Research also suggests that you are more likely to copy the yawn of someone socially or genetically close to you.

研究也指出，人們傾向模仿地位或基因上與自己相似的人打哈欠。

Even dogs are more likely to copy the yawns of their owner as opposed to the yawn of a stranger.

即便是狗狗，也傾向模仿主人打哈欠，而不模仿陌生人。

Finally, mirror neurons also play a role.

最後，鏡像神經元也扮演一個重要角色。

In our brain, mirror neurons fire when we perform a specific action, view someone else doing the action or even just hear someone talk about the action.

當我們做出特定動作、觀察別人，甚至聽到別人談論這個動作時，我們大腦中的鏡像神經元會啟動。

They are important brain cells that are used for learning, self-awareness and relating to others.

在學習、自我察覺、人際交往方面、它們是很重要的腦細胞。

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.

當我們看到別人打哈欠時，腦中的鏡像神經元也以相同的方式啟動，因此我們會跟著別人打哈欠。

So, although yawning may occur in people who are literally 'hot-headed', contagious yawning allows us to be cool with the people around us.

雖然打哈欠會發生在真正「一頭熱」的人身上，哈欠的傳染也幫助我們跟著身邊的人一起降溫。

If you haven't already checked out our Yawn-O-Meter, go try it out and let us know in the comments how long you were able to last.

如果你還沒看過我們的「YAWN-O-METER」的影片，快去試試看並留言讓我們知道你能撐多久。

And subscribe for more weekly science videos!

然後訂閱更多每週的科學影片吧！