我们为什么打哈欠

请注意，点开这篇文章可能会让你打哈欠！这并不是因为我们的节目太无聊或者你会被传染上哈欠，而是因为只要我们看到别人打哈欠，甚至只是想想这个行为，都可能引发我们来一个大哈欠！

It's a question that has puzzled scientists for a long time: why exactly do we yawn? 

In a recent breakthrough, scientists have discovered the part of the brain responsible for contagious yawning. It's called the primary motor cortex, and controls movement in the body. When triggered it makes us yawn. But we can't choose to yawn – it's an involuntary imitation of other people. You know how it goes: your friend yawns, you yawn. 

Knowing which section of the brain is involved is only part of the answer. There is still a deeper question of 'why'. And we don't only yawn when others do, we sometimes yawn alone.

So, let's start with the physiological theories behind yawning. As far back as 400BC, Greek physician Hippocrates speculated that yawning released harmful gases from the body, like "steam escaping from a cauldron". His colourful theory is now, sadly, discounted. 

A more recent idea suggests that we yawn in order to cool down the brain. Just like a busy computer processor, our brains can overheat. Psychologist Gordon Gallup from the University at Albany has argued that the act of yawning delivers a rush of oxygen to the brain, helping us think more clearly. 

What about the connection between yawning and tiredness? We certainly yawn more when we're feeling sleepy or sluggish, but why is this? Theories that it helps give the body extra energy are largely discredited, as we don't yawn when we exercise, a time when we definitely need more get-up-and-go! 

Psychologist Catriona Morrison from the University of Leeds says yawning is an ancient reflex.  She told the BBC that it may have started in "the days of cavemen when someone had to be on watch all the time - yawning increases cerebral blood flow so can improve alertness." 

What about our friends' contagious yawning? Some think it comes down to empathy. While all vertebrate mammals yawn, only humans and chimpanzees 'catch' yawns from others. Studies have shown we yawn more frequently when in the company of close relatives and friends. Could yawning actually help us cement our closest relationships? 

If it does, next time you yawn in company, don't feel too embarrassed. You're just showing off your people skills.