I get up a little after 5:00 in the morning to write this blog. And I have two kids. So as you might imagine, I tend to yawn a lot. So do a lot of the people I see every day on public transportation, in meetings, right around 3:00 when you need some coffee, and so on.
But what is the purpose of a yawn? Why do we do it? Surprisingly enough, given how common an action it is, the reasons for yawning are pretty murky. And not for lack of looking…a quick search of PubMed (a database of the biomedical literature) turned up just under 1,000 papers on yawning. (That’s after filtering out papers by researchers with the name Yawn, which is a horrible name for a scientist.) 2010 even marked the First International Conference on Yawning.
Before we can talk about why we yawn, we should define what a yawn is. The act of yawning (also called oscitation) is a coordinated movement and stretching of the diaphragm (the muscle above your stomach and beneath your ribs that inflates and deflates your lungs), several muscles in the thorax (chest), and the larynx (throat) and palate (roof of your mouth). You inhale and then exhale deep, which also forces your eardrums to stretch.
Speaking of stretching, that stretch you sometimes do when you yawn? Arcing your back and raising your arms up and back over your head? That has a name, too: pandiculation.
Yawning is considered by some to be a reflex — a rapid and involuntary response to a stimulus. When you’re getting a checkup, and your doctor whacks your leg just below your knee with that little rubber hammer? He’s testing your patellar or knee-jerk reflex. There’s a whole long list of reflexes, some of which are only seen in babies, some that are seen in people of all ages.
What’s the stimulus for a yawn? That’s a good question, and while many parts of the brain seem to be involved, no one knows for sure what the cause is. But we can make associations. Fatigue and tiredness, that’s one association. Boredom too. Seeing someone else yawn can trigger a “contagious” yawn. Which only happens, by the way, in people, dogs, and chimps. And so far, dogs seem to be the only animals that can “catch” a yawn from another species…namely, us. (Surprisingly, young children and those with moderate to severe autism don’t “catch” yawns.)
Speaking of dogs and chimps, it’s not just people that yawn. Yawning is a common action among all vertebrate animals (animals with a backbone). Dogs do it. Gorillas do it. Birds, rats, fish, turtles…you get the idea. And animals don’t necessarily yawn when they’re tired. Among Adelie penguins, for instance, yawning is part of a courtship ritual. (Imagine if that were the case in people? “She just yawned, she must be really into me!”)
And unlike some reflexes, we yawn for our entire life. Even babies in the womb yawn, an action recently caught on tape using a technology called 4D ultrasound. Here’s a quick clip:
All this background has has taken us away from the basic question: Why do we yawn? Some see it as a social cue, like “You’re boring the crap out of me” or “I’m tired and I might fall asleep on you.” Studies among pack animals suggest it’s a way of warning the group to stay alert.
But there may be physiologic reasons as well. One long-held theory was that it helped give the body an oxygen boost, though that was refuted in the 1980s. It may help keep the air sacs in our lungs moist by distrubting surfactant (the lubricant that keeps our lung open). The most recent theory is that yawning helps cool the brain. The thought is that a yawn stretches the sinuses like a bellows, drawing air past the membranes along the blood vessels that feed the brain and carrying heat away. However, the jury is still out on that one.
Whatever the reasons we yawn, I hope I haven’t made you yawn…out of boredom. In the event I have, let me make it up to you with some animal yawns: