Here's what science has to say:
Full disclosure: We just yawned. Please excuse this obvious conflict of interest.
We all know what a yawn looks like: The jaw opens wide, with a deep inhalation, and then breathing halts for a hiccup of a moment. Then, a forced exhalation sends air rushing out, often with that signature sort-of-sigh. Yawning seems to be pretty much ubiquitous in the animal kingdom, and for some animals — many humans included — it really is contagious. But why does this mechanism exist in the first place?
The truth is that the jury is still out. But scientists have some ideas.
"This is of interest, but it’s not something that is going to save human lives," Daniel Barone, a sleep expert at Weill Cornell Medicine and New York-Presbyterian, told The Washington Post. "So it doesn’t get as much research attention."
For a long time, the prevailing theory on yawning was that the deep, forceful breathing involved gave a boost of oxygen to your bloodstream, refreshing a tired brain. But this has been pretty thoroughly debunked.
"Even when you do yawn, the amount of oxygen in your blood doesn’t really go up," Barone said. "And just think about it. If that's what yawning is for, why don't you yawn when you're exercising? That's when you need oxygen most."
Thankfully, we don't generally yawn on our afternoon jogs (morning jogs are another matter entirely). And these days, the limited research on yawning suggests that the reflex actually helps to cool down our brains. One study even found that holding hot or cold packs up to a subject's forehead could increase or decrease the rate of yawning, respectively.
Yawning may not increase the oxygen level in your blood, but it does slightly lower the temperature of your brain. The motion of opening your jaw wide sends more blood up to your brain, while the deep inhalation of cool, ambient air ensures that the blood flow is relatively cool. Intriguingly, yawning behavior decreases when the ambient air is warm, as it is in summer — because that air would fail to cool down your blood and brain.
"When you're tired, the brain is a bit warmer," Barone explained. "So the theory here is that your body is trying to reverse that."
That could also help explain why yawning is so contagious among certain animals: Oxygen levels are generally a personal thing, but it's the ambient temperature that dictates whether a yawn will effectively cool down an active brain or not. So if one member of the group starts yawning to cool down, it makes sense that others would unconsciously take the hint. Come on in, guys, the air is fine.
“If I see a yawn, that might automatically cue an instinctual behavior that if so-and-so’s brain is heating up, that means I’m in close enough vicinity, I may need to regulate my neural processes,” Steven Platek, a psychology professor at Georgia Gwinnett College, told Smithsonian Magazine. Platek's research suggests that humans are more likely to yawn in response to others if they score high on tests of empathy, although other studies have failed to find a correlation. Some data suggests that you're more likely to catch a yawn from a close friend or family member than from a stranger. That doesn't seem to be the case for other animals that experience "yawn contagion," which makes some researchers believe that contagious yawning is tied to the sort of interpersonal empathy that separates us from much of the animal kingdom.
Not all scientists have accepted that yawns exist to cool the brain down: Some argue that the lack of yawning during warm periods is a count against the thermoregulation theory, suggesting that the mechanism is purely psychological — not physiological. In other words, it's just some kind of empathetic social cue we've evolved to signal . . . something. It seems unlikely that loads of animals would evolve a physical mechanism to signal tiredness or boredom to their families, but maybe it's a vestige of some other social cue.
But while the origins and purposes of yawning remain mysterious, researchers pretty much agree that yawns make you feel less tired. It's less of a "time for sleep" thing and more of a "no no no NOT time for sleep" thing. So instead of resenting your officemate for inducing a few more yawns than you'd like, thank them for the refreshing jolt of cool air. It's certainly cheaper than a latte.