The short answer is that no one really knows.
The long answer is that no one really knows, but there are plenty of interesting theories:
1. The idea that we yawn to get rid of carbon dioxide and take in more oxygen has been disproved by research, but persists as the “common wisdom” answer. According to this theory, people breathe more slowly when they’re bored or tired and less oxygen gets to the lungs. As CO2 builds up in the blood, the brain reflexively prompts a deep, oxygen-rich breath.
The problem with this theory is a 1987 study by Dr. Robert Provine, who is regarded as the world’s foremost yawn expert. Provine set up an experiment in which volunteers breathed one of four gases that contained varying ratios of CO2 to O2 for 30 minutes. Normal air contains 20.95% oxygen and 0.03% carbon dioxide, but neither of the gases in the experiment with higher concentrations of CO2 (3% and 5%) caused the research subjects to yawn more.
2. Last year, a team of researchers at the University of Albany proposed that the purpose of yawning is to cool the brain. They conducted an experiment similar to Provine’s and again found that raising or lowering oxygen and carbon dioxide levels in the blood did not change the amount or length of yawns.
Subsequent experiments focused on two well-established brain cooling mechanisms: nasal breathing and forehead cooling. When you breathe through your nose, it cools the blood vessels in the nasal cavity and sends that cooler blood to the brain. Likewise, when you cool your forehead, the veins there, some of which are directly connected to the brain, deliver cooler blood. The researchers found that their test subjects with warm or room temperature towels pressed against their heads yawned more than those with cold towels. Subjects who breathed through their noses during the experiment did not yawn at all.
The researchers said their evidence suggests that taking in a big gulp of air with a yawn cools the brain and maintains mental efficiency.
3. Another theory says that yawning has more to do with sociology than physiology and also tackles the question of contagious yawning.
Almost all vertebrates yawn spontaneously, but only humans, chimps and macaques yawn as a result of watching another individual do it. Given that these are social creatures that live in groups, the contagious yawn may have evolved as a way to coordinate behavior and maintain group vigilance. When one individual yawned, the group took that as evidence that their brain temperature was up and their mental efficiency was down. If all members of the group then yawned, the overall level of vigilance in the group was enhanced. In humans, who have color-coded charts to signal how vigilant they should be, yawns may still be contagious as a vestigial response.
While yawns are still largely a mystery, here are some things we know for certain:
• The average yawn lasts about six seconds.
• In humans, the earliest occurrence of a yawn happens about 11 weeks after conception – while we’re still in the womb.
• Your heart rate can rise as much as 30% during a yawn.
• 55% of people will yawn within five minutes of seeing someone else yawn.
• Blind people yawn more after hearing an audio tape of people yawning.
• Reading or even thinking about yawning can cause you to yawn.
• While researching and writing this story, I yawned 37 times.
Comments (35)
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Actually birds yawns are contagous too, at least parrots are. I’ve heard that it’s a coordination thing, like the third theory.
posted by Em on 5-7-2008 at 12:52 pm
I think I yawned about 12 times just while reading this.
posted by Pearl on 5-7-2008 at 12:53 pm
“In humans, who have color-coded charts to signal how vigilant they should be…”
hahaha.
posted by Johnny Cat on 5-7-2008 at 12:58 pm
*YAWN*
posted by RodneyB on 5-7-2008 at 1:59 pm
All it took was the photo of the dog to set me off on a yawn-a-thon.
Remember the episode of Get Smart when the bad guys were holding 86 and 99 hostage, and 99 put them to sleep by starting a yawning chain? Classic spy work!
posted by elizabutt on 5-7-2008 at 2:03 pm
I figured there would be a catch about contagious yawning at the end, so I purposely tried *not* to yawn… and I didn’t! I win!
posted by Katie on 5-7-2008 at 2:20 pm
we yawn in order to equalize the air pressure on both side of our diaphagm.
posted by Bill Gragg on 5-7-2008 at 2:36 pm
All I had to do was read the title of the post and I yawned.
posted by Lindsey on 5-7-2008 at 2:44 pm
Actually, we know why we yawn: it is to reverse the atelectasis of alveoli and to push more surfactant into the alveoli by the type II pneumocytes.
posted by Stewie on 5-7-2008 at 2:45 pm
OK, so I totally remember seeing a quick little animation about why we burp on Sesame Street as a kid. They ended it with saying that they were going to tackle yawns on the next episode, but then it never happened. I remember being highly disappointed by this. I guess that making an animation with the conclusion of “I don’t know” isn’t all that intriguing…
Well, now I guess I can lift that grudge against the show. Thanks!
posted by Bethany on 5-7-2008 at 2:51 pm
My favourite explanation for yawning, and why it’s conatagious is: “You yawn to equalize the pressure on your eardrums. This pressure change outside your eardrums unbalances other people’s ear pressures, so they must yawn to even it out.”
posted by Mike on 5-7-2008 at 2:52 pm
I had a former boss that is a Scientologist and they believe that if you yawn while reading, studying or listening to someone, it means you didn’t understand or don’t fully grasp a word or concept that you just read/heard.
posted by Laura on 5-7-2008 at 3:17 pm
Serioulsy – I yawned 3 times reading this…funny.
posted by Christian on 5-7-2008 at 3:47 pm
It made me yawn too, and not in the “Oh, this is boring” kind of way. Contagious indeed.
posted by Aemi on 5-7-2008 at 5:09 pm
I swear I yawned like 10 times while reading this! Oops, just did another!
posted by Kelly on 5-7-2008 at 9:36 pm
How come sometimes when I yawn my eyes will water so much it looks as if I am crying?
…like now, when I can’t stop yawning! LOL
posted by KLeaux on 5-7-2008 at 11:35 pm
How come sometimes my eyes water when I yawn to the point where it looks as if I am crying?
..like now when I can’t stop crying! LOL
posted by KLeaux on 5-7-2008 at 11:37 pm
I yawned 3 times while reading this!
posted by QMYM on 5-8-2008 at 6:41 am
I have often thought that yawning might have something to do with getting focused, mainly because when I’m really nervous, I tend to yawn a lot. Anyone else?
The word “yawn” has officially lost all meaning to me after all this repetition!
posted by Annie on 5-8-2008 at 9:16 am
i yawn when i’m nervous.
posted by Aeden on 5-8-2008 at 9:53 am
My dog yawns every time I start petting and cuddling him on my couch. My mom and I think hes doing it out of pleasure haha
posted by chalart on 5-9-2008 at 9:33 pm
Weird..I didn’t yawn at all while reading this. I was sure I would have after seeing the picture of the baby yawning!
posted by Dawn on 5-10-2008 at 8:20 pm
wow, just reading the title made me yawn. this is a pretty neat article!
posted by Oisin on 5-12-2008 at 10:15 pm
I thought I got through the whole article without yawning and then I finally went while reading the last blog…..I’m very dissappointed in myself :(
posted by Josh on 7-25-2008 at 3:40 pm
so if it is lack of oxgyen, what can you do?
posted by brenda on 2-7-2009 at 9:24 pm
Just drink oxygenated water. You know the brain needs glucose, oxygen and water. Or drink a cup of hot cereal (especially cereal with glucose).
posted by Yanto on 5-24-2009 at 2:28 pm
In response to a previous comment of: “Actually, we know why we yawn: it is to reverse the atelectasis of alveoli and to push more surfactant into the alveoli by the type II pneumocytes.” My reply: I don’t think so. This reply may be and in fact is quite likely why we “sigh” but not yawn.
posted by Tim on 7-27-2009 at 11:09 am
Why do I yawn mostly when I’m talking???
posted by Leslie on 7-8-2010 at 3:06 pm
I yawn because I don’t get enough entertaining Matt Soniak posts.
posted by Rajiv on 9-8-2010 at 8:50 am
I’ve been wondering recently when the yawn actually becomes contagious. I’ve noticed with my three month old that she does not react to my yawn. I have to believe that contagious yawning is a learned trait. Every time she yawns though, I do too.
posted by John on 9-8-2010 at 9:02 am
The Myth Busters actually did a study on this. They had people come into a trailer and when they were entering Kari would yawn while greeting them and taking them to thier rooms. If i remember correctly the test (including others i think) really did not prove or disprove that yawning is contagous
Other than that-this was a really cool post, thank you!
posted by Jennifer on 9-8-2010 at 9:43 am
It seems like I heard that dogs will yawn not only because they’re sleepy but also when they’re nervous or worried. My dogs don’t yawn that much, but they don’t have alot to be worried about either! They’re dogs!
posted by Beth on 9-8-2010 at 9:54 am
I was wondering why I kept yawning while reading this article…until the last sentence. I’d have to say I think the best yawns I have are the ones that include a little stretch.
posted by Kristina the bureaucrat on 9-8-2010 at 11:18 am
It required a conscious effort, but I’m proud to say that I didn’t yawn once while reading this article. Doesn’t mean that I don’t still want to though…
posted by Paul on 9-9-2010 at 6:31 pm
I yawned approximatly 11 times while reading this lol. oops, there goes another one.
posted by tatyana on 8-21-2011 at 8:05 pm