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We all do it, and we all know it has at least something to do with how tired we feel. But unlike sleep apnea or laptops in the bedroom, yawning is an aspect of sleep that researchers haven't quite figured out just yet.
That doesn't mean we're totally in the dark when it comes to catching flies. Here are a few of the facts we know for sure when it comes to yawning.
There Are Many Theories, But Little Proof
There's little research to support any of a number of theories as to why we yawn. FIrst off, we don't only do it when we're tired. It also probably doesn't reflect a lack of oxygen, although that theory isn't a totally nutty one. The idea likely blossomed from the fact that too-shallow breathing can cause problems, says Michael Decker, Ph.D., associate professor at the Frances Payne Bolton School of Nursing at Case Western Reserve University and a spokesperson for the American Academy of Sleep Medicine.
The bottom lobes of the lungs aren't usually called upon when we're in our resting state. It isn't until we exercise that we typically use more of our lung capacity, but such deep breathing helps keep the lungs healthy, he says. In cases of surgery patients, some have been known to lose lung function after developing pneumonia due to shallow breathing after anesthesia. "Yawning would be like a homeostatic response to not breathing deeply" if this theory were to hold up, says Decker, but there's little proof to suggest it's the primary reason for yawning.
Yawning does seem to increase with boredom, at least according to a small 1986 study of college students who yawned more when shown a pattern of colors than when shown a 30-minute rock video.
The most recent research on yawning suggests that it exists to cool down the brain. That open-mouthed yawn causes sinus walls "to expand and contract like a bellows, pumping air onto the brain, which lowers its temperature," National Geographic reported. The study found that people were more likely to yawn during the winter, when the exterior air is obviously cooler, than in the summer, when yawns won't do much in terms of bringing cold air inside, Healthy Living reported.
Yawning Really Is Contagious
It's true! One study found that when shown videos of yawning, around 50 percent of people also began yawning. It even happens among animals! A 2004 study observed the catching nature of yawns between chimpanzees and baboons and macaques. Perhaps most impressive, though, are dogs, who might start to yawn after just hearing their owners let one slip. Even merely thinking -- or reading! -- about yawning can trigger one (did we get you yet?).
Turns out, it's not really that strange of a reaction, Robert Provine, a psychology and neuroscience professor at the Unversity of Maryland, Baltimore County, told WebMD. Other very human reactions are equally "contagious" -- think about the last time you witnessed someone laughing! A number of studies have tied this catching nature of yawns to empathy, says Decker. "The yawning becomes more of a social phenomenon than a physiological phenomenon," he says, and helps explain why we yawn when we're not tired.
Yawning Is More Contagious Between Besties
Not just anyone will pass a yawn onto you. According to 2012 research, yawns are most contagious among the closest of pals. "Researchers discovered that the closer you are to someone genetically or emotionally, the more likely it is that you'll 'catch' their yawn," HuffPost Science reported. Makes sense given the empathy theory, says Decker, since closer friends and family will have even stronger feelings toward each other.
Yawning May Be A Sign Of Disease
It isn't usually the first symptom of anything serious, but excessive yawning can in some instances signal there's something wrong beyond severe sleep deprivation. In some people, excessive yawning could be a reaction caused by the vagus nerve, according to the National Institutes of Health, which could indicate a heart problem. In other rare cases, it could also signify a number of brain problems.
Even A Fetus Can Yawn 
No one knows exactly why yet, but unborn babies do yawn. While researchers have previously disputed imagery of open-mouth fetuses, a 2012 review of 4D scans was able to distinguish between a developing baby opening its mouth and a "non-yawn mouth opening," HuffPost Science reported. It may have something to with brain development, the researchers posited, and could potentially be used as a marker of normal development, LiveScience reported.
The Average Yawn Lasts 6 Seconds
There might not be a scientific study to back this one up, but a number of news outlets peg yawn length at about six seconds. During those six seconds, heart rate increases significantly. A 2012 study examined the body before, during and after yawns and found that a number of the physiological changes that take place during those six seconds -- or however long you yawn for -- are unique to yawning, and were not replicated when study participants were simply asked to take a deep breath.
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