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.

Related on HuffPost:

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  • Tiny Horse

    Tonto, a guide horse, yawns before going on a walk with his owner Shari Bernstiel, March 19, 2004 in Lansdale, Pennsylvania. Tonto, a miniature horse who went through one year of training, acts as Bernstiel's seeing eye dog and is one of three working guide horses in the United States.

  • Yawn!

    <em>Flickr photo by <a href="http://www.flickr.com/photos/pauerkorde/3306367077/" target="_hplink">PauerKorde Photo</a></em>

  • Uggie The Dog

    Uggie the dog yawns as he sits in front of actor Jean Dujardin at a ceremony honoring the cast and crew of "The Artist" with the "Made In Hollywood" honor, January 31, 2012 at RED Studios in Hollywood, California. Parts of "The Artist" were filmed at RED Studios.

  • Rusty Yawns

    <em>Flickr photo by <a href="http://www.flickr.com/photos/corsinet/5342327363/" target="_hplink">corsi photo</a></em>

  • Malayan Tiger

    <em>Flickr photo by <a href="http://www.flickr.com/photos/kro-media/3579987729/" target="_hplink">KRO-Media</a> </em>

  • Baby Gorilla

    Hasani, a six month-old Western Lowland Gorilla, yawns as he plays in the gorilla exhibit during his first public viewing at the San Francisco Zoo June 5, 2009 in San Francisco, California. Hasani, who was born on December 8, 2008, has been hand-raised by a zoo staff after being rejected by his mother at birth. A surrogate gorilla mother was trained to care for Hasani and has accepted the newborn.

  • Tired

    <em>Flickr photo by <a href="http://www.flickr.com/photos/toolmantim/2490219129/" target="_hplink">toolmantim</a></em>

  • Yawning Lion

    <em>Flickr photo by <a href="http://www.flickr.com/photos/15265630@N02/1779054142/" target="_hplink">tkspencer</a></em>

  • Hippo Yawn

    <em>Flickr photo by <a href="http://www.flickr.com/photos/wwarby/4915772994/" target="_hplink">wwarby</a></em>

  • Yawn!

    <em>Flickr photo by <a href="http://www.flickr.com/photos/realheightz/4273476439/" target="_hplink">real.heightz</a></em>

  • A Meishan Pig Yawns

    A Meishan pig yawns on June 11, 2008 at the Tierpark Friedrichsfelde zoo in Berlin. Some Meishan piglets were born on May 3, 2008 at the zoo. The animals are domestic pigs originating from China.

  • Oliver Yawn and Stretch

    <em>Flickr photo by <a href="http://www.flickr.com/photos/mr_t_in_dc/3510666375/" target="_hplink">Mr. T in DC</a></em>

  • A Sea Lion Yawns

    A sea lion yawns during a swim in it's old enclosure as it waits for the construction of a new sea lion and penguin enclosure.

  • Camel Yawning

    <em>Flickr photo by <a href="http://www.flickr.com/photos/mr_t_in_dc/3869390774/" target="_hplink">Mr. T in DC</a></em>

  • Yawn!

    <em>Flickr photo by <a href="http://www.flickr.com/photos/8802700@N03/3602786434/" target="_hplink">NaturesFan1226</a></em>

  • Yawning

    <em>Flickr photo by <a href="http://www.flickr.com/photos/joshuajamesgross/152493247/" target="_hplink">hermmermferm</a></em>

  • Yawning Baby Tortoise

    <em>Flickr photo by <a href="http://www.flickr.com/photos/yeomans/81200979/" target="_hplink">sly06</a></em>

  • Corsac Fox Yawning

    <em>Flickr photo by <a href="http://www.flickr.com/photos/spencer77/4897753209/" target="_hplink">spencer77</a></em>

  • A Baboon Yawns

    A baboon yawns in its enclosure at Berlin's Zoo on June14, 2011.

  • Knut

    The world's most famous polar bear, Knut, yawns in his enclosure at the zoo in Berlin on December 10, 2010.

  • A White Tiger Cub Yawns

    A white tiger cub yawns in front of his mother, Radjah, on May 20, 2010 at the Maubeuge Zoo, northern France. The seven-year-old Radjah gave birth on May 9, 2010 to two white tigers belonging to a relatively rare species which only exist in captivity.

  • A Leopard Yawns

    A Leopard yawns in his enclosure at the zoo of the northern town of Hanover on March 19, 2010.

  • Big Yawn

    <em>Flickr photo by <a href="http://www.flickr.com/photos/basykes/175515247/" target="_hplink">basykes</a></em>

  • The Kamchatka Brown Bear

    The Kamchatka brown bear named Mikhailo Potapovich yawns in his cage after awakening from the winter hibernation at the Moscow's Zoo.

  • Smarty Jones

    Horse Smarty Jones yawns in his stall after arriving from Philadelphia on June 2, 2004 at Belmont Racetrack in Elmont, New York.

  • Homer Yawns

    Homer the elephant seal yawns as he lies on the boatramp in the port of Gisborne.

  • Red Panda Yawns

    A 15-month-old, female red panda yawns as a tranquilizer dart takes effect. The red panda was spotted making its escape from the zoo grounds and searchers spent most of the day searching the surrounding trees for the animal.

  • "This Random Animal"

    <em>Flickr photo by <a href="http://www.flickr.com/photos/lawtonjm/4668510627/" target="_hplink">lawtonjm</a></em>

  • Peaches

    <em>Flickr photo by <a href="http://www.flickr.com/photos/vivarin/4577137962/" target="_hplink">Vivian Chen [陳培雯]</a></em>

  • tired of walking, mom

    <a href="http://www.huffingtonpost.com/social/erynmarch"><img style="float:left;padding-right:6px !important;" src="http://s.huffpost.com/images/profile/user_placeholder.gif" /></a><a href="http://www.huffingtonpost.com/social/erynmarch">erynmarch</a>:<br />my 5-year-old mini pomeranian, banana, on a walk next to the train tracks in my neighborhood.

  • Sleepy Poodle

    <a href="http://www.huffingtonpost.com/social/Elizabeth_Ann"><img style="float:left;padding-right:6px !important;" src="http://graph.facebook.com/1434426283/picture?type=square" /></a><a href="http://www.huffingtonpost.com/social/Elizabeth_Ann">Elizabeth Ann</a>:<br />My Toy Poodle Nelson is so tired!

  • Obi getting ready to start his lazy day

    <a href="http://www.huffingtonpost.com/social/CaitlinLOrton"><img style="float:left;padding-right:6px !important;" src="http://graph.facebook.com/517959727/picture?type=square" /></a><a href="http://www.huffingtonpost.com/social/CaitlinLOrton">CaitlinLOrton</a>:<br />Obi is a bichon/terrier mix, who actually yawns more than he barks

  • Violet yawns

    <a href="http://www.huffingtonpost.com/social/Wendy_Whelan_Vega"><img style="float:left;padding-right:6px !important;" src="http://graph.facebook.com/1426663943/picture?type=square" /></a><a href="http://www.huffingtonpost.com/social/Wendy_Whelan_Vega">Wendy Whelan Vega</a>:<br />Violet is so bored she can't hold back a yawn.

  • The Yawners

    <a href="http://www.huffingtonpost.com/social/wwildlife"><img style="float:left;padding-right:6px !important;" src="http://s.huffpost.com/images/profile/user_placeholder.gif" /></a><a href="http://www.huffingtonpost.com/social/wwildlife">wwildlife</a>:<br />Micco (lion) & Kira (Bengal tiger) were raised together and played all day. By afternoon, they were ready for naps and just yawned & yawned & yawned. One would start and the other would yawn then the other would yawn, too. Micco continued to yawn throughout his life.

  • Micco (lion) - Big Yawner

    <a href="http://www.huffingtonpost.com/social/wwildlife"><img style="float:left;padding-right:6px !important;" src="http://s.huffpost.com/images/profile/user_placeholder.gif" /></a><a href="http://www.huffingtonpost.com/social/wwildlife">wwildlife</a>:<br />Micco grown up and still yawning all the time.

  • Sleepy Pup

    <a href="http://www.huffingtonpost.com/social/Maggie_Goldstein"><img style="float:left;padding-right:6px !important;" src="http://graph.facebook.com/40100515/picture?type=square" /></a><a href="http://www.huffingtonpost.com/social/Maggie_Goldstein">Maggie Goldstein</a>:<br />Long day at the dog park = big yawn

  • Dasha Yawns

    <a href="http://www.huffingtonpost.com/social/JohannaAnn"><img style="float:left;padding-right:6px !important;" src="http://s.huffpost.com/images/profile/user_placeholder.gif" /></a><a href="http://www.huffingtonpost.com/social/JohannaAnn">JohannaAnn</a>:<br />Dasha is my family's mixed terrier. I snapped this photo of her a few months ago, sitting in her favorite spot in my parents' home--on my old bed, looking out the window.

  • Sleepy Cat

    <a href="http://www.huffingtonpost.com/social/nazdancecoach"><img style="float:left;padding-right:6px !important;" src="http://s.huffpost.com/images/profile/user_placeholder.gif" /></a><a href="http://www.huffingtonpost.com/social/nazdancecoach">nazdancecoach</a>:<br />Midnight yawning while trying to sleep on top of the couch.

  • Woodruff

    <a href="http://www.huffingtonpost.com/social/catrina353"><img style="float:left;padding-right:6px !important;" src="http://s.huffpost.com/images/profile/user_placeholder.gif" /></a><a href="http://www.huffingtonpost.com/social/catrina353">catrina353</a>:<br />Woody is tired because he hasn't had his required 26 hours of sleep per day.

  • Lazy Day Nap Time

    <a href="http://www.huffingtonpost.com/social/kpglo"><img style="float:left;padding-right:6px !important;" src="http://s.huffpost.com/images/profile/user_placeholder.gif" /></a><a href="http://www.huffingtonpost.com/social/kpglo">kpglo</a>:<br />Bruno prepares for his daily nap with his sisters.

  • Leo

    <a href="http://www.huffingtonpost.com/social/Robert_Boulanger"><img style="float:left;padding-right:6px !important;" src="http://graph.facebook.com/1077237682/picture?type=square" /></a><a href="http://www.huffingtonpost.com/social/Robert_Boulanger">Robert Boulanger</a>:<br />I'm saying AHHHH Doctor!

  • Sometimes You Just Have To YAWN It Out

    <a href="http://www.huffingtonpost.com/social/Andie_Amit"><img style="float:left;padding-right:6px !important;" src="http://images.huffingtonpost.com/google_profile_img/3251439.png" /></a><a href="http://www.huffingtonpost.com/social/Andie_Amit">Andie Amit</a>:<br />

  • Sleep Milo

    <a href="http://www.huffingtonpost.com/social/Robin_Stevens_Clark"><img style="float:left;padding-right:6px !important;" src="http://graph.facebook.com/1291711166/picture?type=square" /></a><a href="http://www.huffingtonpost.com/social/Robin_Stevens_Clark">Robin Stevens Clark</a>:<br />This is Milo, he is my 5th child. He is very photogenic and I just happened to snap this pic mid yawn. Hope you enjoy it.

  • Dusty, the tired reindeer

    <a href="http://www.huffingtonpost.com/social/Eileen_Morse"><img style="float:left;padding-right:6px !important;" src="http://graph.facebook.com/5130966/picture?type=square" /></a><a href="http://www.huffingtonpost.com/social/Eileen_Morse">Eileen Morse</a>:<br />

  • Morning on the Ranch

    <a href="http://www.huffingtonpost.com/social/JoJo_Barnett"><img style="float:left;padding-right:6px !important;" src="http://graph.facebook.com/721371497/picture?type=square" /></a><a href="http://www.huffingtonpost.com/social/JoJo_Barnett">JoJo Barnett</a>:<br />Firelight South Ranch, 501(c) non-profit horse rescue and sanctuary. Alpine, Texas. Yawning before feeding.

  • Anxiety yawn

    <a href="http://www.huffingtonpost.com/social/Dollie_Goldman"><img style="float:left;padding-right:6px !important;" src="http://graph.facebook.com/1515597486/picture?type=square" /></a><a href="http://www.huffingtonpost.com/social/Dollie_Goldman">Dollie Goldman</a>:<br />Yawning Honeybear wants to go home from the vet.

  • Boris the Dog

    <a href="http://www.huffingtonpost.com/social/purplluck13"><img style="float:left;padding-right:6px !important;" src="http://images.huffingtonpost.com/aol_profile_img/3256881.png" /></a><a href="http://www.huffingtonpost.com/social/purplluck13">purplluck13</a>:<br />Boris is a lab/sheltie mix and he was super sleepy after waking up from a doggie nap

  • Carmen

    <a href="http://www.huffingtonpost.com/social/cgemmati"><img style="float:left;padding-right:6px !important;" src="http://s.huffpost.com/images/profile/user_placeholder.gif" /></a><a href="http://www.huffingtonpost.com/social/cgemmati">cgemmati</a>:<br />I think Carmen had too much alcohol that night from too much partying and is either very sleepy or she's about to throwup!

  • Sleepy at Work

    <a href="http://www.huffingtonpost.com/social/NixieMarie"><img style="float:left;padding-right:6px !important;" src="http://s.huffpost.com/images/profile/user_placeholder.gif" /></a><a href="http://www.huffingtonpost.com/social/NixieMarie">NixieMarie</a>:<br />

  • Snowfire

    <a href="http://www.huffingtonpost.com/social/enlea"><img style="float:left;padding-right:6px !important;" src="http://images.huffingtonpost.com/google_profile_img/3258767.png" /></a><a href="http://www.huffingtonpost.com/social/enlea">enlea</a>:<br />Snowfire, a Siamese-Manx rescue cat, yawns after roughhousing with his buddy Calvin, a Yorkshire Terrier.

  • Gigi yawning

    <a href="http://www.huffingtonpost.com/social/AnimaSola"><img style="float:left;padding-right:6px !important;" src="http://graph.facebook.com/1313972655/picture?type=square" /></a><a href="http://www.huffingtonpost.com/social/AnimaSola">AnimaSola</a>:<br />I caught Gigi the Mini-Schnauzer in mid-yawn.