Yawning: Everyone does it (even animals!), but no one really knows why.
In general, researchers tend to fall into one of two camps, WebMD reports. There are those who believe yawning must have some physical cause and purpose and those who believe yawning is a form of social communication.
Both sides have recent research supporting their opinions. A 2011 study suggested that yawning could be the body's way of cooling down the brain. "Brains are like computers," Princeton University researcher Andrew Gallup, told Discovery News. "They operate most efficiently when cool, and physical adaptations have evolved to allow maximum cooling of the brain." A gaping, open-mouthed yawn draws in air (which is more often than not cooler than our internal body temperatures) and turns the brain's thermostat down a few notches.
But there's no denying the social nature of the yawn. Notoriously catchy -- how many times have you yawned just reading this? -- yawning is even more contagious between close friends and family members, according to research from earlier this year.
In the attempt to explain why we yawn, however, let's not neglect the fact that most of us really do need more sleep. About one in five adults don't get enough shut-eye, according to the American Academy of Sleep Medicine. But whether it's hectic work schedules, an active night life, early-morning workouts or one-too-many cups of coffee cutting into your sleep, skimping can have serious effects, on everything from your cardiovascular health to your waistline.
Despite their glamorously busy schedules, sleep is important to many celebrities, too. In fact, some have even spoken openly about their problems catching enough winks. Plenty more haven't spoken openly about the importance of sleep, but judging by these photos, we're willing to bet quite a few could use some more sleep. Check out the 30 celebs below trying to stifle (or not!) a big yawn.
Recording artist Miley Cyrus yawns during an early-morning appearance at the 16th annual EIF Revlon Run/Walk for Women on May 9, 2009 in Los Angeles, California.
Actress Lucy Liu attends the premiere of "Kill Bill Vol. II" at the Arclight's Cinerama Dome April 8, 2004 in Hollywood, California.
Serena Williams of the USA yawns during the men's final match between Fernando Gonzalez of Chile and Roger Federer of Switzerland on day fourteen of the Australian Open 2007 at Melbourne Park on January 28, 2007 in Melbourne, Australia.
John Paul II yawns as he presides over his traditional weekly general audience at the Vatican, January 14, 2004.
Tibetan spiritual leader The Dalai Lama yawns while at the Central School for Tibetans in Kalimpong on December 14, 2010.
The NBA's Shaquille O'Neal yawns as he sits on the bench during a Lakers-Jazz game in Inglewood, California.
Former President Bill Clinton yawns before teeing-off during a golf tournament to benefit Martha's Vineyard Hospital at Farm Neck Country Club, August 22, 1993.
Actress Kirsten Dunst yawns on the movie set of "Mona Lisa Smiles" January 13, 2003 in the West Village, New York City.
Charles Barkley yawns during Live TNT NBA Studio Show Media Opportunity for TNT 2004 NBA All-Star week at Magic Johnson Theatres in Compton, California, United States.
Brett Favre, of the Minnesota Vikings, yawns during warmups prior to an NFL preseason game against the Denver Broncos at the Mall of America Field at Hubert H. Humphrey Metrodome, on September 2, 2010 in Minneapolis, Minnesota.
Jerry Seinfeld takes a break while filming a promo October 26, 2004 in New York City.
Venus Williams of the United States yawns during the opening ceremony on day one of the World Team Challenge 2009 tournament held at Victoria Park, January 7, 2009 in Hong Kong, China.
Kate Moss yawns at a West Village restaurant in New York City, April 6, 2006.
Rafael Nadal of Spain yawns during practice on Day Ten of the Wimbledon Lawn Tennis Championships at the All England Lawn Tennis and Croquet Club on June 30, 2011 in London, England.
Apolo Anton Ohno of the United States yawns after the Short Track Speed Skating Men's Semi-Final on day 9 of the Vancouver 2010 Winter Olympics at Pacific Coliseum on February 20, 2010 in Vancouver, Canada.
Newt Gingrich yawns as he leaves the stage after taking part in The Republican Presidential Debate at University of South Florida in Tampa, Florida, January 23, 2012.
Sasha Obama yawns during her father Barack Obama's speech as Michelle Obama holds her head on day four of the Democratic National Convention (DNC) at Invesco Field at Mile High, August 28, 2008, in Denver, Colorado.
FC Barcelona's Argentinian player Lionel Messi yawns during a training session at the Athens Olympic stadium, one day before a Champions League football game against Panathinaikos on November 23, 2010.
Nomar Garciaparra, formerly of the Chicago Cubs, pauses to yawn during the game against the New York Mets on September 26, 2004 at Shea Stadium in New York City.
President of Cuba Fidel Castro yawns during discourse of President Nicanor Duarte on August 15, 2003 in Asuncion.
Former President George Bush yawns as former President Gerald Ford, former First Lady Barbara Bush and former First Lady Nancy Reagan listen to speeches at the Republican National Convention at the First Union Center in Philadelphia, Pennsylvania, August, 1 2000.
Former British Prime minister Margaret Thatcher yawns on November 19, 1990 in Paris, as she attends the Conference on Security and Cooperation in Europe.
British actor and director Charlie Chaplin stretches and yawns in a full-length promotional portrait for his film "The Tramp".
Michelle Wie of the USA yawns on the 18th hole during day one of the LPGA Thailand at Siam Country Club on February 17, 2011 in Chon Buri, Thailand.
German Chancellor Angela Merkel yawns during the 67th sitting of the Bundestag, the lower house of parliament in Berlin, October 27, 2010.
O.J. Simpson yawns during a break in his trial in District Court at the Clark County Regional Justice Center September 25, 2008 in Las Vegas, Nevada.
Brazilian President Dilma Rousseff yawns during the launching of PRONACAMPO -- a Federal Government program to improve education policies in rural areas where the illiteracy rate is high, at Planalto Palace in Brasilia, on March 20, 2012.
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