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.
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