字幕表 動画を再生する 英語字幕をプリント Here's a question people are always wondering about: why do people yawn? We do it every day, especially when we're in a boring meeting or we didn't get enough sleep. So it may not surprise you that we don't really understand all the reasons behind why we yawn, and it's not just humans, birds do it; jaguars do it; even fetuses in the womb do it. Oscitation, the act of yawning, happens with almost all vertebrates. The explanation you may be familiar with is that a great big gaping yawn is the brains way of ordering you to take a deep breath. Supposedly it gets rid of extra carbon dioxide and brings in more oxygen. The problem with that is that there is absolutely no truth that yawning really affects your oxygen levels. Instead, recent studies show that yawning is your brain's natural air conditioning system, bringing cooler blood to chill out your overheated, overtired grey matter. So your brain actually heats up as it uses some 40% of your body's metabolic energy. Just like your laptop gets warm up when it's been playing video games for hours. Body temperature also rises and falls as part of your circadian rhythm, reaching its highest point just before we fall asleep. Which is why a series of yawns at 1am is your body's way of telling you to stop going on Reddit and just go to bed. So just like your computer has a fan to cool things down, a yawn does the same thing to your brain, allowing you to continue processing information effectively. How's that work? Well, since you asked, yawning has two parts. First, stretching your jaws when you yawn increases the rate of blood flow to your skull. Second, the inhalation sends a gulp of air into your upper nasal and oral cavities, which have mucus membranes covered with tons of blood vessels that project up into the forebrain. This is the big part of the brain responsible for receiving and processing sensory information, producing and understanding language, and controlling motor functions, many of the things involved in actually being you. So the cool, refreshing air entering your sinuses changes the temperature of the blood that's now hustling up to the brain, making you more alert, and helping you walk and talk and think more effectively. Don't believe us? Scientist's studied the brain temperatures of mice, observing increased temperatures before yawning and measuring a dip in temperature afterward. And a more simple experiment on humans asked volunteers to hold hot or cold packets to their heads to see if changing the temperature of their brain cases triggered yawning. Turns out that the hot headed participants yawned 41% of the time, while those who chilled out yawned only 9% of the time. So next time you can't stop yawning as you're falling asleep in a sweltering classroom in the middle of the afternoon, remember that it's literally how your brain keeps its cool. Thanks for asking and thanks especially to our Subbable subscribers who keep these answers coming. If you have a Quick Question, let us know on Facebook or Twitter or down in the comments below, and if you want to keep getting smarter with us you can go to youtube.com/scishow and subscribe.