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Human Senses Image Gallery

The average yawn lasts about six seconds. See more human senses pictures.

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Next time you're in a meeting, try this little experiment: Take a big yawn, cover your mouth out of courtesy and watch to see how many people follow suit. There's a good chance you'll set off a chain reaction of deep breaths and wide-open mouths. And before you finish reading this article, it's likely you'll yawn at least once. Don't misunderstand, we aren't intending to bore you, but just reading about yawning will make you do it, just as seeing or hearing someone else yawn makes us do it, too.

­So what's behind this mysterious epidemic of yawning? First, let's look at what this bodily motion is: Yawning is an involuntary action that causes us to open our mouths wide and breathe in deeply. We know it's involuntary because we do it even before we're born: According to Robert Provine, a developmental neuroscientist at the University of Maryland, Baltimore County, research has shown that 11-week-old fetuses yawn. And while yawning is commonly associated with relaxation and drowsiness, your heart rate can rise as much as 30 percent during a yawn, and yawning is a sign of arousal, including sexual arousal [sources: Alexander, The Stress of Life].

Many parts of the body are in action when you yawn. First, your mouth opens, and your jaw drops, allowing as much air as possible to be taken in. When you inhale, the air taken in is filling your lungs. Your abdominal muscles flex, and your diaphragm is pushed down. The air you breathe in expands the lungs to capacity and then some of the air is blown back out.

Now that we know what a yawn is, let's look at what causes us to do it. On the next page, we'll discuss four popular theories that explain why we open wide and breathe deep.