Why do people yawn when they're tired?

May Cheng
23 January 2012

Birds do it, bees probably don't, but almost all mammals do, including humans. That's right...we all yawn. In fact, humans have been known to yawn even before birth — ultrasound recordings have detected yawns in fetuses as early as 11 weeks into a woman's pregnancy. Clearly, if everyone does it, yawning must have some important role in human and animal physiology, right? Probably, but the problem is that while there are many theories as to why it happens, we still don't really have a good reason.

Did You Know?
The average yawn takes about six seconds. This includes the time for the gaping of the mouth, the long, deep inhalation and the short exhalation.

One of the long-held ideas about why we yawn is that people don't breathe as deeply when they're tired, resulting in a decrease in the amount of oxygen intake and an increase in the amount of carbon dioxide in the blood. Eventually, these changes in blood oxygen and carbon dioxide levels trigger the brain to make us take a deeper breath, resulting in a yawn and the deep inhalation that comes with it.

As reasonable as the above argument may sound, a considerable amount of research suggests that it's not true. As mentioned above, babies yawn before they've even been born and taken their first breath. Also, studies where people were made to inhale air with different levels of oxygen and/or carbon dioxide to mimic the effects of changing blood oxygen and carbon dioxide levels did not find that the probability of yawning over a certain time frame was dependent on the amount of these gases.

Did You Know?
Yawning is an involuntary, subconscious behaviour — you don't have to think about it for it to happen. But seeing others do it, or simply thinking or reading about it can cause yawning.

So what are some of the other theories as to why people yawn? As recently as 2007, researchers have suggested that yawning helps cool the brain down and increase alertness. In fact, when test subjects held cold packs to their foreheads (presumably to keep their brains cool), they tended to yawn less often, and were less likely to "catch" yawns when they saw others do it. Perhaps when we're tired and our brains are warmer and not working as efficiently, yawning helps cool down our brains and wakes us up. Sort of like the cooling fan on a computer helping it run better.

The cooling idea of yawning might help explain why paratroopers yawn before they jump, and why Olympic athletes do it before they compete in their event — it's pretty unlikely that it's because they're tired or bored! It might also point to the reason why yawns are contagious. Some people think that humans, and maybe animals, evolved to yawn as a form of communication. Perhaps "catching" a yawn from a tired member of a group helped increase the alertness of everyone around, or signaled a change in behaviour.

Did You Know?
Yawning increases the heart rate, which may also help keep you awake when you're tired by increasing the blood flow to the brain.

So, the simple answer is, we still don't know for sure why we yawn when we're tired, but it seems likely that it helps keep us awake. The next time you yawn, try thinking about whether you feel more alert afterwards.

References and Further Information

1. Argiolas A. and Melis MR. (1997) The neuropharmacology of yawning . Eur. J. Pharmacol. 343: 1—16.

2. Neuroscience For Kids: Yawning...And Why Yawns Are Contagious.

3. Yawning may boost brain's alertness .

4. How Stuff Works: What makes us yawn ?

5. World Science: Why is yawning contagious ? 6. Scientific American: Why do we yawn when we are tired? And why does it seem to be contagious ?

About the Author

May Cheng studies the properties of cardiac potassium channels (proteins that help regulate your heartbeat) as part of her PhD training at the University of British Columbia. She also received her Master's and Bachelor's degrees from UBC. When she's not in the lab, she enjoys researching her other great love — food.

May Cheng

I am a PhD student in the Department of Cellular and Physiological Sciences at UBC, where I am investigating the electrical properties of cardiac potassium channels. When not in the lab, I'm probably cooking up a storm, immersed in a book, or catching a movie.

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