What Is Yawning For? Why Scientists Still Disagree

1. The Idea That Yawns Bring In Oxygen Did Not Hold Up

In the 1987 study, Robert Provine and colleagues had young adult volunteers breathe compressed air, pure oxygen, or air with 3% or 5% carbon dioxide. Breathing sped up with the extra gases, but neither one changed how often people yawned. In a second test, exercise roughly doubled breathing rate, and yawning did not go up[1]. The authors concluded that yawning does not have breathing as its main job.

This is not a perfect knockout. A later commentary points out that the volunteers wore hand-held masks that could leak and counted their own yawns, and that breathing too little oxygen was never tested for safety reasons[1]. Still, the “oxygen top-up” idea predicted that more breathing would mean more yawns, and that did not happen.

2. The Brain-Cooling Idea: What the Experiments Showed

In 2007, Andrew and Gordon Gallup suggested that a yawn helps cool the brain. The idea is that a deep breath of cooler air, together with the stretching of the jaw and face, sends cooler blood toward the head. A brain that is too warm, they argued, does not work as well[2].

To test it, they showed volunteers videos of other people yawning and counted who yawned back. About half of the people who breathed normally or through the mouth did. Nobody told to breathe through the nose did. In a second experiment, people held a pack against the forehead: those with a cold pack yawned far less than those with a warm or room-temperature one. Summaries of the paper give roughly 9% for the cold pack against 41% for the others[2].

These results are interesting, but they come with limits. The researchers did not measure brain temperature. They measured how often people caught a yawn from a video, which may not work like a yawn that comes by itself when you are tired. And the idea came from the same team that ran the tests, so other groups need to repeat it[3].

3. Animals With Bigger Brains Have Longer Yawns

If yawns cool the brain, a bigger brain, which makes more heat, might need a longer yawn. A team led by Jorg Massen at Utrecht University, with Andrew Gallup, checked this. They gathered 1,291 yawns from 101 species: 55 mammals and 46 birds. Yawns were longer in species with heavier brains and more brain cells. Body size alone did not explain the pattern. Mammals also yawned longer than birds with similar-sized brains[4].

The authors say this fits the cooling idea. It does not prove it. It shows two things that rise together, yawn length and brain size, without measuring whether any yawn cooled anything. The yawns came from zoo observations and online videos, so they may not be a perfectly even sample[5].

4. A Dugong Yawning Underwater Shows a Yawn May Not Need a Breath

Dictionaries describe a yawn as opening the mouth slowly while breathing in, then a short breath out. A team at Mie University in Japan watched a captive dugong at Toba Aquarium for about 20 hours. A dugong is a large, plant-eating sea mammal, and it does not breathe underwater. The team recorded 14 yawn-like events, mostly while the animal was resting: the mouth opens slowly, stays wide open, and then closes quickly[6].

The researchers took this as support for the view that yawning does not necessarily require breathing, and they suggested a definition that leaves breathing out. It was one animal and an observation, not a test of the cooling idea[6].

5. Other Scientists Think a Yawn Is Mostly a Signal to Others

In 2010, a Swiss team led by Guggisberg reviewed the experiments on why we yawn. They concluded that explanations based on body function, such as breathing, alertness or temperature, lacked solid experimental support. They favored a social explanation: yawning may tell the people around you that you are sleepy, bored or a little stressed[3]. Andrew Gallup replied in the same journal that the review neglected the temperature evidence, and that yawning may have more than one job. The Swiss team answered in 2011 that the only effect of a yawn they could show for certain was that it is contagious in people, some other primates, and perhaps dogs[3].

So the picture today is this: oxygen is out, brain cooling has some support but no direct proof, and the social explanation has support of its own. They are not strict opposites, since a yawn could do more than one thing. Kininarunote also has a separate article on why yawns spread from person to person.

6. Try It: Keep a Yawn Notebook

For a few days, whenever you yawn, jot down whether the room felt warm or cool, how tired you were, and whether someone nearby had yawned. Just watch. Do not hold a cold pack to your face to copy the experiment, and never try breathing unusual gases.

You will find it hard to tell why any one yawn happened, and that is the point. Researchers face the same problem, and it is why the question is still open.

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