September 30, 2026
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Everyone yawns, including babies, dogs, and chimpanzees. You may even feel one coming on while reading this. Yet researchers have not settled on a single conclusive answer to why we yawn. What science does offer is a handful of well-studied ideas, plus a clear mechanical explanation for why your eyes often water when you yawn. Here is what the research says. Medical News Today

The Oxygen Myth

The most familiar explanation is that we yawn to take in more oxygen or clear out carbon dioxide. It sounds sensible, but it has not held up. Researchers writing in Frontiers for Young Minds note that studies conducted about 30 years ago firmly rejected this idea. A ScienceDaily report on yawning research adds that earlier work never found a link between yawning and blood oxygen levels. Healthline likewise describes the oxygen idea as mostly debunked. www.frontiersin.org +2

The Brain-Cooling Hypothesis

The theory with the most support is that yawning helps regulate brain temperature. Scientific American explains that a deep breath brings in air that slightly cools the brain, and jaw stretching also boosts blood flow to the brain. Scientific American

Several findings back this up:

  • Bigger brains, longer yawns. A study in Biology Letters found that yawn duration was a strong predictor of a mammal’s brain weight and cortical neuron count, regardless of skull or jaw size. Scientific American
  • Yawning follows the seasons. Work led by psychologist Andrew Gallup found that people were more likely to yawn in winter, when the air is cool, than in summer, when they would be inhaling warm air. The same source notes that parakeets more than doubled their yawning when the room got warmer. SubstackSubstack
  • A thermal window. A 2014 study in Physiology & Behavior proposed that if yawning cools the brain through heat exchange with inhaled air, it should occur only within an optimal range of temperatures. PubMed

This also fits with why we yawn when drowsy. The Frontiers researchers explain that yawns are triggered when brain temperature rises, and after a yawn the temperature drops back down. frontiersin

Yawning, Sleep, and Alertness

Yawning is closely tied to shifts between mental states. According to ScienceDaily, we tend to yawn before sleep and after waking, when we are bored or under-stimulated, when anticipating important events, and when stressed. Healthline suggests another reason may be that the body is trying to wake itself up. ScienceDailyhealthline

Boredom has its own proposed explanation. One health article notes that when we are bored, breathing becomes slower and shallower, which may prompt a yawn. Overall, yawning seems to be less a sign of tiredness than a reaction that helps the brain adjust to changing conditions. Substack

Why Yawns Are Contagious

Few behaviors spread as easily. The Frontiers authors point out that we yawn when we see others yawn, and even when we read about yawning. That is why this article may already have worked on you. frontiersin

Contagious yawning develops over time. Spontaneous yawning starts early, since it has been recorded in fetuses as young as 11 weeks. In contrast, children do not start catching yawns until about age 4, and that is around when they develop theory of mind, the understanding that others have different thoughts and feelings. The pattern shows up in other species too. Dogs older than seven months catch human yawns, while younger dogs do not, and infant chimpanzees were immune to yawn contagion while juveniles were not. Why contagious yawning does not (yet) equate to empathy – ScienceDirect +4

Is Contagious Yawning a Sign of Empathy?

This idea is popular, but the evidence is mixed. Health-line cites a Baylor University study finding that people with less empathy were less likely to yawn after seeing someone else yawn. NPR reported on research suggesting that people are more likely to copy the yawn of someone they know. healthlineNPR

Other researchers disagree. A critical review in Physiology & Behavior found inconsistent and inconclusive evidence linking contagious yawning to empathy, with confounds such as visual attention and social inhibition. A dog study in Proceedings of the Royal Society B found no evidence that dogs yawn more for familiar people, a prediction of the empathy hypothesis. And skipping a contagious yawn says nothing bad about you. Healthline is explicit that not catching a yawn is not evidence of psychopathic or sociopathic tendencies. A thermal window for yawning in humans: Yawning as a brain cooling mechanism – ScienceDirect +2

Why Do Your Eyes Water When You Yawn?

Now for the second question. There are two main mechanical explanations, and they probably work together.

1. Muscles Squeeze the Tear Glands

Tears are made in the lacrimal glands, which sit at the upper outer corner of each eye. When you yawn, your face contorts, and your eyes usually shut tight. According to Medical News Today, this action can pull on and stimulate the lacrimal glands, and the effect depends on how forcefully you yawn and how much your facial muscles stretch. One eye-care blog puts it vividly, saying the muscles wring out the glands like a beach towel. Why Do Your Eyes Water When You Yawn? The Science Behind Yawn Tears +2

2. Drainage Gets Interrupted

Tears normally leave the eye through small ducts that lead to the nose. Educational notes on the topic explain that the facial muscle movement briefly compresses these ducts, so tears pool on the eye’s surface and spill over when the eyes open. After the yawn, the muscles relax, and the extra tears drain away or run down the cheek. UnacademyScienceABC

 3. Reflex Tearing

Yawning can also directly trigger the tear pathway. A lens-care resource notes that published research on lacrimation lists yawning among the non-emotional triggers that can stimulate the tear-producing pathway. This is why a big yawn can leave you looking as though you have been crying, even though no emotional tears are involved. Lens.comLens.com

Why Some People Tear Up More Than Others

  • Tiredness and dryness. An eye clinic explains that tired eyes struggle to stay open, and on top of the need to fight dryness, tightly closing your eyes during a yawn can press on the tear glands. Sleep loss matters too, since experiments have found prolonged sleep deprivation can impair lacrimal gland function. CeentaLens.com
  • Dry eyes can cause watery eyes. It sounds backward, but irritation from an unstable tear film can signal the glands to release extra tears to compensate. Lens.com
  • Hydration and tension. One sleep-advice site suggests that being well hydrated makes watery eyes during yawning more likely, while being relaxed reduces the pressure on the tear glands. Bensons for Beds
  • Drainage speed. People whose tear drainage is slower may notice more overflow during a yawn. Lens.com

When to See an Eye Doctor

Teary eyes from yawning are normal. As one source puts it, neither yawning nor watery eyes usually signals a medical problem. However, if you notice excessive tearing whether or not you are yawning, it could point to blocked tear ducts, allergies, or dry eye syndrome. In that case, see an optometrist or GP. Eyeconic likewise advises booking an appointment with an eye doctor if your eyes water more than usual. Why do my eyes water when I yawn? +2

Key Takeaways

  1. Yawning is not mainly about oxygen. That theory has been largely discarded.
  2. Brain cooling is the best-supported explanation, though it is not fully proven.
  3. Yawning seems to help the brain shift between states such as sleepy, bored, and alert.
  4. Contagious yawning appears around age 4, but its link to empathy is still debated.
  5. Watery eyes come from facial muscles pressing on the tear glands and briefly disrupting drainage.
  6. Persistent watery eyes outside of yawning are worth a check-up.

Sources

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