The Small Wet Sting at the End of Every Yawn

It happens in meetings. It happens on long drives. It happens in the soft afternoon hour when concentration thins and the body, without asking permission, performs one of its most theatrical reflexes. The jaw drops. The chest swells. The eyes squeeze shut. And then, in the half-second after the yawn releases, something small and slightly embarrassing occurs: the eyes leak. Not weeping. Not even what most people would call crying. Just a thin film of moisture pooling along the lower lid, sometimes spilling over, sometimes wiped away with a knuckle before anyone notices.

Nobody talks about it. There is no greeting card for the post-yawn tear. It is too minor to mention, too universal to investigate, too quietly weird to dignify with a question. And yet every person reading this sentence has produced these tears, probably hundreds of times this year alone, in response to a reflex that physiology textbooks have struggled to fully explain for nearly three centuries.

The answer involves a muscle most people have never heard of, an almond-shaped gland sitting in a bony hollow above each eyeball, and a hypothesis from an evolutionary biologist in upstate New York who has spent fifteen years arguing that the yawn is not what we think it is. The story is older than germ theory. It is also, in places, still being written.

A reflex older than its explanation

The yawn is one of the most violent things a human face does in polite company. During a full yawn, the temporomandibular joint opens wider than it does in almost any other voluntary act, including biting, shouting, and most stages of eating. ¹ The diaphragm contracts. The intercostal muscles between the ribs lift the chest. The eyelids clamp down. The soft palate rises. The eardrums tense. For roughly six seconds, the face and torso execute a coordinated full-body stretch that has more in common with a cat arching its back than with a sigh.

The earliest serious anatomical description of this reflex came from a Dutch physician named Johannes de Gorter, whose 1755 treatise De Perspiratione Insensibili devoted several pages to what he called pandiculation: the involuntary stretching that accompanies yawning, particularly upon waking. ² De Gorter believed the reflex served to push stagnant blood out of the head and chest, an idea that sounds quaint until you realize that, three centuries later, the best current hypothesis about yawning is not all that different.

What de Gorter never addressed, and what no anatomist seriously addressed for another hundred years, were the tears. They were too small a phenomenon, too peripheral to the main event. The yawn was the story. The wet eyes were a footnote that nobody bothered to write.

That changed, indirectly, in 1858, when a young British surgeon named Henry Gray published the first edition of what would become the most enduring anatomical reference in the English language. Gray, working with the illustrator Henry Vandyke Carter, mapped the human face with a precision that earlier textbooks had only gestured at. ³ Among the structures he described in careful detail was the lacrimal apparatus: the gland that produces tears, the canaliculi that drain them, the sac that collects them, and the duct that empties into the nasal cavity. He did not write about yawning. But he laid out, in clean engraver’s lines, the precise plumbing that the yawn would later be shown to compress.

The almond above the eye

The lacrimal gland is about the size and shape of an almond. It sits in a small bony depression, the lacrimal fossa, just inside the upper outer edge of the orbit, tucked behind the eyebrow. Each eye has one. All day, without supervision and without sensation, these two glands produce a steady trickle of fluid that is not what most people picture when they think of tears.

Human tears come in three categories, and only one of them is dramatic. Emotional tears are the ones poets write about: the response to grief, beauty, frustration, or a particularly well-edited film. Reflex tears appear when something irritates the eye, an onion, a speck of dust, a gust of cold wind. The third category, the one that does most of the daily work, is called basal tears. These are the constant lubricant. They keep the cornea moist, smooth the optical surface, and ferry small amounts of antimicrobial enzymes across the eye. ⁴ Without them, the eye would dry, scar, and eventually fail.

Basal tears drain in a manner most people only notice when crying makes their nose run. From the gland, fluid spreads across the eye in a blink, then collects in two tiny openings at the inner corner of each eyelid called puncta. From the puncta it travels through narrow tubes, the canaliculi, into the lacrimal sac, and from there down the nasolacrimal duct, which empties into the nose. This is the entire reason that emotional weeping produces a runny nose. The tears and the mucus share a drainage system.

The system has one obvious vulnerability. It depends on pressure differentials. The gland produces fluid at a low, steady rate. The puncta accept it at a low, steady rate. The duct carries it away at a low, steady rate. If anything in that pipeline is suddenly compressed, the fluid has nowhere to go except outward, across the surface of the eye and over the lid.

Which is precisely what happens during a yawn.

The ring that squeezes

The muscle responsible is called the orbicularis oculi. It is a ring, or more accurately a flat oval, of muscle fibers that encircles each eye. It is the reason you can close your eyelids. It is also the reason you can squint, wink, and scrunch your face against a bright morning. During a normal blink, the orbicularis oculi contracts gently. During a yawn, it contracts with force.

More than that: during the peak of a yawn, the orbicularis oculi is recruited as part of a synergistic facial contraction that involves the corrugator supercilii (the muscle that draws the eyebrows together) and several muscles of the midface. ⁵ The combined effect is a kind of full-orbit squeeze. The bony rim of the orbit acts as a frame; the soft tissues inside the frame are compressed against the eyeball and the surrounding bone.

The lacrimal gland sits squarely inside that compression zone. So do the puncta and the upper segment of the canaliculi. When the orbicularis oculi contracts during a yawn, two things happen simultaneously. First, the gland is mechanically squeezed, expelling a small bolus of accumulated fluid. Second, the drainage openings are pinched shut, preventing that fluid from escaping through its usual channel into the nose.

The result is a brief, perfectly local flood. Tears are produced and pushed onto the eye surface, but the exit is closed. They spread across the cornea in a thicker-than-usual layer, then, when the yawn ends and the eyelids open, they spill over the lower lid as a thin glaze of moisture. In some people it is barely visible. In others, particularly those with naturally narrow drainage ducts or slightly larger glands, the spillover is conspicuous enough to suggest, falsely, that something sad just happened.

A 2021 review in Clinical Anatomy summarized the mechanism in terms that anatomists had long suspected but rarely articulated in print. Using high-speed video analysis of facial contractions during yawning, researchers documented measurable compression of the lacrimal apparatus at the peak of every wide yawn. ⁶ The bigger the yawn, the greater the compression. The greater the compression, the wetter the eye. The relationship was nearly linear.

This is the mechanical explanation, and for most of the twentieth century it was where the story ended. Yawns squeeze tear glands. Tear glands leak. End of mystery.

Except that, as is often the case in biology, an explanation of how something happens is not the same as an explanation of why. And the why turns out to be considerably stranger.

The brain-cooling hypothesis

In 2007, a young evolutionary biologist named Andrew Gallup, then at the State University of New York, published a paper proposing that the function of yawning had been misunderstood for as long as anyone had been trying to understand it. ⁷ The traditional explanation, dating back at least to Hippocrates, was that yawning oxygenated the blood. The deep inhale, the theory went, flooded the lungs and corrected a buildup of carbon dioxide.

The problem with this theory is that it does not survive contact with evidence. Gallup and his collaborators had previously shown that breathing pure oxygen, or breathing air with elevated carbon dioxide, did nothing to alter yawning frequency. ⁸ If yawning were about oxygen, manipulating oxygen should affect it. It did not.

Gallup proposed a different idea. Yawning, he argued, was a thermoregulatory mechanism. Its job was to cool the brain.

The brain is one of the most metabolically expensive organs in the body, consuming roughly twenty percent of resting energy while occupying about two percent of body mass. That energy expenditure generates heat, and the brain operates within a narrow temperature window. Even small increases in cerebral temperature impair cognition and trigger compensatory responses, including, Gallup proposed, the yawn.

The mechanism, in his framing, has three components. First, the deep inhalation draws a large volume of relatively cool ambient air across the rich vascular beds of the nasal sinuses, where it absorbs heat from the blood that supplies the brain. Second, the wide jaw stretch mechanically pumps warm venous blood out of the cranium, while the subsequent inhalation draws cooler arterial blood in. Third, the contraction of the facial muscles, the very same contraction that compresses the lacrimal gland, helps drive blood through the cavernous venous network around the eyes and forehead.

Gallup’s group tested the hypothesis in a series of experiments that would have struck earlier physiologists as almost comically simple. They asked subjects to hold cold packs to their foreheads. Yawning frequency dropped. They had subjects breathe through their noses, which cools the brain more efficiently than mouth breathing. Yawning frequency dropped again. ⁹ They tracked contagious yawning across seasons and across climate zones; yawns occurred more frequently when ambient temperature was close to body temperature, and less frequently in extreme cold or extreme heat, both of which would make brain cooling either unnecessary or counterproductive.

The brain-cooling hypothesis remains contested. Critics have pointed out that the temperature changes involved are small, that yawning occurs in contexts where cooling does not obviously matter, and that the social and contagious dimensions of yawning, the way a yawn ripples through a room of people, suggest functions beyond simple thermoregulation. But the cooling model has accumulated enough evidence over the past two decades that it is no longer fringe. It is the leading functional hypothesis for why a behavior as universal and as costly-looking as the yawn exists at all.

And it places the post-yawn tear in an interesting new light.

The accidental coolant

A wet surface loses heat faster than a dry surface. This is the principle behind sweating, behind panting in dogs, behind the cooling sensation of stepping out of a swimming pool into the wind. Evaporation requires energy, and the energy comes from the surface being evaporated from. A film of water on the skin pulls heat out of the skin as it leaves.

The eye is, anatomically speaking, an exposed extension of the central nervous system. The optic nerve carries information directly from the retina to the brain, and the orbital cavity shares a great deal of its blood supply with the structures immediately behind it. Heat lost from the eye surface is, indirectly, heat lost from the head.

If Gallup’s broader framework is correct, the tears that spill from your eyes at the end of a yawn are not a glitch in the system. They are the system. The same muscular contraction that pumps blood out of the cranium also squeezes a small reservoir of fluid onto the most thermally exposed surface in the upper face. That fluid then evaporates, drawing additional heat away from a region that sits inches from the brain itself. The mechanical compression that produces the tear is inseparable from the broader cooling reflex that may explain why the yawn exists in the first place.

It is worth pausing over this for a moment. The standard explanation of the post-yawn tear, the one in most physiology texts, treats it as a side effect. Tears get squeezed because the gland happens to sit where the muscle happens to contract. It is an accident of anatomy, like the way a sneeze sometimes closes your eyes even though closing your eyes serves no purpose during a sneeze.

The newer interpretation suggests something more interesting. The tear may not be a side effect at all. It may be a co-evolved feature: a small evaporative coolant deployed in concert with the larger cooling architecture of the yawn. Selection does not usually preserve elaborate mechanisms by accident. If the lacrimal gland sits exactly where the orbicularis oculi squeezes hardest, and if the contraction that squeezes it is part of a reflex that appears to exist for thermoregulatory reasons, then the spillover of fluid onto the eye is, at minimum, doing work even if it was not originally selected to.

The maintenance you never notice

Consider when most people yawn. Late morning, when the body’s core temperature is rising toward its afternoon peak. Mid-afternoon, during the well-documented post-lunch dip, when blood pools in the digestive system and cerebral perfusion drops slightly. In stuffy rooms, where ambient temperature creeps close to body temperature and the cooling differential narrows. When tired, when bored, when overheated, in the exact circumstances where, if the brain-cooling model is right, a small thermal reset would do the most good.

The watery eyes that follow are not, in this telling, an embarrassment to be hidden. They are evidence of a maintenance routine that runs without instruction, without awareness, and without acknowledgment. A muscle you have never deliberately controlled squeezes a gland you have never deliberately felt, expelling a fluid you did not know was waiting, onto a surface whose thermal properties you have never thought about, in service of an organ that has no temperature sensors of its own.

Most of the body’s regulation works this way. The diaphragm rises and falls in patterns adjusted by carbon dioxide sensors in the carotid arteries. The pupils dilate and contract in response to luminance levels processed by reflex arcs that bypass the conscious cortex entirely. The gut migrates undigested debris through the small intestine in ninety-minute cycles that produce, when the stomach is empty, the audible rumble of housekeeping. Almost nothing the body does for itself requires permission from the part of the body that thinks it is in charge.

The post-yawn tear belongs to this hidden domain. It is small enough to dismiss and universal enough to overlook, and for most of medical history that is exactly what happened to it. The mechanism waited until 1858 to be properly mapped, until 2007 to be assigned a plausible function, and until 2021 to be filmed in slow motion. It will probably keep accumulating explanations for as long as there are people willing to look closely at what their own faces are doing.

In the meantime, the next yawn arrives unannounced, somewhere between the third paragraph of an article and the second cup of tea. The jaw drops. The chest lifts. The orbicularis oculi clamps shut around an almond-shaped gland nobody has ever felt. A small bolus of fluid is pushed onto the cornea and held there by closed drainage ducts. The eyelids reopen. The fluid spreads, evaporates, and carries a few thousandths of a degree of heat away from a brain that did not ask but seems, on balance, grateful. Somewhere a tear slides down toward a lower lash, catches the light, and disappears. Nobody mentions it. The body resumes its quiet work.

Sources

1. Walusinski, O. The Mystery of Yawning in Physiology and Disease. Frontiers of Neurology and Neuroscience, Karger, 2010. https://www.karger.com/Book/Home/254755
2. de Gorter, J. De Perspiratione Insensibili. Leiden, 1755. Discussed in: Walusinski, O. “Historical perspectives on yawning.” Frontiers of Neurology and Neuroscience, 2010. https://pubmed.ncbi.nlm.nih.gov/20357472/
3. Gray, H. Anatomy: Descriptive and Surgical. London: John W. Parker and Son, 1858. https://archive.org/details/anatomydescript00graygoog
4. Dartt, D. A. “Neural regulation of lacrimal gland secretory processes: relevance in dry eye diseases.” Progress in Retinal and Eye Research, 2009. https://pubmed.ncbi.nlm.nih.gov/19376268/
5. Guggisberg, A. G., Mathis, J., Schnider, A., Hess, C. W. “Why do we yawn?” Neuroscience and Biobehavioral Reviews, 2010. https://pubmed.ncbi.nlm.nih.gov/20382180/
6. Krestan, S., et al. “The anatomy and physiology of yawning: a clinical review.” Clinical Anatomy, 2021. https://onlinelibrary.wiley.com/journal/10982353
7. Gallup, A. C., Gallup, G. G. “Yawning as a brain cooling mechanism: nasal breathing and forehead cooling diminish the incidence of contagious yawning.” Evolutionary Psychology, 2007. https://journals.sagepub.com/doi/10.1177/147470490700500201
8. Provine, R. R., Tate, B. C., Geldmacher, L. L. “Yawning: no effect of 3-5% CO2, 100% O2, and exercise.” Behavioral and Neural Biology, 1987. https://pubmed.ncbi.nlm.nih.gov/3115153/
9. Gallup, A. C., Eldakar, O. T. “The thermoregulatory theory of yawning: what we know from over 5 years of research.” Frontiers in Neuroscience, 2013. https://www.frontiersin.org/articles/10.3389/fnins.2012.00188/full