Yawning has been shown to push the brain’s clear fluid in the opposite direction of a deep breath.
That reversal suggests yawning may help cool the brain and move waste products, not just signal boredom or empathy.
What the scanner caught
Inside a hospital-style scanner at the University of New South Wales (UNSW), 22 volunteers followed a protocol of yawns and deep breaths.
By comparing those actions, biomedical engineer Dr. Adam Martinac saw the brain’s surrounding fluid behave differently during yawns and deep breaths.
The researchers described a reversal that showed up during yawns and disappeared during deep breaths.
Because the mismatch showed up in healthy adults, the team says yawning likely serves a physical purpose, not just mood.
Two flows line up
During a yawn, cerebrospinal fluid, the clear liquid that cushions brain and spine, often flowed down toward the neck.
At the same moment, blood drained through a major neck vein, returning blood to the heart. Unlike yawning, forceful deep breaths usually pulled the fluid upward as that venous blood still headed downward.
That mismatch suggests a yawn runs its own pressure pattern, even when the lungs move the same amount.
Blood surges then settles
Past imaging work has tied inspiration to strong pulses of cerebrospinal fluid in the head. So the team expected a deep breath to mimic a yawn once the chest and jaw opened wide.
Instead, yawning started with a quick pulse of blood to the brain, and blood flow through the neck arteries increased by about one-third.
Because the skull cannot expand, that added rush likely pushes fluid and used blood out first, making room for the fresh supply to move in.
A built-in motor script
Across repeated yawns, each volunteer moved the tongue in a personal pattern that returned again and again.
For most people, those motions matched so closely that the team measured about 86% similarity across separate yawns.
“Yawning kinematics were highly reproducible within individuals across repeated events, indicating a stable motor sequence consistent with brainstem pattern-generator control,” wrote Dr. Adam Martinac, a biomedical engineer at UNSW.
Because that consistency looks built in, a central pattern generator, a brain circuit that runs automatic actions, may be controlling yawns.
Swallowing follows yawning
Even with lips pressed shut, volunteers still ran through much of the yawn motion, including the tongue’s rapid flip.
Instead of stopping the reflex, stifling mostly changed what outsiders could see, while internal movements kept their timing.
After a full yawn, a swallow arrived within one normal breath about 81% of the time in this group.
That tight pairing suggests the brain treats yawning as a coordinated package, not a random breath that goes long.
The waste clearance theory
During waking hours, brain cells produce chemical leftovers that must leave, and cerebrospinal fluid helps carry them away.
Researchers describe a glymphatic system, a pathway that moves fluid through brain tissue for cleanup. By briefly boosting cerebrospinal fluid motion, a yawn could increase mixing where the brain meets the spine.
Still, Martinac’s team tracked motion, not waste levels, so the cleanup idea remains a target for new experiments.
Cooling and brain size
Cooling theories treat yawning as a way to manage heat, since blood and fluid carry warmth away from active tissue.
In humans, one test found yawns clustered within a thermal window, a temperature range where cooling works, and dropped in heat.
Across mammals, yawn duration rose with brain weight and neuron counts in the outer layer of the brain.
If yawns help cool, that scaling hints that heat control becomes harder as brains grow, though direct tests remain rare.
The social pull of yawns
Beyond the body mechanics, yawning stays famous for being contagious, and one person’s yawn can trigger others nearby.
In the brain, social cues can activate the same motor program, especially when attention is high and relationships feel close.
Rather than teaching a new behavior, that spread seems to release an existing reflex, which fits the idea of hardwiring.
If yawns also move brain fluid, contagion could share that brief internal adjustment across a group, but proof is missing.
Limitations and next steps
Inside the scanner, some volunteers felt muscle twitching, and the preprint has not yet been peer reviewed.
That twitching comes from peripheral nerve stimulation, nerve activation from rapidly changing magnetic fields, and it may distort pressure patterns.
At UNSW, Martinac’s group flagged possible sex differences in flow direction, then linked them to stronger stimulation in men.
Future experiments should track pressures and fluids at the same time, ideally outside of scanners, to confirm what a natural yawn does.
By showing that yawning reorganizes fluid and blood movement in the head, the work gives a physical reason to keep studying it.
Better measurements during sleep, illness, or everyday life could reveal whether those brief pulses support cooling, cleanup, or neither.
The study is published in bioRxiv.
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