Yawning has an unexpected effect on the fluid inside your brain

Yawning isn’t just a deep breath indicating fatigue or boredom, but a process that reorganizes the flow of fluids from the brain, according to an MRI scan that also suggests that we all yawn in slightly different ways.

Most vertebrates yawn, yet the exact purpose of this behavior remains a mystery. Theories to explain yawning include the belief that it brings more oxygen to the lungs, helps regulate body temperature, improves fluid circulation in the brain and controls levels of the hormone cortisol.

“Crocodiles yawn and dinosaurs probably yawned. It’s an incredibly evolutionarily conserved behavior, but why is it still with us?” he asks Adam Martinac at Neuroscience Research Australia, a non-profit medical institution.

To try to solve the mystery of exactly how yawning works and what effects it has on the body, Martinac and his colleagues recruited 22 healthy adults, equally divided between men and women.

All volunteers were then given an MRI scan while performing four different breathing maneuvers – normal breathing, yawning, voluntary yawn suppression and a strong deep breath.

When team members began analyzing the data, they were shocked by the results. Their hypothesis was that yawning and taking a strong, deep breath would cause cerebrospinal fluid (CSF), the fluid that fills the empty spaces of the brain and covers its surface, to move out of the brain.

“But yawning caused the cerebrospinal fluid to move in the opposite direction than taking a deep breath,” says Martinac. “And we’re just sitting there and we’re like, wow, we definitely didn’t expect that.”

More specifically, they found that CSF and venous blood flow became strongly directionally coupled during yawning, often moving away from the brain and toward the spine. This suggests a distinct reorganization of neurofluid dynamics compared to deep breathing, where CSF and venous blood flows typically move in opposite directions, with venous blood flowing out of the brain while CSF flows in.

The exact mechanism of how cerebrospinal fluid moves out of the brain during yawning is still unclear, as is the amount of cerebrospinal fluid that moves — though it’s estimated to be only a few milliliters per yawn, Martinac says. He hopes the volume will be quantified as part of the next phase of research.

“We think it may be the neck muscles, as well as the tongue and throat, that coordinate the withdrawal of this fluid,” he says.

Another key finding is that yawning increased carotid inflow by more than a third compared to deep breathing. This is probably because yawning causes both cerebrospinal fluid and venous blood to flow out of the cranial cavity—rather than venous blood flowing out and CSF flowing in—making room for this extra arterial inflow.

Each volunteer also had a unique and distinct yawn in terms of tongue movement. “Each individual seems to have what appears to be an individual yawning signature,” says Martinac.

Another puzzle the team wants to further solve is the benefit of this CSF movement to our bodies.

“Maybe it’s thermoregulation, maybe it’s waste removal, or maybe it’s none of those things,” he says. “You could probably survive without yawning, but there are maybe six or seven or eight different very small effects that just cumulatively help us regulate waste removal, thermoregulation, and even the emotional group dynamics of yawning.”

The fact that yawning is so contagious is also a mystery—though it was crucial to the experiment because the researchers encouraged participants to yawn using a screen inside the MRI scanner that showed videos of other people yawning.

“Whenever we have my lab meeting or I give a presentation, I always have to go last because when I start talking about my research, everyone starts yawning,” says Martinac.

Andrew Gallup from Johns Hopkins University in Maryland says the study has a number of important findings that add significantly to the understanding of yawning. He also says that the researchers downplayed some of their findings — particularly that work suggests that yawning has an important thermoregulatory role.

“The fact that the internal carotid artery increased by 34 percent during … yawning is a really important finding that seems to be overlooked or at least downplayed in the current version of the paper,” says Gallup.

He also points out that the study looked at contagious yawning rather than spontaneous yawning and suggests that the impact of spontaneous yawning may be even greater.

“There is reason to expect that spontaneous yawning will cause even greater changes in CSF and blood flow than described here,” he says. “Indeed, the videos suggest that the contagious yawns were relatively short compared to the average duration of a spontaneous yawn in humans, which is around six seconds.”

Yossi Rathner at the University of Melbourne in Australia agrees that the team underestimated some of their findings, but strongly disagrees with the argument for thermoregulation.

Rathner says this may be because, as sleep pressure increases, a chemical compound called adenosine— which has links to the regulation of sleep and wakefulness – accumulates in the brainstem. “Yawning can induce fluid movements in the brainstem that flush out adenosine, temporarily relieving sleep pressure and increasing alertness,” he says. “This is not a direct finding of the study, but a possible implication of the data.”

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