Yawning has an unexpected influence on the fluid inside your brain

Yawning is not just deep breathing indicating fatigue or boredom, but a process that reorganizes the flow of fluids out of the brain, according to MRI scans that also suggest we each yawn in a slightly different way.

Most vertebrate animals yawn, but the exact purpose of this behavior remains a mystery. Theories explaining yawning suggest that it brings more oxygen to the lungs, helps regulate body temperature, improves the flow of fluids around the brain, and manages levels of the hormone cortisol.

“Crocodiles yawn and dinosaurs probably yawn. It’s this incredibly evolutionarily conserved behavior, but why is it still present in us?” asks Adam Martinac of Neuroscience Research Australia, a non-profit medical institution.

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

All volunteers then underwent an MRI while performing four different breathing maneuvers: normal breathing, yawning, voluntary suppression of a yawn, and deep, forced breathing.

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

“But yawning triggered a movement of the CSF in the opposite direction to that of deep breathing,” explains Martinac. “And we’re just sitting there like, whoa, we definitely didn’t expect that.”

Specifically, they found that CSF and venous blood flow became strongly directionally coupled during yawning, often moving away from the brain toward the spine. This suggests a distinct reorganization of neurological fluid dynamics relative to deep breathing, when CSF and venous blood flows typically move in opposite directions, with venous blood flowing out of the brain while CSF flows into it.

The exact mechanism by which CSF is moved out of the brain during a yawn is not yet clear, as is the amount of CSF displaced – although it is estimated to be just a few milliliters per yawn, Martinac says. He hopes to quantify the volume as the next stage of research.

“We think it could be due to the muscles in the neck, the tongue as well, as well as the throat, coordinating to extract this fluid,” he says.

Another key finding is that yawning increases carotid arterial flow by more than a third compared to deep breathing. This is likely because yawning prompts both CSF and venous blood to flow out of the cranial cavity – rather than venous blood flowing out and CSF entering – thereby creating space for this additional arterial inflow.

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

Another puzzle the team wants to solve next is how this movement of the CSF benefits our body.

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

The fact that yawning is so contagious is also a mystery – although it was crucial to the experiment, as the researchers encouraged participants to yawn by using a screen inside the MRI scanner that showed video footage of other people yawning.

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

Andrew Gallup, of Johns Hopkins University in Maryland, says the study has many important findings that make an important contribution to the understanding of yawning. He also claims that the researchers downplayed some of their findings – particularly that this work strengthens the argument that yawning plays an important thermoregulatory role.

“The fact that internal carotid arterial flow increased by 34 percent during… yawning is a very important finding that appears to be overlooked or at least downplayed in the current version of the paper,” Gallup explains.

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

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

Yossi Rathner of the University of Melbourne, Australia, acknowledges that the team underestimated some of its findings, but strongly disagrees with the arguments for thermoregulation.

Rathner says it could be that as sleep pressure increases, a chemical compound called adenosine – which has links to sleep-wake regulation – builds up in the brainstem. “Yawning can trigger fluid movements in the brainstem that clear adenosine, temporarily alleviating sleep pressure and increasing alertness,” he says. “This is not a direct conclusion of the study, but a possible implication of the data.”