Yawning could have an overlooked role in regulating fluids in the brain, according to a groundbreaking new study, which also sheds fresh light on what happens when someone tries to stifle a yawn.
It was previously thought that the behaviour had evolved mainly to regulate oxygen levels. Alternative theories suggested yawning was meant to signal tiredness to other members of the species.
The new study used MRI scans to reveal that yawning reorganised the flow of the cerebrospinal fluid out of the brain. It’s known that this fluid helps remove waste and move key chemicals from the brain, thereby maintaining pressure balance and supporting the organ’s overall health.
The study also notes that each person yawns in a slightly different way.
“Yawning appears to be a highly adaptive behaviour and further research into its physiological significance may prove fruitful,” the study, published in the journal Respiratory Physiology & Neurobiology, says.
Yawning involves a coordinated motion of the jaw, head and neck in a consistent, repeatable pattern. These movements, the study says, influence the flow of the cerebrospinal fluid around the brain and spinal cord.
The study assessed how yawning affected the fluid-flow pathways near the brainstem and upper spine of 22 healthy participants as compared to movements like normal and deep breathing as well as stifled yawns.
Researchers found yawns increased cerebrospinal fluid flow compared to regular breathing, indicating that it had a “functional physiological purpose” and wasn’t just a social cue to indicate tiredness.
While deep breaths also seemed to increase the flow of the fluid, yawning was “more frequently” associated with cerebrospinal fluid outflow.
Deep breathing, in contrast, showed counter-directional cerebrospinal fluid flow.
Even contagious yawning elicited a marked flow of the cerebrospinal fluid during the expiratory phase that was not apparent during deep or normal breathing, the study found.
Researchers found that muscle actions were nearly the same each time a person yawned, confirming that it was an involuntary movement controlled by the brainstem.
Notably, even stifled yawns lasted nearly the same duration as uninhibited yawns, meaning that stifling did not affect the underlying process.
“Once initiated, yawning proceeds as a structured sequence that can be partially masked but is difficult to fully interrupt,” scientists explained.
The flow pattern observed in the study during yawns also suggests it influences solute transport and heat exchange in the brain.
“Alignment of cerebrospinal and venous blood flow, and increased carotid arterial blood inflow, during yawning may also optimise heat exchange, contributing to cooling of the brain,” the study notes.
If confirmed with more targeted brain studies, the findings can provide further insights into conditions involving impaired CSF flows such as migraine.