Recent MRI studies reveal that yawning is not simply a sign of fatigue or boredom; it reorganizes fluid flow in the brain, indicating that yawning is unique for each individual.

Yawning is observed in most vertebrates, yet its precise purpose remains largely unclear. Theories suggest that yawning enhances oxygen intake, regulates body temperature, boosts fluid circulation in the brain, and modulates cortisol hormone levels.

“Crocodilians yawn, and even dinosaurs likely did too. This behavior has evolutionary significance, but why does it persist today?” queries Adam Martinac from Neuroscience Research Australia, a non-profit medical organization.

To understand yawning’s mechanisms and its impact on the body, Martinac and his team involved 22 healthy participants, evenly divided by gender, in their study.

Participants underwent MRI scans while performing four distinct breathing actions: regular breathing, yawning, voluntarily suppressing yawns, and deep breathing.

The data analysis revealed surprising findings. The initial hypothesis was that yawning and deep breathing would similarly facilitate the movement of cerebrospinal fluid (CSF) out of the brain.

“However, yawning caused CSF to flow in the opposite direction compared to deep breathing,” states Martinac. “We were genuinely surprised by this outcome.”

Specifically, the study discovered a strong directional coupling between CSF and venous blood flow during yawning, both moving away from the brain toward the spine. This stands in contrast to deep breathing, where CSF and venous blood typically travel in opposing directions—CSF flows in while venous blood flows out.

The specific mechanisms governing CSF movement during yawning, including the volume expelled, remain unclear. Current estimates suggest a mere few milliliters of CSF are moved per yawn. Future research aims to quantify this further.

“It’s likely that neck, tongue, and throat muscles collaborate to facilitate this fluid movement,” he adds.

Another noteworthy finding is that yawning augmented carotid artery inflow by over one-third compared to deep breathing. This is presumably because yawning clears CSF and venous blood from the cranial cavity, allowing for increased arterial inflow.

Each participant exhibited a distinct “yawn signature,” showcasing variability even in tongue movements. “It seems that everyone has a unique pattern to their yawns,” says Martinac.

One intriguing area for future research is the physiological benefits arising from CSF movement during yawning.

Theories suggest that this could relate to thermoregulation, waste removal, or potentially other unexplored functions. “It is possible to live without yawning, but there are several subtle effects that likely assist in waste management, temperature control, and even the social dynamics of yawning,” he explains.

The contagious nature of yawning adds another layer of mystery and proved essential for this study, as video footage of yawns was shown to participants while they were inside the MRI scanner.

“In our lab meetings, I always have to speak last because my discussion of this research triggers yawning in everyone else,” Martinac shares.

Researchers like Andrew Gallup from Johns Hopkins University highlight the significant findings of the study, emphasizing its contributions to our understanding of yawning. He also noted that some of the findings have been understated, particularly those affirming yawning’s role in temperature regulation.

“The observed 34% increase in internal carotid artery flow during yawning is a critical finding that deserves more attention,” Gallup asserts.

He further noted that the study focused on contagious yawns versus spontaneous yawns, indicating that spontaneous yawns may induce even greater changes in CSF and blood flow.

“The video suggests contagious yawns are shorter than the average spontaneous yawn, which lasts about six seconds,” he notes.

Professor Yossi Rathner from the University of Melbourne agrees the team may have underestimated certain findings but opposes some claims concerning thermoregulation.

“Increased sleep pressure can elevate levels of a compound called adenosine that accumulates in the brain stem. Yawning seems to facilitate fluid movement in the brain stem, helping to flush out adenosine, temporarily alleviating sleep pressure and boosting alertness,” Rathner explains. “While this isn’t a direct conclusion from the study, the data strongly implies this relationship.”