The Surprising Effects of Yawning on Brain Fluid Dynamics

A recent study has uncovered an unexpected effect of yawning on the movement of cerebrospinal fluid (CSF) that protects the brain, though the significance of these findings remains unclear.

Researchers from the University of New South Wales in Australia suggest that these results could help shed light on why humans and various other species have the ability to yawn.

The team used MRI scans to examine the heads and necks of 22 healthy individuals while they were instructed to yawn, take deep breaths, suppress yawns, and breathe normally.

Considering that yawning and deep breathing involve similar processes, the researchers anticipated that the scans would yield similar results. However, they were surprised to discover a significant difference: unlike deep breathing, yawning caused CSF to flow away from the brain.

“The yawn was triggering a movement of the CSF in the opposite direction than during a deep breath,” said neuroscientist Adam Martinac, sharing his surprise at the findings.

Notably, this effect wasn’t consistent across all participants and appeared less frequently in men, although the researchers warned that this variability might have been influenced by the MRI scanner itself.

The analysis also indicated that both yawning and deep breathing enhanced blood flow away from the brain, effectively making more room for fresh blood. Interestingly, while the direction of blood flow didn’t change with yawning, it did see an initial increase of about a third during the carotid arterial flow into the brain, hinting at multiple potential reasons for the behavior.

All participants exhibited distinct yawning patterns that followed a consistent rhythm, suggesting that each person possesses a unique central pattern generator for yawning.

“This flexibility might explain the variations in yawning patterns among participants while still retaining recognizable, individual-specific traits,” the researchers noted. They imply that these yawning patterns are innate rather than learned.

The next intriguing question is what all this actually means. Why does yawning differ so markedly from deep breathing in terms of CSF movement, especially since CSF is crucial for maintaining the nervous system’s operations by providing nutrients and eliminating waste?

One hypothesis presented by the researchers is that yawning might play a role in “cleaning out” the brain. Another theory posits that it could serve a cooling function for the brain.

Moreover, yawning seems to have a close connection to brain size; typically, larger brains lead to longer yawns, which is a fun fact to share the next time you catch yourself yawning.

Yawning remains a puzzling phenomenon, with its purpose not fully understood. It’s a behavior seen across many species and often appears to be contagious among both humans and animals.

“Yawning appears to be a highly adaptive behavior, and further exploration of its physiological significance could be beneficial for understanding central nervous system balance,” the researchers concluded.

This research has not yet undergone peer review, but the findings are accessible on bioRxiv.