Research has uncovered that yawning plays a significant role in the movement of fluids within the brain, challenging previous assumptions about this common behavior. A study led by researchers at Neuroscience Research Australia utilized MRI scans to explore how yawning affects cerebrospinal fluid (CSF) and venous blood flow. The results suggest that yawning is not merely a sign of fatigue or boredom, but may serve crucial physiological functions.
The investigation involved 22 healthy adults, evenly split by gender. Participants underwent MRI scans while performing various breathing techniques, including normal breathing, yawning, voluntary yawn suppression, and forceful deep breaths. Researchers were surprised to find that yawning triggered a movement of CSF in the opposite direction to that of deep breaths. According to Adam Martinac, a lead researcher at the institute, “The yawn was triggering a movement of the CSF in the opposite direction than during a deep breath.”
During yawning, CSF and venous blood flow became closely coupled, moving together away from the brain and toward the spinal column. This contrasts with deep breathing, where CSF flows into the brain while venous blood flows out. Although the precise mechanisms for this fluid movement remain unclear, it is estimated that only a few milliliters of CSF are moved during a yawn.
Another notable finding from the study is that yawning increased the inflow of blood from the carotid artery by over a third compared to deep breathing. This suggests that yawning may create additional space within the cranial cavity, allowing for increased blood flow. Each participant exhibited a unique “yawning signature,” indicating that individuals may yawn differently in terms of tongue movement and other factors.
The researchers aim to further understand the potential benefits of CSF movement during yawning. Martinac speculates that it could relate to thermoregulation, waste clearance, or other physiological processes. “You could probably survive without yawning, but maybe there are several small effects that collectively aid in regulating waste clearance and thermoregulation,” he said.
The contagious nature of yawning also poses an intriguing question. To encourage yawning among participants, the researchers displayed videos of others yawning during the MRI scans. Martinac noted the tendency of people in his lab meetings to yawn when discussing his research, highlighting the phenomenon’s social aspect.
Experts in the field have praised the study for its contributions to understanding yawning. Andrew Gallup, a researcher at Johns Hopkins University, emphasized the importance of the finding that internal carotid arterial flow increased by 34% during yawning. He believes this aspect was somewhat downplayed in the study’s presentation. Gallup also suggested that spontaneous yawning might yield even larger changes in CSF and blood flow compared to the contagious yawns examined in the study.
In contrast, Yossi Rathner from the University of Melbourne expressed reservations about linking yawning to thermoregulation. He proposed an alternative theory that yawning may help flush out a chemical compound called adenosine, which is associated with sleep-wake regulation, thereby temporarily alleviating sleep pressure.
This study opens new avenues for further research into the physiological roles of yawning. With the next phase of research, the team hopes to quantify the volume of CSF moved during yawning and explore its implications for human health. As yawning continues to be a subject of fascination, understanding its underlying functions may eventually uncover deeper insights into its evolutionary significance and biological importance.
