Yawning is often perceived as a simple sign of fatigue or boredom. However, new research employing MRI scans suggests this common behavior is a complex process that actively reorders the flow of fluids within the brain. These findings also indicate that each individual’s yawn is unique in its execution.
Across the animal kingdom, from crocodiles to likely dinosaurs, yawning is a widespread behavior among vertebrates. Despite its prevalence, its precise function remains elusive. Various theories propose that yawning might increase oxygen intake, assist in body temperature regulation, improve cerebrospinal fluid circulation, or help manage cortisol levels.
Adam Martinac from Neuroscience Research Australia chairs this ongoing investigation into the purpose and bodily effects of yawning. His team, comprising 22 healthy adults (equally divided between men and women), aimed to clarify these functions.
Participants underwent MRI scans while engaging in four distinct respiratory actions: normal breathing, yawning, voluntarily inhibiting a yawn, and a forceful deep inhalation. The researchers’ initial expectation was that both yawning and a deep breath would cause cerebrospinal fluid (CSF) – the fluid bathing the brain and spinal cord – to move away from the brain. This fluid fills the brain’s cavities and covers its surface.
The analysis of the MRI data yielded surprising results. Contrary to their hypothesis, the scans revealed that yawning drove CSF movement in the opposite direction compared to a deep breath. This unexpected outcome prompted significant reevaluation of their assumptions.
Specifically, the study observed a strong directional coupling between CSF and venous blood flow during yawning, often moving in unison away from the brain and toward the spinal column. This pattern signifies a distinct reorganization of neurofluid dynamics, markedly different from deep breathing. In deep breaths, CSF and venous blood typically flow in opposing ways: venous blood exits the cranium while CSF enters it.
The exact mechanism by which CSF is expelled from the brain during a yawn is still under investigation. The volume of fluid moved also remains an estimation, though preliminary figures suggest it is only a few milliliters per yawn. Martinac hopes to quantify this volume in the research’s subsequent phases.
The research team speculates that the coordinated action of neck muscles, the tongue, and the throat might facilitate this fluid expulsion. Another significant finding demonstrated that yawning boosted carotid arterial inflow by over a third when compared to deep breathing. This increase is likely a consequence of yawning facilitating the simultaneous outward movement of both CSF and venous blood from the cranial cavity, thereby creating space for augmented arterial influx.
Moreover, the study uncovered a personalized aspect to yawning. Each volunteer exhibited a unique pattern in tongue movement during their yawn, suggesting what Martinac described as an “individual yawning signature.”
The team is now focused on determining the physiological benefit of this CSF fluid shift. Potential explanations range from thermoregulation and waste clearance to more subtle, cumulative effects impacting various bodily functions and even emotional group dynamics within social contexts.
The contagious nature of yawning, a phenomenon crucial for the experiment itself, remains a separate enigma. Researchers encouraged participants to yawn by displaying videos of others yawning within the MRI scanner. Martinac humorously shared that he often presents his research last in lab meetings to avoid inducing widespread yawning.
Andrew Gallup from Johns Hopkins University acknowledges the study’s significant contributions to understanding yawning, suggesting the researchers may have understated certain findings. He specifically highlighted the substantial increase in internal carotid arterial flow, which he believes warrants more emphasis and could further support the role of yawning in thermoregulation.
Gallup also noted that the study focused on contagious yawns, positing that spontaneous yawns might induce even more pronounced changes in CSF and blood flow. He pointed out that the observed contagious yawns were shorter than the typical six-second duration of spontaneous human yawns.
Yossi Rathner from the University of Melbourne concurs that some findings might be underplayed but questions the thermoregulation hypothesis. Rathner proposes an alternative theory: as sleep pressure builds, the accumulation of adenosine in the brainstem might be implicated. Yawning, he suggests, could trigger fluid movements that flush away adenosine, thereby temporarily reducing sleepiness and enhancing alertness. While not a direct finding, this remains a plausible implication of the study’s data.
Reference: Biorxiv DOI: 10.64898/2025.12.17.695005