New Research Reveals Yawning is a Cerebrospinal Fluid “Self-Cleaning” Process
A seemingly ordinary yawn may be helping your brain “drain waste.” A recent imaging study from the University of New South Wales (UNSW Sydney) shows that during a yawn, cerebrospinal fluid and venous blood flow out of the cranial cavity simultaneously, a phenomenon that may be linked to brain waste clearance, aging, and neurodegenerative diseases.
Yawning is an extremely common instinctive reaction, often appearing when tired, bored, or simply seeing someone open their mouth wide. However, the scientific community has yet to reach a conclusion on “why we yawn.” This research, a collaboration between UNSW and the Australian Neuroscience Research Institute NeuRA, proposes an intriguing possibility: yawning may play a role in promoting fluid flow in and around the brain and assisting in maintaining a stable brain environment.
The research team used real-time magnetic resonance imaging (MRI) to observe what happens inside the head and neck of subjects while yawning, normal breathing, and deep breathing. Led by Professor Lynne Bilston of the UNSW School of Biomedical Engineering, the study was published in the journal *Respiratory Physiology & Neurobiology*. The study included 22 participants, and despite the small sample size, revealed an unexpected fluid dynamics pattern: the direction of cerebrospinal fluid flow differed drastically during a true yawn versus deep breathing.
Cerebrospinal fluid is a clear liquid that surrounds and cushions the brain and spinal cord, like water surrounding a floating object. It protects the central nervous system from impact and is responsible for transporting nutrients and removing waste. The study found that when subjects yawned, cerebrospinal fluid and venous blood flowed out of the skull synchronously; whereas during deep breathing, cerebrospinal fluid flowed back into the cranial cavity, while venous blood continued to flow out of the skull and back to the heart as usual. Even “pretend yawning” deep breaths resulted in cerebrospinal fluid flowing into the cranial cavity; only yawns triggered by “contagious yawning” caused both cerebrospinal fluid and venous blood to flow outward.
Professor Bilston stated that they observed that the physical act of yawning significantly alters the flow patterns of fluids around the brain. Previously, the academic community speculated that yawning might help clear brain waste, but lacked direct evidence. The results of this new study suggest that yawning likely plays a role in the brain’s fluid “cleaning” process, especially during the period leading up to bedtime. Changes in the efficiency of brain waste clearance are widely believed to be associated with Alzheimer’s disease, Parkinson’s disease, and other dementias—diseases that are often linked to abnormal accumulation of metabolic waste in and around the brain, which is often associated with impaired cerebrospinal fluid flow.
One of the paper’s authors, Dr. Adam Martinac, a postdoctoral researcher at NeuRA, pointed out that yawning is an extremely ancient behavior in evolution and has been observed in many species, such as crocodiles. Researchers speculate that dinosaurs likely yawned as well. He emphasized that these animals are unlikely to be yawning because they are “bored by their companions,” so there is likely a more fundamental physiological drive behind it. Interestingly, human fetuses exhibit yawning behavior in the early stages of development, but overall, scientific research related to yawning remains relatively scarce.
To induce genuine yawns, the research team showed subjects videos of humans and even animals yawning to trigger “contagious yawning.” Subsequently, the researchers acquired MRI images at the C3 segment of the cervical spine—the “transport hub” where cerebrospinal fluid and blood enter and exit the cranial cavity. They compared images of subjects yawning with images of deep breathing (but not actually yawning) to clearly distinguish the flow direction of cerebrospinal fluid and venous blood in different states.
The study also provided support for another mainstream hypothesis—that yawning may be involved in regulating brain temperature. The temperature of human brain tissue is typically about 1 degree Celsius higher than other parts of the body, and the temperature of venous blood leaving the brain is typically about 0.2 to 0.3 degrees Celsius higher than that of arterial blood entering the brain. Martinac pointed out that during yawning, cooler arterial blood can be observed flowing into the cranial cavity to compensate for the changes caused by the simultaneous outflow of cerebrospinal fluid and venous blood, providing evidence for the speculation that “yawning has a certain brain cooling function.” Researchers emphasized that while they are not yet willing to assert how much yawning contributes to temperature regulation, overheating of the brain can increase the risk of cell damage, seizures, and cerebral edema, and the brain has extremely narrow and precise “homeostatic” requirements for temperature maintenance.
Humans have evolved multiple mechanisms to regulate brain temperature, such as regulating cerebral blood flow and sweating. The proportion of these complex mechanisms occupied by yawning remains inconclusive. However, this study shows that yawning at least “intervenes” in fluid flow and heat exchange, and more in-depth experiments and clinical studies may emerge in this direction in the future.
In addition to fluid and temperature regulation, the research team also presented a rather interesting observation: human yawns seem to have a “personal fingerprint.” Through continuous sagittal MRI sequences, researchers found that everyone’s tongue movement pattern during yawning is very complex, but highly consistent for the same person, and significantly different for different individuals. Martinac said that this unique tongue movement trajectory is almost like a “biological fingerprint,” and theoretically, it may even be possible to identify individuals based on the dynamic characteristics of their yawns.
Although the researchers also acknowledge that the explanations regarding yawning helping to clear brain waste and regulate brain temperature are still speculative, this work expands people’s imagination of this everyday action. In the future, with larger-scale samples, longer-term follow-up, and comparisons with patients with neurodegenerative diseases, perhaps one day we will discover that those yawns we consider “casual” are actually part of a sophisticated self-maintenance program operating behind the scenes in the brain.