Yawning is an involuntary reflex action that is widely considered to be a sign of fatigue, boredom, or stress. However, a study published in the Respiratory Physiology & Neurobiology, titled Biomechanics of contagious yawning: Insights into cranio-cervical fluid dynamics and kinematic consistency, has revealed something unexpected about the effect of yawning on the flow of fluid that protects the brain.
While scientists are not clear about the impact of the findings, researchers from the University of New South Wales in Australia believe that the discovery can provide clues as to why humans and other animals evolved the capacity to yawn in the first place.
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There were 22 healthy participants in the study, who were asked to yawn, take deep breaths, stifle yawns, and breathe normally. The research team used MRI to scan their heads as they performed the action.
How yawning shows up in MRI scans
Since the processes of yawning and deep breathing share many similarities, scientists expected them to show up similarly in MRI scans. However, the images obtained in the study revealed that the cerebrospinal fluid (CSF) moves away from the brain while yawning in some cases, and the opposite way during deep breaths.
The odd phenomenon apparently occurred less often in men, and researchers caution that it may be because of interference from the scanner itself. The analysis did reveal that the flow of blood leaving the brain increases during both yawning and deep breathing, which allows fresh blood to be pumped in.
Unique yawning patterns
The researchers also discovered that each participant had a unique yawning pattern that they closely followed each time they yawned.
According to neuroscientist Adam Martinac, who spoke with New Scientist on the subject, “Each person yawns in a unique way – so the tongue motion during the yawn is different between people, but very consistent for each person. It’s almost like a fingerprint, so you could possibly identify someone just based on how they yawn.”
This is likely a sign that all individuals have their own central pattern generator that determines their yawn.
What do the findings mean?
While researchers are yet to clearly establish the reason why yawning and deep breathing affect the CSF differently, they have reportedly been considering two possibilities.
The CSF is known to deliver nutrients and remove wastes from the central nervous system. As such, yawning can play a specific role in clearing the brain. On the other hand, it may also support the cooling function of the brain.