The results, based on a small-scale group of 22 participants and published in Respiratory Physiology & Neurobiology, showed that yawning triggered a specific manoeuvre in which cerebrospinal fluid (CSF) and venous blood moved out of the skull together, whereas during deep breathing cerebrospinal fluid flowed into the skull.
The researchers admit it is surprising that there is still such mystery surrounding the specific reason why people yawn given that it is such a basic action that everyone does on a regular basis.
“Yawning remains very mysterious, even though it’s a primordial process that has been preserved throughout evolution. We know that crocodiles yawn, so we think dinosaurs also yawned," Adam Martinac, a Postdoc and corresponding author of the paper, said.
“It’s unlikely that crocodiles and dinosaurs are yawning because of a social response, such as being bored by other crocodiles or dinosaurs!
"So there is likely something more fundamental going on. But nobody has really worked it out for certain.
“We also know that human fetus’s yawn during that very early development stage, but overall the science of yawning is surprisingly understudied.”
Cerebrospinal fluid flow
Cerebrospinal fluid is a clear liquid that surrounds the brain and spinal cord, filling the space around them like water around a floating object.
It is important because it cushions and protects the brain and spinal cord from injury and also helps carry nutrients in and waste products out.
The fact that CSF and venous blood flows away from the skull during yawning, but CSF flows in the opposite direction when deep breathing, was a big surprise to the researchers.
“We observed that yawning is a body movement that can influence the flow of fluids around the brain,” said Prof. Bilston, from UNSW’s School of Biomedical Engineering.
“There has been speculation that yawning can help clear waste from the brain, but so far there has not been solid proof.
“Our research suggests that yawning can play a role in cleaning brain fluid, which would most likely happen close to bedtime.”
This finding could be important for further studies into neurodegenerative diseases such as Alzheimer's, Parkinson’s and dementia — all of which have been potentially linked to the build-up of waste products in and around the brain that can be a result of impaired CSF flows.
“We think there's something here really worth investigating further. Those neurodegenerative diseases are associated with an accumulation of waste and the older you get the more waste there can be,” said Martinac.
“We don’t know how strong the link is related to how CSF is cleared, but in the last 10 years there have already been a lot of investigations into that area, and this can be another element.”
One of the key observations was that CSF flow when people yawned was the opposite of what happened when they simply took a deep breath.
The team showed volunteers videos of people, and even animals, yawning in order to trigger so-called contagious yawns.
MRI scans were taken at the level of their C3 vertebra, a crossroads in the upper neck where blood and CSF pass as they travel to and from the brain. The scans of the subjects contagiously yawning were then compared to those when they simply took a deep breath as if pretending to yawn.
In normal breathing the venous blood, that which has delivered oxygen, drains out of the skull and back towards the heart. The CSF flows in the opposite direction during that physiological process.
Even when the volunteers were pretending to yawn, the venous blood flowed out and CSF flowed into the skull.
Only when they were really yawning, via those contagious yawns, did venous blood and CSF flow out of the skull together.