A new imaging study from UNSW Sydney suggests yawning may help move important fluids out of the brain, prompting further research into links with ageing and neurodegenerative disease.
A simple yawn may feel like the most ordinary of human acts – a reflex triggered by tiredness, boredom, or seeing someone else's mouth stretch wide.
But amazingly scientists still cannot say with certainty why we do it.
Now, new research from academics at UNSW Sydney and Neuroscience Research Australia (NeuRA), suggests that yawning may play a subtle but intriguing role in moving fluids in and out of the brain.
Although the researchers acknowledge the idea is speculative, they say their work introduces an interesting avenue for understanding the physiological functions of yawning.
Using real-time MRI scans, the team led by Professor Lynne Bilston were able to see what happens inside the head and neck when people yawn, and compare it to the effect of normal and deep breathing.
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."
