“Crocodiles yawn and dinosaurs probably yawned. It’s this incredibly evolutionarily conserved behaviour, but why is it still with us?” asks Adam Martinac at Neuroscience Research Australia, a not-for-profit medical institution.

To try to solve the mystery of exactly how yawning functions and what effects it has on the body, Martinac and his colleagues recruited 22 healthy adults, equally split between men and women.

All the volunteers were then given an MRI scan while performing four different breathing manoeuvres – normal breathing, yawning, voluntary suppression of a yawn and a forceful deep breath.

When the team members began to analyse the data, they were shocked by the results. Their hypothesis had been that yawning and a forceful deep breath would both prompt the movement of cerebrospinal fluid (CSF), the liquid that fills the brain’s empty spaces and covers its surface, out of the brain.

“But the yawn was triggering a movement of the CSF in the opposite direction than during a deep breath,” Martinac says. “And we’re just sitting there like, whoa, we definitely didn’t expect that.”

More specifically, they found that CSF and venous blood flow became strongly directionally coupled during yawning, often moving together away from the brain and towards the spinal column. This suggests a distinct reorganisation of neurofluid dynamics compared with deep breathing, when CSF and venous blood flows typically move in opposing directions, with venous blood flowing out of the brain while CSF flows in.

The exact mechanism for how the CSF is moved out of the brain during a yawn is still unclear, along with how much CSF is moved – though it is estimated to be just a few millilitres per yawn, says Martinac. He hopes to quantify volume as part of the next stage of the research.

“We think it might be the neck muscles and the tongue as well, and the throat all coordinating to pull this fluid out,” he says.

Another key finding is that yawning boosted carotid arterial inflow by over a third compared with deep breathing. This is probably because yawning prompts both CSF and venous blood to flow out of the cranial cavity – rather than venous blood flowing out and CSF flowing in – creating space for that extra arterial influx.

Each volunteer also had a unique and distinct yawn in terms of the movement of their tongue. “Each individual seems to have what looks like an individual yawning signature,” says Martinac.

Another puzzle the team wants to solve next is the benefit to our bodies of this movement of CSF.

“Maybe it’s thermoregulation, maybe it’s waste clearance or maybe it’s none of these things,” he says. “You could probably survive without yawning, but maybe there’s like six or seven or eight different very small effects, and they just cumulatively help us basically regulate waste clearance, thermo-regulation and even the emotional group dynamics of a yawn.”

The fact that yawning is so contagious is also a mystery – although it was crucial to the experiment, as the researchers encouraged participants to yawn by using a screen inside the MRI scanner that displayed video footage of other people yawning.

“Whenever we have my lab meetings or I do a presentation, I always have to go last because if I start talking about my research, everyone starts yawning,” says Martinac.

Andrew Gallup at Johns Hopkins University in Maryland says the study has numerous important findings that make an important contribution to understanding yawning. He also says the researchers have downplayed some of their findings – particularly that the work adds to the case for yawning having an important thermoregulatory role.

“The fact that internal carotid arterial flow increased by 34 per cent during… yawning is a really important finding that seems to be overlooked or at least downplayed in the current version of the paper,” says Gallup.

He also points out that the study examined contagious yawns rather than the spontaneous kind and suggests that the impact of spontaneous yawning may be even greater.

“There is reason to expect that spontaneous yawns produce even larger changes in CSF and blood flow than described here,” he says. “Indeed, the videos suggest that the contagious yawns were quite short in comparison to the average duration of spontaneous yawns in humans, which is around six seconds.”

Yossi Rathner at the University of Melbourne, Australia, agrees that the team has underplayed some of its findings but strongly disagrees with the case for thermoregulation.

Rathner says it might be that as sleep pressure builds, a chemical compound called adenosine – which has links to sleep-wake regulation – accumulates in the brainstem. “Yawning may trigger fluid movements in the brainstem that flushes the adenosine away, temporarily alleviating the sleep pressure and increasing alertness,” he says. “This is not a direct finding of the study, but a possible implication of the data.”