Yawning to cool the brain

Why do we yawn? We tend to yawn before sleep and after waking, when we are bored or under stimulated. We yawn in the anticipation of important events and when we are under stress. What do all of these have in common? Researchers from the University of Vienna, Austria, and the Nova Southeastern University and SUNY College at Oneonta, USA highlight a link with thermoregulation, and in particular, brain cooling. The results of their study have been published in the scientific journal “Physiology & Behaviour”.

Common belief is that yawning helps to increase the oxygen supply. However, previous research has failed to show an association between yawning and blood oxygen levels. New research by a team of researchers led by Psychologist Andrew Gallup of SUNY College at Oneonta, USA now reveals that yawning cools the brain.

Sleep cycles, cortical arousal and stress are all associated with fluctuations in brain temperature, Yawning subsequently functions to keep the brain temperature balanced and in optimal homeostasis. According to this theory, yawning should also be easily manipulated by ambient temperature variation, since exchange with cool ambient air temperature may facilitate lowering brain temperature. Specifically, the researchers hypothesized that yawning should only occur within an optimal range of temperatures, i.e., a thermal window.

To test this, Jorg Massen and Kim Dusch of the University of Vienna measured contagious yawning frequencies of pedestrians outdoors in Vienna, Austria, during both the winter and summer months, and then compared these results to an identical study conducted earlier in arid climate of Arizona, USA. Pedestrians were asked to view a series of images of people yawning, and then they self-reported on their own yawning behavior.

Results showed that in Vienna people yawned more in summer than in winter, whereas in Arizona people yawned more in winter than in summer. It turned out that it was not the seasons themselves, nor the amount of daylight hours experienced, but that contagious yawning was constrained to an optimal thermal zone or range of ambient temperatures around 20o C. In contrast, contagious yawning diminished when temperatures were relatively high at around 37o C in the summer of Arizona or low and around freezing in the winter of Vienna. Lead author Jorg Massen explains that where yawning functions to cool the brain, yawning is not functional when ambient temperatures are as hot as the body, and may not be necessary or may even have harmful consequences when it is freezing outside.

While most research on contagious yawning emphasizes the influence of interpersonal and emotional-cognitive variables on its expression, this report adds to accumulating research suggesting that the underlying mechanism for yawning, both spontaneous and contagious forms, is involved in regulating brain temperature. In turn, the cooling of the brain functions to improve arousal and mental efficiency. The authors of this study suggest that the spreading of this behavior via contagious yawning could therefore function to enhance overall group vigilance.

Why Do We Yawn and Why Is It Contagious?

Snakes and fish do it. Cats and dogs do it. Even human babies do it inside the womb. And maybe after seeing the picture above, you’re doing it now: yawning.

Yawning appears to be ubiquitous within the animal kingdom. But despite being such a widespread feature, scientists still can’t explain why yawning happens, or why for social mammals, like humans and their closest relatives, it’s contagious.

As yawning experts themselves will admit, the behavior isn’t exactly the hottest research topic in the field. Nevertheless, they are getting closer to the answer to these questions. An oft-used explanation for why we yawn goes like this: when we open wide, we suck in oxygen-rich air. The oxygen enters our bloodstream and helps to wake us up when we’re falling asleep at our desks.

Sounds believable, right? Unfortunately, this explanation is actually a myth, says Steven Platek, a psychology professor at Georgia Gwinnett College. So far, there’s no evidence that yawning affects levels of oxygen in the bloodstream, blood pressure or heart rate.

The real function of yawning, according to one hypothesis, could lie in the human body’s most complex system: the brain.

Yawning—a stretching of the jaw, gaping of the mouth and long deep inhalation, followed by a shallow exhalation—may serve as a thermoregulatory mechanism, says Andrew Gallup, a psychology professor at SUNY College at Oneonta. In other words, it’s kind of like a radiator. In a 2007 study, Gallup found that holding hot or cold packs to the forehead influenced how often people yawned when they saw videos of others doing it. When participants held a warm pack to their forehead, they yawned 41 percent of the time. When they held a cold pack, the incidence of yawning dropped to 9 percent.

The human brain takes up 40 percent of the body’s metabolic energy, which means it tends to heat up more than other organ systems. When we yawn, that big gulp of air travels through to our upper nasal and oral cavities. The mucus membranes there are covered with tons of blood vessels that project almost directly up to the forebrain. When we stretch our jaws, we increase the rate of blood flow to the skull, Gallup says. And as we inhale at the same time, the air changes the temperature of that blood flow, bringing cooler blood to the brains.

In studies of mice, an increase in brain temperature was found to precede yawning. Once the tiny rodents opened wide and inhaled, the temperature decreased. “That’s pretty much the nail in the coffin as far as the function of yawning being a brain cooling mechanism, as opposed to a mechanism for increasing oxygen in the blood,” says Platek.

Yawning as a thermoregulatory system mechanism could explain why we seem to yawn most often when it’s almost bedtime or right as we wake up. “Before we fall asleep, our brain and body temperatures are at their highest point during the course of our circadian rhythm,” Gallup says. As we fall asleep, these temperatures steadily decline, aided in part by yawning. But, he added, “Once we wake up, our brain and body temperatures are rising more rapidly than at any other point during the day.” Cue more yawns as we stumble toward the coffee machine. On average, we yawn about eight times a day, Gallup says.

Scientists haven’t yet pinpointed the reason we often feel refreshed after a hearty morning yawn. Platek suspects it’s because our brains function more efficiently once they’re cooled down, making us more alert as result.

A biological need to keep our brains cool may have trickled into early humans and other primates’ social networks. “If I see a yawn, that might automatically cue an instinctual behavior that if so-and-so’s brain is heating up, that means I’m in close enough vicinity, I may need to regulate my neural processes,” Platek says. This subconscious copycat behavior could improve individuals’ alertness, improving their chances of survival as a group.

Mimicry is likely at the heart of why yawning is contagious. This is because yawning may be a product of a quality inherent in social animals: empathy. In humans, it’s the ability to understand and feel another individual’s emotions. The way we do that is by stirring a given emotion in ourselves, says Matthew Campbell, a researcher at the Yerkes National Primate Research Center at Emory University. When we see someone smile or frown, we imitate them to feel happiness or sadness. We catch yawns for the same reasons—we see a yawn, so we yawn. “It isn’t a deliberate attempt to empathize with you,” Campbell says. “It’s just a byproduct of how our bodies and brains work.”

Platek says that yawning is contagious in about 60 to 70 percent of people—that is, if people see photos or footage of or read about yawning, the majority will spontaneously do the same. He has found that this phenomenon occurs most often in individuals who score high on measures of empathic understanding. Using functional magnetic resonance imaging (fMRI) scans, he found that areas of the brain activated during contagious yawning, the posterior cingulate and precuneus, are involved in processing the our own and others’ emotions. “My capacity to put myself in your shoes and understand your situation is a predictor for my susceptibility to contagiously yawn,” he says.

Contagious yawning has been observed in humans’ closest relatives, chimpanzees and bonobos, animals that are also characterized by their social natures. This begs a corollary question: is their capacity to contagiously yawn further evidence of the ability of chimps and bonobos to feel empathy?

Along with being contagious, yawning is highly suggestible, meaning that for English speakers, the word “yawn” is a representation of the action, a symbol that we’ve learned to create meaning. When we hear, read or think about the word or the action itself, that symbol becomes “activated” in the brain. “If you get enough stimulation to trip the switch, so to speak, you yawn,” Campbell says. “It doesn’t happen every time, but it builds up and at some point, you get enough activation in the brain and you yawn.”

Yawning may cool brain when needed

Yawning isn’t triggered because you’re bored, tired or need oxygen. Rather, yawning helps regulate the brain’s temperature, according to Gary Hack, of the University of Maryland School of Dentistry, and Andrew Gallup, of Princeton University.

"The brain is exquisitely sensitive to temperature changes and therefore must be protected from overheating," they said in a University of Maryland news release. "Brains, like computers, operate best when they are cool."

During yawning, the walls of the maxillary sinuses (located in the cheeks on each side of the nose) flex like bellows and help with brain cooling, according to the researchers.

They noted that the actual function of sinuses is still the subject of debate, and this theory may help clarify their purpose.

"Very little is understood about them, and little is agreed upon even by those who investigate them. Some scientists believe that they have no function at all," Hack said in the news release.

The researchers said their theory that yawning helps cool the brain has medical implications. For example, excessive yawning often precedes seizures in people with epilepsy and pain in people with migraine headaches.

Doctors may be able to use excessive yawning as a way to identify patients with conditions that affect temperature regulation.

"Excessive yawning appears to be symptomatic of conditions that increase brain and/or core temperature, such as central nervous system damage and sleep deprivation," Gallup said in the news release.

Fetuses yawn in the womb, according to new research

We know that unborn babies hiccup, swallow and stretch in the womb but new observational research concludes that they also yawn.

The 4D scans of 15 healthy fetuses, by Durham and Lancaster Universities, also suggest that yawning is a developmental process which could potentially give doctors another index of a fetus’ health.

While some researchers have suggested that fetuses yawn, others have disagreed and claim it is simple mouth opening.

But the new research clearly distinguished ‘yawning’ from ‘non-yawn mouth opening’ based on the duration of mouth opening. The researchers did this by using the 4D video footage to closely examine all events where a mouth stretch occurred in the fetus.

Using their newly developed criteria, the research team found that over half of the mouth openings observed in the study were classed as yawns.

The study was carried out on eight female and seven male foetuses from 24 to 36 weeks gestation. The researchers found that yawning declined from 28 weeks and that there was no significant difference between boys and girls in yawning frequency.

Although the function and importance of yawning is still unknown, the study findings suggest that yawning could be linked to fetal development, and as such could provide a further medical indication of the health of the unborn baby.

Bonobos Catch Yawns from Friends

For bonobos, yawning is contagious, but only between friends.

Yawns spread more easily between family and close friends, and from high-status monkeys to those lower on the totem pole, according to a study published online in the journal PLoS ONE. This pattern of social yawning mimics one found in humans and suggests infectious yawning is a byproduct of empathy, which coordinates emotions in a group.

"It underlines that the mechanism of yawn contagion in the two species is the same," said study co-author Elisabetta Palagi, a primate researcher at the University of Pisa in Italy. "One of the possible functions of yawn contagion is to synchronize individuals of a social group. In humans, yawn contagion is extremely important but just between people who share strong bonds."