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Why Do We Yawn and Why Is It Contagious?

Pinpointing exactly why we yawn is a tough task, but the latest research
suggests that our sleepy sighs help to regulate the temperature of our
brains.

[5]Smithsonian Magazine
* Marina Koren

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GettyImages-807688390.jpg

Humans yawn from their earliest days–even babies still inside the womb
can yawn. Photo by Yuricazac / Getty Images.

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 [6]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 [7]Steven Platek, a psychology professor at [8]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 [9]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 [10]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 [11]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 ([12]fMRI) scans, he found that areas of the brain activated
during contagious yawning, the [13]posterior cingulate and
[14]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 [15]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.”

Marina Koren is a staff writer at The Atlantic. Previously, she was a
digital intern for Smithsonian.com.

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This post originally appeared on Smithsonian Magazine and was published
June 28, 2013. This article is republished here with permission.

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