Yawning is common throughout the animal
kingdom, yet we still don't know what purpose it
serves (Science Photo Library)
Yawning has puzzled scientists for more than
two millennia. But could a new theory settle the
question once and for all? David Robson
investigates.
Mid-conversation with Robert Provine, I have a
compelling urge, rising from deep inside my
body. The more I try to quash it, the more it
seems to spread, until it consumes my whole
being. Eventually, it is all I can think about – but
how can I stop myself from yawning?
Provine tells me this often happens when people
are talking to him; during presentations, he
sometimes finds the majority of his audience with
their mouths agape and tonsils swinging. Luckily,
as a psychologist at the University of Maryland,
Baltimore County and author of Curious Behavior:
Yawning, Laughing, Hiccupping, and Beyond, he
isn’t offended. “It makes a very effective lecture,”
he says. “You talk and then the audience starts
yawning. And then you can ask people to
experiment on their yawns – like closing the lips,
or inhaling through clenched teeth, or trying to
yawn with the nose pinched closed.”
It is through experiments like these that Provine
has tried to explore a millennia-old mystery: why
do we yawn? We all know that tiredness,
boredom, or the sight of someone else can all
bring along the almost irrepressible urge – but
what purpose does it serve the body? When he
first started work on so-called “chasmology” in
the late 80s, Provine wrote that “yawning may
have the dubious distinction of being the least
understood, common human behaviour”. Nearly
three decades later, we may be closer to an
answer, but it’s one that has split the field.
Arguably the first studier of yawns was the Greek
physician Hippocrates nearly 2,500 years ago. He
believed that yawning helped to release noxious
air, particularly during a fever. “Like the large
quantities of steam that escape from cauldrons
when water boils, the accumulated air in the body
is violently expelled through the mouth when the
body temperature rises,” he wrote. Different
incarnations of the idea lingered until the 19th
Century, when scientists instead proposed that
yawning aids respiration – triggering a rush of
oxygen into the blood supply, while flushing out
the carbon dioxide. If that were true, you would
expect people to yawn more or less frequently
depending on the oxygen and carbon dioxide
concentrations in the air. Yet when Provine asked
volunteers to breathe various mixtures of gases,
he found no such change.
Many theories have instead focussed on the
strange, contagious nature of yawning – a fact
that I know only too well from my conversation
with Provine. “Around 50% of people who
observe a yawn will yawn in response,” he says.
“It is so contagious that anything associated with
it will trigger one… seeing or hearing another
person, or even reading about yawning.” For this
reason, some researchers have wondered if
yawning might be a primitive form of
communication – if so, what information is it
transmitting? We often feel tired when we yawn,
so one idea is that it helps set everyone’s
biological clocks to the same rhythm. “In my
view the most likely signalling role of yawning is
to help to synchronize the behaviour of a social
group – to make them go to sleep more or less
at the same time,” says Christian Hess, at the
University of Bern in Switzerland. With the same
routine, a group can then work together more
efficiently throughout the day.
Yet we also yawn during times of stress: Olympic
athletes often do it before a race , while
musicians sometimes succumb before a concert.
So some researchers, including Provine, believe
that the strenuous movements might have a
more general role in rebooting the brain – when
you are sleepy they make you more alert, or
when you are distracted they make you more
focussed. Spreading through a group, contagious
yawns could then help everyone reach the same
level of attention, making them more vigilant to a
threat, for instance. The mechanism is somewhat
hazy – though one French researcher, Olivier
Walusinski, proposes that yawning helps to pump
cerebrospinal fluid around the brain , which could
trigger a shift in neural activity.
With so many competing and contradictory ideas,
a grand unifying theory of yawning may seem
like a distant speck on the horizon. But over the
last few years, one underlying mechanism has
emerged that could, potentially, appease all these
apparent paradoxes in one fell swoop. Andrew
Gallup, now at the State University of New York at
Oneonta, was first inspired with the idea during
his undergraduate degree, when he realised that
yawning might help to chill the brain and stop it
overheating. The violent movement of the jaws
moves blood flow around the skull, he argued,
helping to carry away excess heat, while the
deep inhalation brings cool air into the sinus
cavities and around the carotid artery leading
back into the brain. What’s more, the strenuous
movements could also flex the membranes of
sinuses – fanning a soft breeze through the
cavities that should cause our mucus to
evaporate, which should chill the head like air
conditioning.
The most obvious test was to see if people are
more or less likely to yawn in different
temperatures . In normal conditions, Gallup found
that around 48% felt the urge to yawn, but when
he asked them to hold a cold compress to their
foreheads, just 9% succumbed. Breathing through
the nose, which could also cool the brain, was
even more effective, completely dampening his
subjects’ urge to yawn – potentially suggesting a
handy trick for anyone facing embarrassment
during a tedious conversation.
Perhaps the best evidence comes from two
troubled women who approached Gallup soon
after he first published his results. Both were
looking for relief from pathological yawning
attacks, sometimes lasting an hour at a time . “It
was extremely debilitating and interferes with any
basic activity,” says Gallup. “They’d have to walk
away and go to a secluded area – it affected their
personal and professional lives.” Intriguingly, one
of the women found the only way to stop the
yawning attack was to throw herself into cold
water. Inspired, Gallup asked them to place a
thermometer in their mouths before and after the
attacks. Sure enough, he saw a slight rise in
temperature just before the yawning bouts, which
continued until it dropped back to 37C.
Importantly, this brain chill might underlie the
many, seemingly contradictory, events that lead
to yawning. Our body temperature naturally rises
before and after sleep, for instance. Cooling the
brain slightly might also make us more alert –
waking us up when we are bored and distracted.
And by spreading from person to person,
contagious yawns could therefore help a whole
group to focus.
Gallup’s unified theory has been somewhat
contentious among yawning researchers.
“Gallup’s group has failed to present any
convincing experimental evidence to support his
theory,” says Hess. In particular, his critics point
out he hasn’t made direct measurements of
temperature changes in the human brain, though
Gallup says he has found the expected
fluctuations in yawning rats. Provine is more
positive, however – believing that it could be one
way in which yawning helps the brain change
state, and focus.
Even if Gallup has managed to find that unified
theory, many mysteries remain. Why do foetuses
yawn in the womb, for instance?
It could just be that they are practicing for life
outside, or perhaps the yawn plays a more active
role in guiding the body’s growth – by helping to
develop articulation in the jaws joints, for
instance, or by encouraging the growth of the
lungs, says Provine. If so, Provine suggests that
yawning’s functions in the womb may be more
important than our attacks as adults.
Provine also points out that yawning – and
perhaps other bodily functions, like sneezing –
shares some strange parallels with sex. The
facial expressions involved are surprisingly
similar, he says – just take a look at this picture
and you can see where he’s coming from.
Like sex, yawns and sneezes involve a build-up
that ends in a pleasant climax. “Once initiated,
they go to completion – you don’t want a
yawnus interruptus,” is how Provine puts it. For
these reasons, he wonders if a shared neural
machinery underlying these different feelings.
“Mother Nature does not reinvent the wheel,” he
says. As evidence, he points to the fact that
certain anti-depressants can lead some patients
to orgasm during a yawn – a rare side effect that
could quickly lose its appeal.
Eventually, the temptation to yawn just proved
too irresistible during my conversation with
Provine. It was a warm summer day, so perhaps
my yawns were stopping my brain from over-
heating during our stimulating conversation.
Whatever function it was serving, the relief was
almost worth the agonising wait.
I’m willing to bet you’ve been stifling a few
yawns yourself by this point – as Provine points
out, reading or even thinking about yawning can
be enough to set us off. So go ahead, let it out –
and do so in the knowledge that you are enjoying
one of life’s most enduring mysteries.