Why Do We Yawn, and Why Is It Catching?

Science isn't entirely sure why humans and other animals yawn, let alone why yawning can be contagious -- but we've got some good hypotheses. Learn about them, from cooling the brain to spreading alertness, in today's episode of BrainStuff, based on these articles: https://science.howstuffworks.com/life/inside-the-mind/human-brain/question572.htm; https://science.howstuffworks.com/life/inside-the-mind/human-brain/contagious-yawn.htm

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Episode Transcript

Speaker 1 (00:01):
Welcome to Brainstuff, a production of iHeartRadio. Hey, brain Stuff,
Lauren Vogelbaum Here. Next time you're in a meeting or
a group discussion, you can try a little experiment. Take
a big yawn, covering your mouth out of courtesy, and
watch to see how many people follow suit. There's a

(00:24):
good chance you'll set off a chain reaction of deep
breaths and wide open mouths, and before you finish listening
to this episode, it's likely that you'll yawn at least once.
Don't misunderstand. We don't intend to bore you here, but
just thinking about yawning can make you do it it
just as seeing or hearing someone else yawn makes us

(00:44):
do it too. My jaw, for the record, is practically
itching right now, holding one back. But why do we
yawn in the first place, and why is it so contagious? Today,
let's talk about what we know and don't know about yawning.
Yawning is an involuntary action that causes us to open

(01:06):
our mouths wide and take a deep breath. Oh, we
know it's involuntary because we do it even before we're born.
Yawning has been reported in fetuses as early as eleven
or twelve weeks old. It's been linked to healthy progress
during early brain and facial muscular development from around twenty
four to thirty weeks in grown adult humans. While we

(01:28):
typically associate yawning with relaxation and drowsiness, your heart rate
can rise as much as thirty percent during a yawn.
When we yawn, we increase the flow of blood to
the brain, so yawning is also assigned or cause of arousal.
Many parts of the body are in action when you yawn.
At first, your mouth opens and your jast stretches powerfully,

(01:52):
allowing as much air as possible to be taken in
an increasing blood flow to the neck, head, and face.
Inhale well slowly and deeply, meaning your abdominal muscles flex,
and your diaphragm contracts and pulls down underneath your ribcage,
allowing your chest to expand above it and your lungs
to fill with air. The stretching in your jaw may

(02:15):
also open up your sinus system and allow airflow there.
You pause, and then you breathe out more quickly than
you breathed in. Your diaphragm relaxes, curving back upward towards
your lungs which deflate a bit, and your jaw relaxes,
and you probably close your mouth. So this isn't just

(02:36):
a deep breath. And we know that our bodies control
yawning and breathing with different mechanisms. Yawning may serve a
number of functions, especially in different animals. After all, humans
are not the only animals that yawn. Other mammals, birds,
and even fish dow too. Some researchers think that yawning

(02:56):
first evolved about four hundred million years ago in jawed fish,
and that pretty much all vertebrates yond spontaneously to help
regulate bodily functions. Though interestingly, the duration of yawns across
different species seems to be positively correlated to how large
and complex that species brain is. Like, researchers analyzed over

(03:21):
a hundred species of birds and mammals and found that
animals with more brains yawn for longer. In humans, fatigue,
sleep deprivation, or boredom easily bring on yns. But scientists
are discovering that there's more to yawning than most people think.
There's a few hypotheses about why we do it spontaneously

(03:42):
and why it's contagious in groups. An old hypothesis that
yawning helps to draw in more oxygen or remove a
buildup of carbon dioxide in our bodies. This would make
a bit of sense in contagious yawning because larger groups
of people off put and more carbon dioxide, which means
our bodies might want to act to draw in more oxygen.

(04:05):
But this one has been disproven. Lots of research has
been done on it, including by Robert Provine, who was
a developmental neuroscientist at the University of Maryland, Baltimore County
and a leading expert on yawning. His team found that
giving people additional oxygen did not decrease yawning, and decreasing
the amount of carbon dioxide in a subject's environment also

(04:28):
didn't prevent yawning. These air makeup changes did affect subject's
breathing rates, however, showing that we breathe and yon for
different reasons. Furthermore, the bodies attempt to regulate blood oxygen
levels couldn't explain all yawning because we know that marine
mammals sometimes yawn when they're underwater and therefore not taking

(04:51):
in air, and human fetuses in the womb aren't using
their lungs to breathe either. They receive oxygen through the
placenta and the millicle cord. More recently, researchers proposed another
hypothesis that yawning is a way humans and other mammals
help regulate our brain temperature. The brain releases a certain

(05:13):
amount of heat, which gets dissipated partially through warmed blood
flowing from the brain being replaced with cooler blood from
the rest of the body. It's dangerous for the brain
to get too warm in humans. Anything above one hundred
fahrenheit or about thirty seven point eight celsius is hazardous,
and anything above one of four fahrenheit or forty celsius

(05:34):
can be deadly. So having an automated mechanism to increase
blood flow and also ventilate the sinuses of cooler air
as happens when you yawn, it seems like an evolutionarily
good play. This has been proven out in rats, in
which arising brain temperatures will trigger yawning, which will then

(05:55):
decrease those brain temperatures, and it's been shown to a
contagious yawning in humans. In the early two thousands, a
team out of the State University of New York Polytechnic
Institute led by one Andrew Gallup found that when subjects
purposefully regulated their brain temperature by breathing through their noses

(06:16):
or by using cold compresses on their foreheads, it suppressed
their contagious yawning. Other participants yawned in response to video
of people yawning about forty eight percent of the time,
but the subjects cooling their brains only yawned contagiously nine
percent of the time. Also, a research published just this

(06:36):
year in spring of twenty twenty six indicated that yawning
may help your system circulate cerebras spinal fluid and thus
help remove waste materials from the brain, maybe to help
get you ready for a good sleep, or maybe to
help flush you out when you've failed to get a
good sleep. But okay, oh, we have to talk more

(06:57):
about that yon contagion thing. Humans are also not the
only species that can catch a yawn. This has also
been observed in other social species, including chimpanzees, bonaboes, lions, parakeets,
and domesticated dogs. Though we should note here that the
individual response of any given animal to seeing or hearing

(07:19):
or thinking about another animal yawn can vary significantly. For example,
adult humans will yawn in response to seeing a yawn
something around sixty percent of the time. A recent study
has shown that a fetus will yawn about half the
time that it's mother yawns about a minute afterwards, perhaps

(07:39):
due to some kind of physical or chemical prompt But
after a child is born, yawns don't seem to be
catching until the kidd in question is four or five
years old. Perhaps coincidentally, this is around when children usually
start developing a sense of empathy, so there's another hypothesis
that contagious year yawning is a test of empathetic response.

(08:03):
This is backed up by the observation that chimpanzee infants
don't yawn contagiously either, and whether bonibo catches a yawn
depends on how close they are to the original yawning individual.
The yawns of friends and family members are much more
likely to elicit a contagious yawning response, and the same

(08:24):
seems to be true of humans. A few studies have
tried to test this based on yon responses in autistic
versus allistic children and adults, working off the concept that
these groups process facial cues and empathetic responses differently. Often
these sorts of tests will show a subject video of

(08:44):
someone moving their mouth versus actually yawning, and then measure
the subject's response from there. But empathy in general and
the autism spectrum, both relatedly and separately, are very complicated,
and the results of these studies really only indicate that
more research is necessary. A leading hypothesis these days has

(09:07):
to do with yawning occurring around states of bodily change,
like between sleeping and waking. It also ties back into
yawning creating increased blood flow to the brain, which can
help us be more alert. The concept goes that yawning
is an involuntary response to stressful situations. Some Olympic athletes

(09:29):
will yawn right before their event. Some dogs will yawn
when they're anxious like at the vets office, or excited
like when you come home. It's a way to sort
of stretch and come into your body and focus, and
evolutionarily seeing that happening to someone in your group and

(09:49):
automatically getting in on it too sounds pretty worthwhile, even
if a yawn just means that your compatriot is sleepy
and trying to wake up, it'd be good for you
to join in on the increased vigilance the aforementioned Andrew
Gallup at a Sunny polytechnic recently ran a study where
they showed subjects video of people yawning or just moving

(10:12):
their mouths, and then showed them images that could include
a threatening stimulus in the form of a snake or
a non threatening stimulus in the form of a frog.
The subjects were then asked to pick out the snakes
and frogs and the images as quickly as possible. After
watching people yawn, the study subjects became able to identify

(10:34):
the snakes faster, and only the snakes. This had no
effect on frog identification. So even though we still don't
know for sure why we yawn, we do know lots
of interesting things about it. By the way, there is
such thing as excessive yawning. It can indicate that something
in your system is off. Usually that's your sleep quality,

(10:58):
but it can also be something more immediately serious, like
a heart attack or stroke, or epilepsy or multiple sclerosis.
So if you suddenly find yourself yawning a lot, especially
if you have any other symptoms or changes that seem
out of place, get in touch with the healthcare provider.
It probably doesn't count, though, if the yawns you're noticing

(11:19):
all happened while you were listening to this episode, though
we hope it wasn't that many. Today's episode is based
on the articles why do We Yawn? By Melanie red
Zeki McManus and why is Yawning Contagious? By Josh Clark,
both on how stuffworks dot Com. Brain Stuff is production

(11:40):
of iHeartRadio in partnership with how stuffworks dot Com and
is produced by Tyler Klang. For more podcasts from my
heart Radio, visit the Aheartradio app, Apple Podcasts, or wherever
you listen to your favorite shows.