Why do we yawn and why is it contagious?

May 21, 2018 by Mark Schier And Yossi Rathner, The Conversation
Yawning increases our alertness. Credit: shutterstock.com

Consider the scenario. You're driving on a long, straight stretch of country highway at about 2pm on a sunny afternoon, and you're desperately keen to reach your destination. You're trying to stay alert and attentive, but sleep pressure is building up.

In response you , sit up straighter in your seat, possibly fidget around a little and engage in other mannerisms that may increase your level of arousal.

Is this the purpose of yawning? Yawning is generally triggered by several things, including tiredness, fever, stress, drugs, social and other psychological cues. These are generally well documented and vary between individuals.

The question of why we yawn evokes a surprising amount of controversy for what is a relatively minor field of study. We don't have evidence that can point us to the exact purpose of yawning.

But there are several theories about the purpose of yawning. These include increasing alertness, cooling the brain, and the evolutionary theory of alerting others in your group that you're too tired to keep watch, and someone else should take over.

1. Helps us wake up

Yawning is known to increase with drowsiness. This has led to the arousal hypothesis of yawning. Associated with the yawning are increased movement and stretching behaviour. The increased fidgeting behaviour may help maintain vigilance as sleep pressure builds.

Also, specific muscles in the ear (the tensor tympani muscles) are activated during yawning. This leads to a resetting of the range of movement and sensitivity of the eardrum and hearing, which increases our ability to monitor the world around us after we may have tuned out before the yawn.

Additionally, the opening and flushing of the eyes will probably lead to an increase in visual alertness.

2. Cools the brain

Another theory for why we yawn is the thermoregulatory hypothesis. This suggests that yawning cools the brain. Yawning causes a deep inhalation that draws cool air into the mouth, which then cools the blood going to the brain.

Proponents of this theory claim a rise in brain temperature is observed prior to yawning, with a decrease in temperature seen after the yawn.

But the research report that gave rise to this theory only shows excessive yawning may occur during an increase in brain and body temperature. It doesn't suggest this has a cooling purpose.

Increased yawning rates are seen when fevers have been experimentally induced, which does suggest a correlation between body warming and yawning. But there is no clear evidence it leads to body cooling – just that body warming seems to be a trigger for yawning.

3. Sentry duty

Yawning-like behaviour has been observed in almost all vertebrates, suggesting that the reflex is ancient. The evolutionary based behavioural hypothesis draws on humans being social animals. When we are vulnerable to an attack from another species, a function of the group is to protect each other.

Part of our group contract has included sharing sentry duties, and there is evidence from other social animals of yawning or stretching signals when individuals are becoming lower in arousal or vigilance. This is important for changing activities to prevent the watch from slipping, or to indicate the need for another sentry.

Neuroscience explanations

The yawning reflex involves many structures in the brain.

One study that scanned the brains of those who were prone to contagious yawning found activation in the ventromedial prefrontal cortex of the brain. This brain region is associated with decision-making. Damage to this region is also associated with loss of empathy.

Stimulation of a particular region of the hypothalamus, which contains neurons with oxytocin, causes yawning behaviour in rodents. Oxytocin is a hormone associated with social bonding and mental health.

Injecting oxytocin into various regions of the stem causes yawning, too.These include the hippocampus (associated with learning and memory), ventral tegmental area (associated with the release of dopamine, the happy hormone) and the amygdala (associated with stress and emotions). Blocking the oxytocin receptors here prevents that effect.

Patients with Parkinson's disease don't yawn as frequently as others, which may be related to low dopamine levels. Dopamine replacement has been documented to increase yawning.

Similarly, cortisol, the hormone that increases with stress, is known to trigger yawning, while removal of the adrenal gland (which releases cortisol) prevents yawing behaviour. This suggests that stress might play a role in triggering yawning, which could be why your dog may yawn so much on long car trips.

So, it seems yawning is somehow related to empathy, stress and dopamine release.

Why is it contagious?

Chances are you've yawned at least once while reading this article. Yawning is a contagious behaviour and seeing someone yawn often causes us to yawn as well. But the only theory that's been suggested here is that susceptibility to is correlated with someone's level of empathy.

It is interesting to note, then, that there is decreased contagious yawning among people on the autism spectrum, and people who have high psychopathic tendency. And dogs, considered to be highly empathetic animals, can catch human yawns too.

Overall, neuroscientists have developed a clear idea of a wide range of triggers for yawning, and we have a very detailed picture of the mechanism underlying yawning behaviour. But the functional purpose of yawning remains elusive.

Back to our road trip, the yawning may be a physiological cue as the competition between vigilance and begins to favour drowsiness. But the overwhelming message is that sleep is winning and encouraging the driver to pull over for a break, and it shouldn't be ignored.

Explore further: Women found to be more susceptible to contagious yawning than men

Related Stories

Women found to be more susceptible to contagious yawning than men

February 3, 2016
(Phys.org)—A trio of researchers with Università di Pisa, in Italy has found via observational study, that women are on average twice as likely to yawn after seeing someone else yawn, than are men. In their paper published ...

Contagious yawning more closely associated with perceptual sensitivity than empathy

September 5, 2017
Contagious yawning is a universal phenomenon, but why it happens remains a mystery.

Yawning frequencies of people vary with temperature of the season, study finds

May 6, 2014
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 ...

Yawning—why is it so contagious and why should it matter?

August 31, 2017
Feeling tired? Even if we aren't tired, why do we yawn if someone else does? Experts at the University of Nottingham have published research that suggests the human propensity for contagious yawning is triggered automatically ...

Psychopaths may not yawn along with you

September 8, 2015
(HealthDay)—People with psychopathic traits are less likely to catch "contagious yawning" than those with higher levels of empathy, a new study suggests.

Recommended for you

How do babies laugh? Like chimps!

November 7, 2018
Few things can delight an adult more easily than the uninhibited, effervescent laughter of a baby. Yet baby laughter, a new study shows, differs from adult laughter in a key way: Babies laugh as they both exhale and inhale, ...

Tongue-in-cheek Nobels honor nutritional analysis of cannibalism, roller-coaster kidney stones treatment

September 14, 2018
A nutritional analysis of cannibalism and treating kidney stones on roller-coasters were research projects honored by tongue-in-cheek awards at Harvard University Thursday, designed to make you laugh first, and think later.

Pediatric robot patient offers new level of realism for doctors in training

September 10, 2018
A team of researchers and engineers at Gaumard Scientific has unveiled a new robot that raises the bar on medical training devices. The robot, called HAL, has been made to look like a five-year-old male patient and offers ...

Why men say they've had more lifetime sexual partners than women

July 25, 2018
The disparity between the number of sexual partners reported by men and women can largely be explained by a tendency among men to report extreme numbers of partners, and to estimate rather than count their lifetime total, ...

Censors jump into action as China's latest vaccine scandal ignites

July 22, 2018
Chinese censors on Sunday deleted articles and postings about the vaccine industry as an online outcry over the country's latest vaccine scandal intensified.

Revenge of a forgotten medical 'genius'

June 30, 2018
It's not an uncommon fate for a pioneering scientist: languishing unrecognised in his time before dying in obscurity. But as his 200th birthday approaches, the life-saving work of a Hungarian obstetrician is finally getting ...

2 comments

Adjust slider to filter visible comments by rank

Display comments: newest first

RobertKarlStonjek
not rated yet May 22, 2018
Yawning most probably is nothing more than a form of spontaneous stretching targeting the facial muscles and lungs rather than the muscles of the long bones as is the usual manifestation of stretching.

Yawning and stretching typically occur at around the same time, near to bedtime or upon waking, and often correlated (stretch and yawn at the same time) and most probably serve the same purpose.

If Yawning was meant to cool the brain then yawning would be done through the nose as the nasal cavity is adjacent to the brain, the mouth and throat are not.

Yawning occurs with the same spontaneity and the same kind of pleasurable sensation as spontaneous stretching and is frequently correlated even in other mammals such as pet dogs.

It is notable that coughing, laughing and stretching are also contagious and this correlation is more likely to answer the question of this behaviour than concentrating on yawning alone.
RobertKarlStonjek
not rated yet May 23, 2018
[from discussion on other forums]
The problem with the cooling hypothesis is twofold:
1) We yawn through the mouth which is much less efficient at cooling the brain than the navel cavity which is adjacent to the brain (it is even possible to do keyhole brain surgery through the nose);
2) We do not always breathe when yawning.

The argument for stretching include:
1) we yawn at similar times to spontaneous stretching;
2) we often stretch and yawn at the same time, especially with the arms;
3) the above is also true of dogs and probably other animals.
4) The reward feelings (pleasurable feelings) for yawning are similar to those for spontaneous stretching;
5) Occam's razor can be taken to those more elaborate attention getting theories unless there is some evidential reason for pursuing them.

Therefore it is highly probable that the primary purpose of yawning is concomitant with the primary purpose of stretching.

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.