A yawn is an involuntary reflex where the mouth is opened wide, and the lungs take in a lot of air. The air is then exhaled slowly. During this time, the eardrums stretch, and the eyes may also close tight, causing them to water.

No thought or action has to be taken to produce a yawn, and the process is similar for everyone. Yawning commonly occurs either before or after sleep, which is why it is usually considered a sign of being tired. Yawning also occurs frequently in people who are doing boring or tedious things.

There is also a social aspect to yawning. Yawning appears to be contagious among humans and other animals, and the contagiousness of a yawn is well documented but hardly understood.

Fast facts on yawning

Here are some key points about yawning. More detail and supporting information is in the main article.

  • Typically, yawning is a response to fatigue or lack of stimulation.
  • Babies can yawn, even in the womb.
  • Yawning is contagious, as part of humans natural empathic response.
  • Yawning serves a social function, communicating boredom.

There is no definitive reason for a yawn yet. Many proposed theories have surfaced and been studied, and they provide some clues.

A change of state

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Whilst yawning is often considered a result of being tired or bored, this may not always be the case.

Yawning is commonly thought to be a sign of sleepiness or boredom, though this is not always the case.

While someone who yawns may be tired, the heart rate quickly rises during a yawn. This increased heart rate suggests yawning can be a sign of alertness rather than sluggishness.

Yawning, in general, may simply be a way for the body to change the state of awareness it is in:

  • Before bed: yawning could be taken as a sign that the body is preparing for sleep.
  • When bored: yawning while doing a boring task may be a sign of the brain transitioning from a high level of alertness to a lower one.
  • After exercise or sport: yawning after an intense sports activity may be a sign of transitioning from high energy to low energy in the brain.

People may also yawn when changing physical states as well, such as moving from an area of high pressure to low pressure. This pressure can build up in the eardrums and may cause the person to yawn to release it.

A respiratory function

Yawning may be a function of breathing. Yawns may be more likely when the blood needs oxygen. A yawn causes a big intake of air and a faster heartbeat, which could theoretically mean that it is pumping more oxygen through the body. So a yawn may be simply designed to help clear toxins out of the blood and provide a fresh supply of oxygen.

To cool the brain

Yawning may cool the brain. A yawn causes the jaw to stretch out, increasing blood flow in the face and neck. The large inhale and rapid heartbeat caused by the yawn also causes blood and spinal fluid to cycle through the body faster. This whole process may be a way to cool down a brain that has gotten too hot.

A study posted to Physiology & Behavior supported this theory. Researchers found that yawns were more likely at around 20 °C, which is the temperature they suggested would be ideal for cooling off the blood and brain.

As a communication tool

Some researchers believe the reason humans yawn has more to do with evolution. Before humans communicated vocally, they may have used yawns to convey a message.

Yawns are considered a sign of boredom or sleepiness, and that could be what early humans were communicating as well. However, early humans may have used yawning to signal their alertness to others, bare their teeth to aggressors, or serve as some other communication tool.

Humans and chimpanzees are not the only animals that yawn. All vertebrates yawn, from fish and birds to wolves and horses.

There are only three species that yawn contagiously, however: humans, chimpanzees, and the family of wolves and dogs.

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Whilst almost all animals can yawn, only a few species have been seen to exhibit contagious yawning. Canines such as dogs and wolves are one of those species.

Yawning is a reflex that does not follow many consistent patterns.

One thing that many people agree on is that yawning appears to be contagious. Seeing another person yawn can cause those watching to "catch" the yawn themselves.

Science has wondered why this happens, and many theories have surfaced including:

  • Time of day: Some researchers have suggested that the time of day or the intelligence of the people who catch the yawn cause contagious yawning, but most people do not think this anymore.
  • Empathy: One of the most common theories is that contagious yawning is a sign of empathy for others. Seeing a person yawn may cause the viewer to yawn, especially if they are close to or comfortable with that person.

A recent study posted to PLOS One showed groups of chimpanzees a video of other chimpanzees yawning. Results revealed that the chimpanzees were more likely to catch the yawn when watching chimpanzees they were familiar with yawn. This supports the idea that empathy and familiarity are involved in the contagiousness of a yawn.

Another study posted to PLOS One found that contagiousness in humans was an individual response. There was little correlation between intelligence, time of day, or empathy in those tested. The biggest factor they found was age. Older people were less likely to catch a yawn from others.

The full reason for contagious yawns is unknown.

Yawning is usually harmless, but it is possible to yawn too much. Excessive yawning can be caused by a few different disorders that require medical attention.

The vagus nerve, which is the nerve connecting the throat and abdomen to the brain, can cause excessive yawning by interacting with the blood vessels. This is called a vasovagal reaction. This response may be a sign of a sleep disorder or brain condition. It can even be a sign of heart conditions, such as a heart attack or problems with the aorta.

Anyone experiencing a lot of yawning with no apparent cause should contact a medical professional as soon as possible.