Why Do We Yawn?

Why Do We Yawn?

The honest answer is that nobody knows for certain why we yawn. Researchers agree on what happens during a yawn, but the purpose of the reflex is still an open question, and several explanations compete.

What is known is that yawning is ancient and widespread. Nearly all mammals yawn, and so do many birds, reptiles and fish. Babies yawn before they are born, and adults yawn most often in the hour before sleep and just after waking. Any explanation has to fit all of these facts, which is one reason the debate is not settled.

A yawn is part of the wider story of respiratory physiology because it is, among other things, an unusually deep breath. But unlike an ordinary breath, it is not controlled by the need for oxygen. The control of breathing that keeps blood gases steady works on its own schedule, while yawns follow sleepiness, boredom and social cues.

Explained simply: a yawn is a big, slow stretch of the mouth, throat and chest that your brain starts by itself. Scientists can describe every step of it, but they are still arguing about what it is for.

What Happens in the Body When You Yawn?

A yawn has three phases: a long breath in with the mouth opening wide, a short peak when the muscles are fully stretched, and a quicker breath out as the jaw closes. A typical yawn lasts about six seconds, and once it starts it is very hard to stop halfway.

• Breath in: jaw-opening muscles pull the mouth wide while the diaphragm moves far down and the intercostal muscles lift the ribs, so the lungs take in much more air than in a normal breath.
• Throat opening: the pharynx widens, the larynx moves down, and the vocal cords spread far apart.
• Peak: the eyes squint, the face and neck muscles stretch, and many people stretch their arms at the same time.
• Breath out: the air leaves faster than it came in, the jaw closes, and the muscles relax.

Several side effects come from the same stretch. The squeezing of the face can press on the tear glands so the eyes water, and muscles of the soft palate can open the auditory tubes, which is why the ears sometimes pop.

What Are the Main Theories About Why We Yawn?

The four best-known theories say that yawning raises oxygen, cools the brain, marks a change in alertness, or sends a social signal, and only the last three are still taken seriously.

The oxygen idea (not supported)

The oldest explanation is that we yawn to take in oxygen or get rid of carbon dioxide. Experiments in the 1980s tested this by having people breathe air with extra oxygen or extra carbon dioxide, and their yawning rate did not change. Exercise, which raises the body's oxygen use sharply, does not make people yawn more either. The page on why we need oxygen explains how oxygen supply is really controlled.

Brain cooling

This theory says a yawn brings cooler air into the mouth and nose and increases blood flow through the face and skull, which may help carry heat away from the brain. Some studies found that people yawn more when the outside air is moderately warm than when it is very hot or cold, which fits the idea, but it remains under debate.

State change and alertness

Yawns cluster at moments when the brain is shifting from one state to another: falling asleep, waking up, getting bored, or getting ready for action. One idea is that the stretch and deep breath help the body move between these states.

Social signal

Because yawns spread from person to person, many researchers think they also help a group share its state, for example signaling that it is time to rest.

Why Do We Yawn When We Are Tired?

We yawn when we are tired because yawning is closely tied to the brain's sleep and wake cycle, although why the two are linked is part of the same unsettled question. Yawns are most frequent in the late evening and in the first hour after waking, the two daily turning points between sleep and wakefulness.

Boredom produces yawns for a similar reason: it is a low-alertness state. Interestingly, people also yawn in situations that are not sleepy at all, such as athletes before a race or performers before going on stage. This supports the idea that yawns mark a change of state rather than tiredness alone.

Networks in the brainstem and the hypothalamus, using chemical messengers such as dopamine and oxytocin, are known to take part in starting a yawn, which links yawning to the same brain regions that manage sleep, arousal and hormone release.

Why Is Yawning Contagious?

Yawning is contagious because the brain responds to other people's yawns with a yawn of its own, most likely as a form of social mirroring, although the exact mechanism is still being studied. Seeing a yawn, hearing one, or even reading about yawning can be enough to start one.

Contagious yawning is not present at birth. Babies yawn spontaneously from the start, but catching yawns from others usually appears only in early childhood, around age four or five, when children are also learning to read other people's feelings. Studies have found that adults catch yawns more easily from family and close friends than from strangers, which links contagious yawning to social closeness and empathy.

Some animals catch yawns too. Chimpanzees catch yawns from other chimpanzees, and pet dogs have been reported to yawn after watching their owners yawn.

Which Parts of the Respiratory System Take Part in a Yawn?

A yawn uses the airway from the mouth to the lungs and the muscles that move air through it. The mouth and the oral cavity open widely, the pharynx and the nasopharynx stretch, and the larynx drops so that the open glottis lets a large volume of air pass.

The inhalation and exhalation of a yawn use the same muscles as ordinary breathing, only much harder: the diaphragm and external intercostals for the breath in, and the elastic recoil of the lungs and chest for the breath out.

Yawning belongs to a small family of breathing reflexes with different triggers. A sneeze clears the nose, a cough clears the lower airways, and a hiccup is a sudden spasm of the diaphragm. A yawn is the only one of these that is regularly triggered by seeing someone else do it.