The average duration of the yawn is 5-6 seconds. It's the involuntary opening of the mouth with a long and deep inhalation through the mouth and nose, followed by a slow exhalation associated with a feeling of well-being. In humans, yawning is often accompanied by stretching of the upper limbs.

Given the universality of this daily reflex, we may be led to think that the mechanisms and purposes that underlie it no longer have secrets.. The truth is that we have always been familiar with it but we have known it recently and not entirely. Let's start with a trivial question: why do we yawn?

AGAINST SLEEP AND BOREDOM

Experience suggests that drowsiness and boredom are the most common stimuli of yawning.. In certain circumstances the mind must make an effort to remain alert and maintain contact with the external environment..

Various studies have identified a increased heart rate, compared to baseline values, at the peak of yawning. Yawning is thought to have, like caffeine, a role in the’cerebral excitation. Yawning movements could mechanically stimulate the carotid body and cause it to become aroused.. This year carotid body is a small mass of peripheral receptors and support cells, located near the bifurcation of the common carotid arteries. Carotid bodies are highly vascularized and their compressions can lead to the release of hormones such as adenosine and catecholamines., which subsequently mediate the response to excitation.

Yawning out of boredom

OXYGENATE THE BRAIN

More recent is the idea that yawning can serve to regulate the brain temperature. How the PC fan lowers the processor temperature, so yawning would lower the temperature of the brain. A study that supports this hypothesis, was conducted by continuously monitoring cortical brain temperature in rats. In doing so it was noticed, that during the 3 minutes before yawning, cortical temperatures had significantly increased. After yawning, the temperature dropped until it returned to baseline values in the three minutes following the act. Furthermore, studies on both animal and human models show, that yawning occurs in the anomalous phenomena of thermoregulation as in heat stroke.

There are also pathological situations such as multiple sclerosis, epilepsy, migraine, stress, anxiety, head trauma, and stroke, in which you yawn excessively with consequent temporary cessation of symptoms. This is because these conditions lead to an increase in temperature, that the body tries to correct transiently through yawning.

A MATTER OF EMPATHY?

You will surely have seen someone respond to your yawn! It's not a mistake or even a coincidence, yawns are really Infectious! Collecting clinical data, psychological and neurological, some researchers argue that susceptibility to contagious yawn, in humans, correlates with empathic skills. Millen and Anderson, based on documented observations, wrote that infants and preschoolers appear largely immune to contagious yawning. The latter can be induced in children only after 4-5 years, since before this age, the neural mechanisms needed to understand the mental state of other individuals are still developing. The empathy hypothesis is also confirmed by evidence, indicating a reduction in susceptibility to contagious yawning, in patients suffering from disorders that affect the ability to interact socially. Examples include schizophrenic patients and people with an autism spectrum. The mirror neurons are the structures identified for the ability to understand and identify that are the basis of the empathic process. It is thanks to the activation of these neurons, through visual and/or auditory stimuli, that yawning is unknowingly imitated.

EAR DEFENSE REFLEX

Several observations have led to the idea that yawning can serve as a “defense reflex” of the ear, which is triggered by rapid changes in altitude or other conditions that lead to air trapping in the middle ear. In these circumstances yawning could be useful to equalize the air pressure in the middle ear with the air pressure outside. This is done through the opening of the Eustachian tubes, duct connecting the middle ear with the upper part of the pharynx. The walls of this duct are normally placed next to each other and the air enters the interior only when they deviate. The tensor muscle of the eardrum and the stapedial muscles, intervene in the opening and closing of the Eustachian tube and they are also solicited by the act of yawning.

However, since the Eustachian tubes can also be opened with swallowing, ear pressure regulation has not been identified as the main function of yawning.

CAPTURING OXYGEN

The most quoted hypothesis in recent centuries, recognized at yawn the ability to trigger the’oxygenation of the brain when oxygen levels decreased and carbon dioxide concentration increased. This belief was discarded following the results of a study that monitored the frequency of yawning in subjects breathing air mixtures containing more CO2 than normal or pure O2.. No frequency change was found!

NON-VERBAL COMMUNICATION

Modern theories hold that yawning is an act of non-verbal communication. Within a group, yawning you can send messages different from other members. The magnitude of the message may vary depending on the situations and also on the species. Yawning can communicate fatigue, boredom, hunger, difficult digestion and also sexual attraction.

The scientific community has not yet come to a yawning theory that everyone agrees on.. It is always stunning to discover how in physiology nothing is trivial, not even a yawn!

You have come to read this far? Good, we hope we haven't bored you too much, otherwise you are allowed to release a regenerating yawn!

Sources:

https://www.lescienze.it/news/2014/03/24/news/sbadiglio_contagioso_chiave_empatia-2065562/

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3678674/