What causes a Yawn

Yawning, a reflex or involuntary action, involving inhalation of large amounts of air followed by its exhalation, is often associated with fatigue, sleepiness, overwork, stress or simply boredom. What causes yawning or why do people yawn are questions that have given rise to a number of answers over a period of time. There are many different theories which explain the reasons behind yawning. It may also be possible that yawning is triggered not by a single, but by multiplicity of factors.

Out of all possible factors that can stimulate yawning, two are considered by scientists as primary reasons. One important reason behind yawning is thought to be the increase in the concentration of carbon dioxide or low levels of oxygen in the blood or lungs, especially the alveoli of the lungs. So, yawning is a reflex action that helps the body to increase the intake of oxygen into the lungs and consequently to the bloodstream. However, many researchers do not regard this as a satisfactory explanation, as according to them, oxygen intake is less while yawning than during normal respiration.

A second opinion in this regard is that yawning controls the temperature of the brain and thereby helps in cooling the brain. It generally occurs when one is tired or has not had enough sleep, as both these conditions have been found to increase brain temperature. While some believe yawn to be a mechanism to cool the body, others think, it occurs to stabilize pressure on either side of the eardrums. Yawning is also regarded as an action quite similar to stretching, as it also flexes the muscles and joints of the body, besides increasing blood pressure and heart rate.

Yawning is believed to be induced when the level of glucose supplied to the brain decreases, as in the state of hunger. It is also thought to be a mechanism of clearing the lymphatic system, mainly before and after sleep. Another assumption in this regard is that yawning is caused by some chemicals found in the brain, like, serotonin, dopamine, glutamine, glutamic acid and nitric oxide. These chemicals are also responsible for affecting emotions, mood, appetite and certain other states. Activation of these chemicals in the brain is found to increase the frequency of yawns, while some other chemicals such as opioid neurotransmitters, e.g., endorphins, can reduce yawning.

A yawn is contagious, in the sense that it can easily spread to the person who is observing the other person yawning. Besides observing, even reading or thinking about it is also found to stimulate yawning in some individuals. Some are of the opinion that it may be caused by mirror neurons present in the frontal cortex of the brain, which imitates the action of others. These neurons get activated when exposed to a stimulus or action performed by another individual usually from the same species. As yawning keeps the brain cool and alert, it is also thought to be an evolutionary development, through which a group of animals can be kept alert and vigilant against possible danger. Many researchers also believe that yawning is an empathic behavior that happens naturally. Contagious yawning is also considered to be a herd instinct used to communicate tiredness and time to sleep, to other individuals in a group.

Thus, yawning is not only associated with tiredness, sleepiness and some physiological conditions, but is also a form of non-verbal communication that can be interpreted to mean several different things. Apart from these, it has given rise to some really interesting beliefs or superstitions. To the ancient Greeks and Mayans, yawning was a sign which indicated that the soul of the person yawning was trying to run away from the body. Another common belief is that one should always cover the mouth while yawning, otherwise the soul may escape from the body. These beliefs may have originated to discourage public yawning, and also to maintain public health, as yawning might facilitate the spread of diseases.