In spite of this till date, “yawning” still remains shrouded in a veil of mystery with no clear understanding of as to why or what are the mechanisms behind this activity. Provine, who is considered as the pioneer of yawning research, remarked in the late 1980s that “yawning may have the dubious distinction of being the least understood and common human behavior.” Yawning, is a behavior that is observed across an evolutionarily diverse array of animals from fishes and birds to mammals and from the fetal to the old stages of life. In human beings, yawning has been observed as early as 12 weeks of fetal life. It is a phylogenetically old yet orogenically new and stereotyped motor behavior that has not undergone many changes over the course of evolution. And any trait that has survived evolution is bound to have some significance for survival. It is also important to learn about the different mechanisms behind yawning because excessive yawning has been associated with certain conditions like narcolepsy, uremia, side effects of medications that are used to treat depression or anxiety such as selective serotonin reuptake inhibitors or even various forms of encephalopathy. In this review, we will try to explore the various mechanisms behind this seemingly ubiquitous phenomenon. The first documented account of yawning dates back to 400 BC where Hippocrates wrote in de flatibus liber (a treatise on wind) that “yawning precedes a fever because the large quantity of air that has accumulated ascends all at once, lifting with the action of a lever, and opening the mouth; in this manner, the air can exit with ease. Like the large quantities of steam that escape from cauldrons when water boils, the accumulated air in the body is violently expelled through the mouth when the body temperature rises.” Although as we shall see subsequently in this article in the absence of any teleological or scientific evidence, this notion was later discredited by the researchers in the late twentieth century, but the idea prevailed for a long time.
Various theories have been put forward behind yawning. Some try to answer the purpose, while others attempt to explain the mechanisms behind it. And both homeostatic roles and communicative role have been suggested. They range from respiratory, arousal, and thermoregulatory to its role as a component of action repertoire of advanced empathic and social skills.
A specific type of yawning has also been observed in relation to certain emotional states and described by ethologists as “emotional yawns.” Although not much comprehensive in its explaining in humans, this theory states that yawn is generated by the paleomammalian brain and helps to pacify after stress.
Is There a Biochemical Basis? Evidence of a possible biochemical basis was put forward primarily from animal experiments, primarily in the last decades of the previous century. Dopaminergic and Cholinergic agonists, ACTH-MSH and oxytocin, were observed to produce yawning in rats. Intraventricular injection of ACTH, α-MSH, and oxytocin was found to induce yawning and stretching in these animals.
The phenomenon of yawning has fascinated the human beings for ages and the truth is, the complete neurobiology behind it is still unknown. The unique, universal, ephemeral, and stereotypical qualities of yawning have invited boundless speculation about its evolution, function, and significance from the scientific community. With more and more evidences and experiments with newer methods like neuroimaging, older concepts have changed and newer theories about various aspects have emerged. However, its full neurophysiological mapping is yet to be completed.
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