By Mariam Melkumyan
After a long day of traveling to Maryland and back, I started yawning every few minutes, but I didn’t feel that sleepy yet, so I was confused as to why I was yawning so excessively. That got me wondering, why do we yawn at nighttime, or when bored, or while reading or thinking about yawning, like you probably are now (or will be in a bit).
Humans and animals both experience yawning, yet no one really knows the exact cause or reason for yawning. Gupta and Mittal very eloquently define yawning as the opening up of the mouth accompanied by a long inspiration, with a brief interruption of ventilation, followed by a short expiration1. They suggest a few reasons why we yawn, although the precise reason and mechanism are still unknown.
One reason why we yawn is drowsiness and boredom. Boredom or drowsiness lead to the stimulation of the sleep-generating system (neuronal population in the brainstem that shut down the arousal system2), and the mind is forced to make extra efforts to maintain contact with the external environment1,3. Therefore, our bodies yawn to increase arousal level, leading to an increase in heart rate and skin conductance (an indirect measure of the autonomic nervous system function associated with attention)1. Next time you yawn, pay attention to how you feel before and after. You may find that before the yawn you were zoned out and slightly out of touch with reality, while after the yawn you may find yourself more present in your situation. Yawning seems to be a mechanism for us to become more aroused and more aware of our surroundings.
Another interesting reason why we yawn is that yawning might regulate the temperature of the brain. In 2010, researchers measured the brain temperature of rats before, during, and after yawning and found that there was an increase in the number of yawns when the brain temperature was increased4. Even more fascinating is that the brain temperature was restored to baseline after yawning and stretching, suggesting that yawning may be responsible for the thermoregulation of the brain4. Remarkably, an overall increase in the surrounding temperature is also associated with an increase in the frequency of yawns, while a cooler temperature is associated with a decrease in the frequency of contagious yawns5.
Contagious yawning is another noteworthy and commonly observed reason for yawning, where watching a person yawn, or reading or hearing about yawning, can trigger a yawn1. This contagious effect of yawning is not observed in infants and toddlers, and seems to begin occurring around ages four to five, which is when the neural mechanisms required for theory of mind, or understanding the mental state of others, start developing6. Since contagious yawning is not seen in infants who lack the neural mechanisms involved in empathy, contagious yawning is thought to be linked to empathy7. The reason behind empathetic yawning may be due to the mirror neuron system, a system of neurons that react similarly to an action executed by oneself and an action observed when others perform it8,9. Additionally, empathetic yawning may be due to the activation of the superior temporal sulcus, a brain region involved in comparing actions made by others to the sensory-motor consequences of actions made by self9,10. The superior temporal sulcus and the mirror neuron system are involved in empathetic yawning specifically, however, other brain regions and neurotransmitter systems are responsible for the action of yawning.
There is no one region of the brain responsible for yawning, however, there are some theories for where the “yawning center” of the brain is located. Early studies suggest that the yawning center is in the medulla of the brainstem, as children that were missing cortical (higher order) structures at birth had an intact yawn-stretch act, suggesting that yawning is not controlled by the cortex9,11. More recent studies confirm that yawning may be due to the central pattern generators in the medulla12. Central pattern generators are neuronal circuits that produce rhythmic patterns such as walking, breathing, and swallowing13. The central pattern generators send feedback from the medulla to the paraventricular nucleus (PVN) of the hypothalamus, leading to an autonomic response12. The PVN is involved in multiple homeostatic functions, including blood pressure and heart rate, feeding and metabolic balance, sexual behavior, and yawning9,14. Yawning is controlled by the oxytocinergic neurons originating in the PVN, which project to other regions of the brain, leading to the behavioral effect of yawning.
Argiolas and Melis in 1998 showed that activation of the oxytocinergic neurons in the PVN by excitatory neurotransmitters leads to yawning behavior, while the inhibition of these neurons prevents yawning9,15. The involvement of the hypothalamus in yawning makes sense, as the hypothalamus is involved in thermoregulation, and as mentioned above, yawning can lead to a reduction in the temperature of the brain5,9. Multiple excitatory and inhibitory neurotransmitter systems act together to induce the yawning behavior through the activation of motor nuclei of multiple cranial nerves, the respiratory neurons in the medulla, and intercostal muscles (the muscles between the ribs that allow the ribcage to expand and contract)9,12,16, but the exact details of how these systems work is out of the scope of this article. Interested readers can take a look at the article by Krestel, Bassetti, and Walusnski (2017)9.
Since numerous brain regions and neurotransmitter systems are involved in the act of yawning, it is possible that excessive yawning (more than 3 yawns in 15 minutes without an obvious cause) can be a sign of pathologies such as anterior and posterior circulation stroke, brain tumors, traumatic brain injury, epilepsy, migraine, and various neurodegenerative diseases9. These conditions can be life threatening and should be taken seriously, however, when I yawn multiple times within 10 minutes, I have to make sure to remember that I am a tired graduate student and multiple yawns in 15 minutes are probably due to my body asking me to take a nap. So, if you yawned multiple times reading this article because of social empathy (and hopefully not boredom), feel free to go take a power nap to refresh before a long day of experiments.
- Humans and animals yawn when we are tired, bored, or not stimulated enough (like when reading very lengthy and dry scientific articles)
- Yawning is important for alertness, arousal, thermoregulation of the brain, and is a sign of social empathy
- The medulla and the oxytocinergic neurons in the hypothalamus control yawning
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