Consider the following scenario. You’re driving down a long, straight stretch of country highway around 2 p.m. on a sunny afternoon, desperate to get to your destination. You’re attempting to remain alert and attentive, but sleep pressure is mounting.
You yawn, sit up straighter in your seat, possibly fidget a little, and engage in other behaviors that may increase your level of arousal in response.
Is this what yawning is for? Yawning is commonly triggered by a variety of factors, including fatigue, fever, stress, drugs, social and other psychological cues. These are generally well documented and vary from person to person.
The question of why we yawn elicits a surprising amount of debate for such a minor field of study. We lack evidence to pinpoint the precise purpose of yawning.
Yawning increases our alertness. from shutterstock.com
However, there are several theories as to why people yawn. These include increasing alertness, cooling the brain, and the evolutionary theory of informing others in your group that you’re too tired to keep watch and that someone else should.
1. Assists us in waking up
Drowsiness is known to increase yawning. This has given rise to the yawning arousal hypothesis. Increased movement and stretching behavior are associated with yawning. Increased fidgeting may aid in maintaining vigilance as sleep pressure rises.
During yawning, specific muscles in the ear (the tensor tympani muscles) are activated. This causes the range of motion and sensitivity of the eardrum and hearing to be reset, increasing our ability to monitor the world around us after we may have tuned out prior to the yawn.
Furthermore, the opening and flushing of the eyes will most likely increase visual alertness.
2. Cools the brain temperature
The thermoregulatory hypothesis is another explanation for why we yawn. This implies that yawning helps to cool the brain. Yawning causes a deep inhalation, which draws cool air into the mouth, cooling the blood traveling to the brain.
According to proponents of this theory, a rise in brain temperature is observed prior to yawning, followed by a decrease in temperature after the yawn.
However, the research report that gave rise to this theory only shows that excessive yawning can occur when brain and body temperatures rise. It makes no mention of a cooling function.
Increased yawning rates are observed when fevers are experimentally induced, indicating a link between body warming and yawning. However, there is no clear evidence that it causes body cooling – only that body warming appears to be a trigger for yawning.
3. Sentry duty
Yawning has been observed in almost all vertebrates, implying that the reflex is ancient. Humans are social animals, according to the evolutionary-based behavioral hypothesis. When we are vulnerable to an attack from another species, one of the group’s functions is to protect one another.
Sharing sentry duties has been part of our group contract, and there is evidence from other social animals of yawning or stretching signals when individuals are losing arousal or vigilance. This is necessary for changing activities in order to keep the watch from slipping or to indicate the need for another sentry.
Explanations from neuroscience
Many brain structures are involved in the yawning reflex.
One study found activation in the ventromedial prefrontal cortex of the brain in those who were prone to contagious yawning. This brain area is linked to decision-making. Damage to this area is also linked to a loss of empathy.
In rodents, stimulation of a specific region of the hypothalamus that contains oxytocin-producing neurons causes yawning. Oxytocin is a hormone that has been linked to social bonding and mental health.
Yawning is also caused by injecting oxytocin into various regions of the brain stem. The hippocampus (associated with learning and memory), ventral tegmental area (associated with the release of dopamine, the happy hormone), and amygdala are among them (associated with stress and emotions). This effect is prevented by blocking the oxytocin receptors.
Patients with Parkinson’s disease yawn less frequently than others, which could be due to low dopamine levels. Dopamine replacement therapy has been shown to increase yawning.
Similarly, cortisol, a stress hormone, is known to cause yawning, while removing the adrenal gland (which produces cortisol) prevents yawning behavior. This suggests that stress may play a role in triggering yawning, which may explain why your dog yawns so much during long car rides.
So it appears that yawning is linked to empathy, stress, and dopamine release.
What makes it contagious?
You’ve probably yawned at least once while reading this article. Yawning is contagious, and seeing someone yawn frequently causes us to yawn as well. However, the only theory proposed here is that susceptibility to contagious yawning is related to someone’s level of empathy.
It’s worth noting, then, that people on the autism spectrum and those with a high psychopathic tendency have less contagious yawning. Dogs, which are thought to be highly empathetic animals, can also detect human yawns.
Overall, neuroscientists have a clear understanding of a wide range of yawning triggers, and we have a very detailed picture of the mechanism underlying yawning behavior. However, the functional purpose of yawning remains a mystery.
Returning to our road trip, yawning may be a physiological cue that the competition between vigilance and sleep pressure is beginning to favor drowsiness. The overarching message, however, is that sleep is winning and encouraging the driver to pull over for a break, and this should not be ignored.
