The age old question of ‘why do we yawn’ may finally have been answered. Researches from Princeton University and University of Arizona recently conglomerated evidence for a thermoregulatory theory of yawning. Conclusions drawn from the thermal window hypothesis and a survey done in Arizona linking yawning frequency to ambient temperature point to a yawning mechanism for cooling the brain.
Yawning has been shown to elicit changes in endocrine, neurotransmitter, and cortical levels which cool the brain and modify arousal levels. The fact that yawning is controlled by the hypothalamus, thermoregulatory region of the brain, was an initial link leading to thermoregulatory theory and thermal window hypothesis. Yawning introduces cooler blood to the brain to promote convection, gives direct heat exchange through nasal orifices again for convection, and ventilates the sinuses promoting cooling through evaporation.
If yawning truly serves to cool the brain, three facts must be true according to thermal window hypothesis (TWH):
– Yawns should increase in frequency with initial rises in ambient temperature. This is due to thermoregulatory mechanisms triggering as brain temperature increases.
– Yawns should decrease in frequency as ambient temperature nears/exceeds body temperature. Air hotter than the brain will not cool it.
– Yawns should stop once ambient temperature is below a specific point. Countercurrent heat exchange could cool the brain below homeostasis.
In support of this, mouse, parrot, and human models showed triggered yawning with initial increases to brain or ambient temperature with a decrease in brain temperature following yawning. In the mouse experiment, yawning started with an increase of 0.11°C to the brain. An equal decrease in temperature was seen post-yawning. The parrot model showed a decreased frequency of yawning once ambient temperatures were 2-6° below their normal body temperature of 40°C, which is also consistent with TWH.
In conditions linked to thermoregulatory dysfunction such as multiple sclerosis (MS), epilepsy, and anxiety, atypical yawning instances were often seen as accompanying symptoms. Cooling of the head has been shown to alleviate MS symptoms and diminish yawning. Physicians may be able to use excessive yawning as a diagnostic tool for identifying abnormal thermoregulation as shown by these cases.
Following to the Arizona study, results showed that subjects were nearly twice as likely to yawn during winter, 22°C, when shown pictures of others yawning as compared to summer of 37°C, equal to human body temperature. This is explained by TWH in conjunction with a group vigilance notion. Because vigilance is increased after yawning, social transmission of this vigilance trigger may increase collective detection of and response to threats.
Piecing together, yawning seems to serve as a thermoregulatory mechanism for cooling the brain. This was evidenced by hypothalamal control of yawning, successive brain cooling following yawning, inhibited yawning post forehead cooling, and strong correlations between ambient temperature and yawning frequency.
“No I’m not bored, my brain is just a little warm.”
Andrew C. Gallup and Omar T. Eldakar. 2013. The thermoregulatory theory of yawning: what we know from over 5 years of research. Frontiers in Neuroscience.