Eli5: What actually is a yawn? Why and how does it happen? What is the purpose of it?
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Yawning is not super well understood, and there are various theories about why it occurs.
A yawn is an involuntary reflex that involves the opening and stretching of the jaw, a sharp inhalation, followed by a hold of the breath and a steady exhalation.
A once popular theory was that yawning helped oxygenate the blood, but various studies involving high and low O2 concentrations have disproved this hypothesis.
There is evidence that yawning can help stimulate the body in ways similar to caffeine intake. Also, studies show that yawning can help reduce brain temperatures, and yawning appears to increase after abnormal thermoregulation (how the body manages temperature).
Many species – humans included – exhibit “sympathetic yawning.” That is, we are more likely to yawn when we see others yawn (even other species yawning), or when we read or talk about yawning. Yawning may have social implications from our early evolution.
So to summarize, there are more than one cause of yawning. The purpose of yawning is not very well understood or studied, but we are aware of some physiological effects of yawning, and we’re aware of some common misconceptions about yawning.
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