Recent research published in PNAS sheds light on an ancient mystery: why is yawning contagious? Why is it hard to hold back a yawn if we see someone yawning? In the past, it was thought that the reason was related to empathy, i.e. the ability to understand and share the emotions of others, but recent studies have shown that the underlying mechanism is quite different. The protagonists of this phenomenon are mirror neurons, a special type of nerve cells located in the brain. Mirror neurons are activated not only when we perform an action, but also when we watch someone else performing it. In the case of yawning, the observation of another person yawning activates our mirror neurons, as if we were unconsciously imitating that gesture. But why do we yawn? Besides being contagious, yawning has several important functions that are not yet fully understood. Yawning may encourage more oxygen to enter the lungs and thus the blood, increasing the amount of oxygen reaching the brain. In some cases, yawning could be used as a form of non-verbal communication, e.g. to signal tiredness, a kind of collective signal indicating bedtime.
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