Why people yawn when they are sleepy? Are there any biological reasons behind yawning?
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Aside all the other excellent answers in the comments section, I’ll add a little something that I’ve been told by my biology teachers a few years ago.
She said that some studies were suggesting that yawning could also have been a way to tell the group it’s time to rest/sleep.
Its contagiosity would be rooted in the need to rest “the weakest” of the group in terms of energy, since it’s getting tired, thus less effective in task achieving. By resting them, it’d also be a two birds with one stone moment: the group gets a shot at resting too, so that everyone resumes rested and ready to go.
Also, it’d be contagious so that the signal is sent to the group at dusk. One starts to yawn out of sleepiness, the others do too, and gradually sleepiness sets in. It’d be so that the group (minus the sentries) rests at once and thus can be effective as a whole.
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