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I've read a new study which suggests that yawning may help you keep a cool head. Also, the findings might hold some hope for sufferers of insomnia, migraines, and even epilepsy.

Is there any conclusion about what the function of yawning is and why do we yawn?

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In an abstract, Brainerd at al. (2004) report the presence of yawn-like behaviors in "cartilaginous and ray-finned fishes, a lungfish, salamanders, caecilians, mammals, turtles, lizards, an alligator and birds".

So clearly the motor patterns necessary for this behavior evolved before air breathing and apparently have been conserved for ~400 million years. They reject the increased oxygen delivery hypothesis and associate yawning with stretching:

Yawning also has been thought to serve a similar physiological function to stretching. We measured the stretching movements of fins and limbs in association with yawns, and found that they follow the same movement pattern as jaw opening and closing. This lends support to the existing hypothesis that yawning and stretching serve a similar function, but the detailed nature of this function remains unclear.

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