Theory 52/Yawning/Rate lens
Yawn Length Follows Cooling
The claim
Yawn duration is set by the rate at which brain temperature can be reduced, not by fatigue.
01Mechanism
A yawn draws cool blood to the brain; its duration reflects how quickly the thermal gradient can be exploited.
02Prediction
Duration should track cooling rate.
03Refutation
Duration tracking tiredness would remove the thermal term.