Re: What causes yawning in humans?

I do know that when a room builds up in carbon dioxide, a lot of people in 
the room, the carbon dioxide causes you to become sleepy and you begin to 
yawn even if you are not tired.  It would seem to me that yawning is the 
streching of the neck and head muscles in order for the veins to constrict 
and move blood more rapidly to the brain.  So hypoxia of the brain or carbon 
dioxide would trigger it.

For further information, study about the veins and how blood is transported 
through them.

John M. Lewis
Mad Scientists