Bored

They have demonstrated that to be always busy, especially during the infancy, it prevents from developing the imagination. 
If we don't do anything is good for the brain.
Apparently of this form a network is activated, this network is the one that makes us dream awake, is busy with connecting experiences and lessons learned in the past with future plans, creates stories on our own life chaining the recollections and it gives wings to the imagination. Between 60 and 80 % of the energy of the brain it devotes itself to support "only" the connection between neurons.

If we yawn spontaneously it is not for boredom, not neither because of the hunger.
  The most probable thing is that we have the too warm "brain", since to biological level the yawn is a mechanism that serves to cool the brain.