I don't tend to yawn that much. When I do it is usually when I am tired, or if someone else is yawning. Ever noticed how it seems to be contagious? Yawning is triggered by lower oxygen and this disparity is fixed by the yawn or gasp of air to increase oxygen levels. This has the effect of increasing alertness.
I am not all that aware of yawning a great deal, except when it's later at night than I am used to and I have visitors who I wish would go home!!!!
I have always found that yawning,like scratching an itch,is very contagious,and we even wrote a skit about these very things when I was at Art School!!! HILARIOUS!!!The entire audience was yawning AND scratching!!! I have also heard that yawning is a way to quickly get more oxygen when it is needed.