How come when you are tired, you yawn, when yawning means there is a lack of oxygen?

Answer 1

We maintain a pressure gradient in our lungs of 22cmH2O and the atmospheric pressure at sea level is 760 cmH20. When you yawn you alter the pressure in your lungs thus creating a shift in the atmospheric pressure around you. As a result others around you must adjust lung pressure to maintain equilibrium. This quantification of pressure changes results in a ripple effect in the atmosphere and all humans and animals will yawn to balance the pressure in thier respective environments. It's all very scientific.

Answer 2

Many people believe that yawning means a lack of oxygen.

Yawning is a release of stress...much like stretching, etc. It doesn't mean you are necessarily tired or lacking oxygen.