In the physical world a yawn is a way of taking in more oxygen. In the spiritual sense it is our inner selves attempting to assimilate more knowledge or power.
Psychological / Emotional Perspective:
In the animal kingdom a yawn often is a warning against aggression, and to actually be yawning in dreams may be a way of controlling our own or other’s abusive or overly assertive behaviour.
Everyday Material Aspects:
If we become conscious of yawning in a dream it can indicate boredom and tiredness in the everyday. We may also be attempting to communicate with someone, but have not yet thought through what we wish to say.