My t never yawns! I would be offended if she did. Some people are really loud, sloppy yawners and let the whole world know, is your t one of these?
Sometimes clients yawn a lot and its drug related, anti depressants, anti anxiety which sometimes they can make you really drowsy.
I had a supervisor last year who fell asleep regularly because he had diabetes. It didn't really bother me and I let him sleep because he was very sick but if he wasn't I probably would have been very offended, I know other counsellors fired him as their supervisor because of his naps.
I wonder if it's a medical issue, might be worth asking your t about? Just so you know it's not personal and you are not boring.