Why do I lose my hearing when I yawn?

This is due to a muscle in your middle ear called the tensor tympani, which is attached to the small ‘hammer’ bone that transmits sound from the eardrum. The muscle automatically contracts to reduce our hearing sensitivity in response to a sudden loud sound like thunder, and it also contracts as we chew, so we aren’t deafened by the sound of our own jaw muscles. Yawning also involves jaw movements that trigger the tensor tympani, though, so a side effect is that we get deafer during a yawn.