A yawn is Nature’s way of correcting your shallow breathing and giving you a quick boost of energy. Remember that breathing is both voluntary and involuntary. If we depended on having to think about each and every breath, we never would have made it to kindergarten.
You have the power to shift into “manual” mode when it’s suitable (as with breathing exercises) but you remain in “automatic” most of the time. Sometimes because you are bored or tired, your meter drops into a “low fuel” range and you yawn to establish a more suitable oxygen/carbon dioxide level.
A yawn also invigorates face and speech muscles. Thinking about a yawn (or trying not to) is usually enough to trigger the process.
A typical yawn shapes your mouth into a vertical oval. Once you have an oval in place, expand on this by stretching that oval outward into a circle. Use a mirror at first if you need visual feedback. This practice yawn may trigger an authentic deep yawn and watering eyes. If so, well done.
One great big yawn is probably more useful than a lot of little stifled ones.
Be well. Breathe beautifully.