What is known/taught about yawning during meditation? At some point in life I learned I could focus on some bodily awareness that resulted in the following experience:


Piloerectus; Not once, but it stays throughout the meditation while I keep on focusing.
A smile is slowly coming on my face (slowly as in it could take 20 seconds or so). It intensifies when I focus on that particular bodily feeling. It feels mechanically. When I take a picture of this I genuinely look happy.
Yawning; I cannot suppress this yawning when I focus on that particular bodily feeling. I yawn almost every minute intensely.
Energy swirls through my body. which can even make pains go away. Like pains from stress as facial muscle cramps or so.


This all can instantly go away when I have thoughts of doubt, fear, resentment or any other negative thought.

The smile and energy swirls I understand. Why piloerectus and yawning occur I do not. I am not feeling tired at all when the yawning happens. And when I stop to focus on this particular bodily feeling, the yawning stops. I do not yawn during a whole day, for instance. To me the yawning is a reproducible sign that indicates whether I am focused or not during meditation. But why does it happen?

In this pubmed article it is described that yawning is controlled by dopamine release (amongst other neurotransmitters). 

I was wondering if there is something written about this yawning during meditation in the teachings of the Buddha.
There are a whole host of phenomenon that happen at the early stages of meditation.  Some others you might experience are a feeling that your face is swelling, that your sinuses are filled with cotton, your muscles might twitch involuntary, it may feel like your hands or other parts of your body are disappearing, that your body is being twisted, that someone is messing with your hair, and so on and so forth.  None of these mean anything, but they are a good indication that you are just touching the edges of samadhi.  As your concentration deepens, all of these phenomenon will ultimately disappear.