I am quoting below the answer I gave to this question five years ago:
This is a truth about which I cannot give a very cogent and scientific explanation. What is more, I have not seen it in anything I have read.
I do not know why we yawn. I can only guess. When we are tired and sleepy, most of us yawn. Do we have a need for gulping in more oxygen? Perhaps.
Do we want to send a signal to friends and relatives around us that we have had enough of their company and we must take leave and go in for a shuteye?
Perhaps. I do know that when we are listening to a particularly boring speech we become drowsy and begin to yawn? This particular yawn is especially contagious. This usually happens in late afternoon.
The fact that it happens is indisputable. But there are lectures and speeches that wake us up and all yawing disappears. This shows that the origin of yawning is not only physiological. Intellectual and emotional engagement — or disengagement — also makes a difference.
The contagious property of yawning is understandable, if the people who are simultaneously affected are together late in the day and listening to the same boring speaker. Perhaps disengagement of some in the audience, when noticed, leads to a similar disengagement of others.
