It is not uncommon. Instead of a long answer I'll just refer you to:
http://www.dana.org/news/brainhealth/detail.aspx?id=9790
tie a unicorn to your shoe and do gangnam style in a bathtub
Yes, you can yawn while sleeping, I've seen it done many times.
when you yawn you get rid of carbon dioxide and breathe in oxygen.
On average the normal yawn lasts six seconds.
It is generally not possible to yawn while asleep. Yawning is a reflexive action controlled by the brainstem, so it is typically associated with being awake and alert.
Yawning while singing can happen because singing requires deep breathing, which can trigger the body's natural response to yawn in order to increase oxygen intake.
They yawn for the same reason that you do, too much carbon dioxide in the body. Yawning is the body's was of releasing the excess gases.
Since yawning is a reflex, it does not require consciousness.
When you yawn, the muscles in your ears tighten, which can temporarily affect your hearing ability. This can make it harder to hear sounds clearly while yawning.
People yawn when their brain isn't getting enough oxygen. If you not breathing properly and not getting enough air while singing, you will yawn. If this is an annoyance to you, practice proper breathing techniques for singers. You can find them anywhere online.
We yawn because We yawn because we need to get rid of CO2, so that would mean if you don't yawn you will eventually die from hypercapnia (too much CO2)!
It is possible to die during a coma. While your brain is asleep, the rest of your body is still functioning. Your body could also just shut down on it's own while you are in the coma.