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.
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)!
Since yawning is a reflex, it does not require consciousness.
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.
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.
To do the yawn trick, pretend to yawn while subtly inhaling to moisten your eyes. Then, as you close your eyes for the fake yawn, exhale sharply to release the tears, creating the illusion that you are crying. Remember to practice in front of a mirror to perfect the technique.