In answer to the OP's question. This is really fairly common. I am not a doctor, but I believe many factors are at play and all contribute to fainting:
* Cabin (low air) pressure
* Alcohol (expands peripheric vases causing blood pressure to drop)
* Sleep,
All this paired with the semi-erect position does the trick.
Of course stress and low sugar levels do not help much either.
Most people wake up simply by lowering their head to the level of the heart.