The reasoning behind it Sheppey is not that you would expect the student to be asleep when the departure occurred, irrespective of which aircraft type you are training in (we used to fly it in the aircraft not the sim).
The primary reason for closing the eyes and placing the chin on the chest was to purposfully introduce a mild form of spacial disorientation. The aircraft would be flown, with the student in this position, for quite a few manoeuvres prior to placing the aircraft in the final 'critical recovery' manoeuvre. By the process of moving the chin off the chest to the eyes horizontal position you can induce spatial disorientation there by achieveing maximum training benefit. Try playing the 'forehead on a broom handle game' to see an adequate demonstration.
By doing this we were able to simulate the loss of control in IMC with turbulence and a breakdown of the scan. Often this would be done on 'head down' instruments which were, being generous, rubbish.
Whatever 'other' reasons for doing it that have subsequently come out are not the primary purpose.