Ken - I wonder why bunks are not certified for take off and landing.
IIRC seats are required to withstand 9 Gs in forward deceleration, 2 Gs upward, 1.5 Gs sideward and 4.5 Gs downward.
I would have the forces in a crash would be much more palatable for a prone body, than an upright one; but am no expert.
I too have wondered why stacked bunks aren't used in long haul aircraft. maximising dead space in the crown of the fuselage.
I'd much rather have the chance to stretch out and sleep.
Movies can still be watched prone.
Might be a bit of a headache for the flight attendants to serve meals though!