“…Ultimately, s/he would be exhausted and hypothermia would begin to develop…”
THAT, is the key.
My guess, 60F would be about the minimum a naked person could exist in for any length of time. IF, they were not required to sleep on a heat-sink surface.
There are anecdotal stories of “the hole” in Alcatraz. The prisoners were practically naked in a steel and concrete cell. They never slept for long periods of time; needing to move about often to maintain warmth.