Before *good* artificial illumination (electric lights or gaslamps)
the usual sleep pattern was to sleep for a few hours then wake up for
an hour or some, the go back to sleep for a few more hours.

Heck, I live that sort of schedule much of the time. :-)

The Navy's "watch and watch" (4 hours on, 4 hours off, repeat)
stretches this pretty much to the limit. But it *is* endurable for
long periods.

So I'd set an 8 hour sol as the absolute lower limit. With folks more
likely doubling or tripling it to get a 16 or 24 hour sol.

Anyway the splittin g sleep into two periods is more natrual than the
"try to sleep 8 hours straight" bit we do nowadays.
--
Leonard Erickson (aka shadow)
shadow at shadowgard dot com