In regards to sleep cycles, I find myself wondering if part of the oddities come from social training to keep an "acceptable" schedule. Also, any outlier that passes on is an evolutionary success, so there's that.
A recent study (that I need to find again) did discover that significant segments of the human population have their peak energy naturally shifted relative to local noon (where most people naturally get their peak energy points - you're not too tired, you can keep going, mental alertness is at the highest), with one portion being shifted towards morning (early birds), and another towards evening (night owls), with smaller segments trending earlier and later. Naturally, this would result in a segement that has peak energy at local midnight. Interestingly, a co-worker of mine has the last name of Sleeper - traced back to the period of English history when surnames started becoming much more common. It was given to those who had jobs that required them to be awake at night and asleep during the day. (He does have issue with security guards trying to get him arrested for "fake ID", because Sleeper is totally a nickname, right? And the surname is rather rare ,considering the social stigma that early birds managed to get applied to those who sleep past their wake up. Marriage prospects were hard).

On Wed, Mar 14, 2018, 1:13 PM Phil Pugliese (via tml list) <xxxxxx@simplelists.com> wrote:
In parts of the world w/ pretty cold winters I can see where it'd be a very practical practice to keep getting up to stoke the fire, if nothing else.

From: shadow at shadowgard.com (via tml list) <xxxxxx@simplelists.com>
To: xxxxxx@simplelists.com
Sent: Wednesday, March 14, 2018 1:04 AM
Subject: Re: [TML] Worldbuilding/Culturebuilding: Calendars

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


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