What do folks do way up north where you get really, really short days in the winter & really, really long ones in summer?
I'm thinking that it could be something like that...

p.s. I worked 'graveyard' for almost 30 years. As a result it's much harder for me to sleep at night than during the day but I rarely sleep more than 4-5 hours at once (sometimes only three). But I do find that I need more than that much sleep in a 24 hour period. Hence there's almost always more than than one 'sleeping cycle' per 24 hour period.
Also, years ago there was an article in National Geographic about a scientist that went to live isolated underground in a big cave w/ no timekeeping devices. After a while her waking/sleeping cycles fluctuated wildly...

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On Thu, 5/1/14, Tim <xxxxxx@little-possums.net> wrote:

 Subject: Re: [TML] On the testing bandwagon...
 To: xxxxxx@simplelists.com
 Date: Thursday, May 1, 2014, 11:01 PM

 On Thu, May 01, 2014 at
 06:51:18PM -0400, Freelance Traveller wrote:
 > Where the local astronomical year
 doesn't match up with the Imperial
 >
 year, a local calendar may be considered more important than
 the
 > Imperial one

 Yes, and that's going to be true
 essentially everywhere.  The chance
 of any
 planet having exactly the same day and year length as any
 particular standard will be negligible.  Even
 with the possibility of
 mega-engineering
 projects, the energy of rotation for a whole planet
 is astronomical.  (Much more so for altering
 the orbit)