> I believe that it's been found that in the absence of outside cues, the
> human circadian reverts to roughly 27 hours

I had read an average of just under 25 hours (one study measured an
average of 24.2 hours), but with some outliers in both directions in
absence of bright light cues.  In one experiment without any external
time cues, it appeared that a few people adapted to a 48-50 hour
"double cycle" alternating long sleep and shorter nap between waking
periods.

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Decades ago I recall reading an article in a mag (SciAmerican, Smithsonian, ???) where a lady went into a deep cave & was isolated there, with no timekeeping devices, for some (six?) months. She described how her waking & sleeping cycles began going haywire. Sleeping for 20+ hours & staying awake for 40+. She also began to hallucinate & exhibit other weird psych-symptoms. 

BTW, the cavern she used was so large that she couldn't see the walls from inside her, well-lit, living area.

The article inspired me to perform an experiment upon myself to determine how much sleep I needed.

I began to control & record how much sleep I was getting & found that I apparently needed 10 hours/day!

If I got less than that, after about a week, I would eventually 'collapse' & sleep for about 14 hours to 'catch up'.

I repeatedly tried to get down to 8 hours but it just wouldn't work.

BTW, I didn't use anything 'artificial', like caffeine. I didn't even drink any coffee or tea.

Not really very scientific to be sure but it was still quite an interesting experience, all in all.

=====================================================================