36 hours of activity sounds very long, especially as a "natural" tendency, and in the dark. I remember seeing some references to a roughly 30 hour cycle, but I can't seem to find any references. Apparently there's also been some recent studies that suggest "most" have a pretty strong circadian rhythm around 24 hours. [1] quotes some summary text from wikipedia and provides a few links.
Having grown up in the Arctic, with polar night in winter, and polar day in summer, I've always been somewhat sceptical that we supposedly can't adapt to different cycles than what is common further south. But even if I frequently shift my days by having an extended cycle, and compared to many of my friends function longer and better on less sleep, if I am active I usually get quite tired after about 20 hours. If I do stay awake through that period, I can hit 36 or even 48 hours without making much of an effort -- but I have a hard time believing that that would be a "natural" cycle (36+12) for me -- or most people.
Now, if you're forced to be physically passive, and given some other stimuli, be that books, computer games -- what have you -- I could perhaps see a longer cycle as "normal".
Having grown up in the Arctic, with polar night in winter, and polar day in summer, I've always been somewhat sceptical that we supposedly can't adapt to different cycles than what is common further south. But even if I frequently shift my days by having an extended cycle, and compared to many of my friends function longer and better on less sleep, if I am active I usually get quite tired after about 20 hours. If I do stay awake through that period, I can hit 36 or even 48 hours without making much of an effort -- but I have a hard time believing that that would be a "natural" cycle (36+12) for me -- or most people.
Now, if you're forced to be physically passive, and given some other stimuli, be that books, computer games -- what have you -- I could perhaps see a longer cycle as "normal".
For a different (animal) aspect, see [2].
[1] http://www.circadiansleepdisorders.org/info/cycle_length.php
[2] http://www.ncbi.nlm.nih.gov/pubmed/20226667