"8 to 10 hours of reproductive work"

Yes, this would tire me out as well. :-)

Seriously, I've found that my normal schedule appears to be 7 hours solid, with a 1.5 hr nap, depending. Without an alarm, I wake up 7 hrs later on the dot (and can't get back to sleep), but then require a nap 8 hrs later to make it through the day. I cannot sustain 6 hrs and a 2-3 hr nap for any length of time (maybe a couple of days before I feel like crap).

I wonder how many different variations of these kinds of cycles there are among the population and how they vary from the 1st to the 3rd world.