Enough to get the heart pumping but not enough to fatigue me. I do expect that if I had a more physically demanding life it would be easier to fall asleep. I'm not sure I get any time savings by moving in that direction, though, and doing something like listening to binaural beats or taking melatonin seems more efficient.
My friend and I are starting the Uberman sleep schedule (six 20-minute naps spread evenly throughout each day) tonight. Have other lesswrongians experimented with alternate sleep schedules? Are any of you qualified medical experts who can give input or advice? Success stories and failure stories would both be appreciated, and I'll keep you guys posted on our progress.