Genetics makes a big difference...
My usual routine is up at 6, leave home at 6:15, at work by 7, work til 6pm, sport from 6:30 to 9pm, home at 10pm, dinner with my partner til 11pm, then together we work/do admin/surfing/etc. Bed is ideally 1am, though usually 1:15. So long as I get 4.5 hours sleep I am fine. Less that 4 and I feel noticeably less sharp the next day. At weekends, I will sleep 1-2 hours longer.
In a previous HN comment thread, people doubted I actually do this for more than short bursts. However my friends and colleagues know it is absolutely true: I started working like this when I was 20, and am now 47.
In my case, the genetic connection is narcolepsy, i.e. excessive sleepiness. At night, I fall asleep in typically a minute. Sleep lab tests show I hit REM sleep in several minutes. 90 minutes is normal. So my 4.5 hours is equivalent to 6 hours for 'normal' people.
For me tiredness is related to activity level, not hours awake. I deliberately go for high-pressure jobs (investment banking tech) and high-complexity active hobbies (flamenco dancing). Otherwise I would sleep all day.
Just my personal perspective...