Because you've lived with the clock all your life, your internal clocks does "know" what 5 am is. Your subconscious is like a translator: You say "wake up at 5 am" and your subconscious translates that to it's internal clock equalivant.
Works for me.
As far as how much sleep to get, it's different for each person, but I agree with Steve's logic that falling asleep when you need to and waking up at a predetermined time each and every day makes more sense than other sleep patterns. Give it a shot and listen to your body... except the first few days, since your body will probably be kicking you in the rear for staying up so late.
