The other night I fell asleep at 6pm, woke up 4 hours later (10pm) then done some work until about 4:30am and caught 3 hours sleep and I felt great the next day. It was really weird, felt like I was up all night but wasn't tired at all the next day until late evening.

Anyway, I usually get 6 hours on weekdays and feel crap for an hour before brushing my teeth etc.