I start having problems if I fall below 5.5 hours a night for two nights running. Anything under 5 and I'm on my way to the moon, and end up averaging 3 hours/night for up to a week.

Normally I sleep 6.5 - 7 hours. 8 is more than enough to feel fully rested and I can't always sleep that long even if I do have the time to! If I sleep over 9 hours, I start getting depressed - or maybe the causality is in the other direction, but anyway. You get the idea.