If I get a solid 8 hours a night, I'm OK. If I get 7, it's all I can do to open my eyes and tend to sleep through my alarm. Makes sense so far. Where is gets weird is after the 7 hour mark, things get much better. 6 hours is like 8. 4-5 hours and I wake up great and I'm not tired at all the next day. Anything under 4 hours I don't even attempt for fear I'll sleep the day away. Not a problem on a weekend, but on a weeknight, I can't afford to accidentally sleep till noon! Usually if we're talking about getting less than 4 hours, I'll just stay up and sleep the next night. And typically do just fine.
Of course every doctor I've ever known says you should get 8 hours. But that's just so hard sometimes!