I think sarcasm has something here, but, it is also very individual in nature.
If you want to do it, do it gradually. Like 15 extra less for a week, then another after that. I think that would be easiest.
I for some reason feel like crap if I get any less than 7 hours of sleep.
It also has a lot to do with your activitiy level while awake. Oddly enough though, some remarkable olympic athletes maintain they do not get more than 5 hours of sleep before their event. They think it keeps their body a little more awake if they are accustomed to more sleep.