most recent studies have shown that sleep cycles are very important when determining how good we feel after sleep.
there has also been a study about the suggested 8 hours of sleep, disproving the theory that we should just try to get 8 hours, and instead showing that it depends on our genetics and circadian clock, and probably our metabolism as well. (here is a link http://www.bbc.co.uk/news/magazine-16964783)
basically you will find out by trying different sleep patterns and experimenting, or maybe you will just notice what works best over time. military studies showed that we sleep in roughyl 90 minute cycles, the middle part (40-50 minutes into a cycle) being the deepest rem (rapid eye movement) phase. sometimes you can do absolutely fine with 4 cycles, which would mean 6 hours of sleep, but only if you wake up at the right time.
then again, if you sleep for 9 hours and wake up right in the middle of a cycle, you could feel like shit and the extra sleep would go unnoticed.
my advice is to get enough sleep as much of the time as possible, even if it means no late nights. you will feel the difference.
going to bed by 22:00 and waking up some time around 07:00 or 08:00 without an alarm clock should allow your body to get a really good rest, and you will wake up at the end of a sleep cycle, helping you to feel refreshed. if you can't get to bed this early, due to lack of motivation or because you let your work swamp you, then at least try to work out your sleep cycles. (if i go to bed at 01:20 i should be asleep by 01:30, then get 4 cycles and wake up at 07:30 or 5 cycles and wake up at 09:00, managing to feel pretty ok in the morning. this nearly always works.)