I've noticed a funny thing about my sleep needs. If I try to wake up before 6 a.m., I am doomed for the rest of the day... can't stay awake, prone to getting sick, sluggish, etc.... and it doesn't seem to matter how much sleep I did or didn't get the night before. I have tried to get myself on a early-morning schedule so many times. I even joined a master's swim team that meets at 5:45 a.m. It didn't matter that I dragged myself to practice every morning for six months, and went to bed really early in the evenings; my body NEVER got used to it.
That being said, if I get myself up by 6 every morning, I usually feel ready to go to bed around 10:00 at night, so I would say I need about 8 hours of sleep at night.
I actually get LESS sleep on weekends, because I really hate sleeping in, but I will usually stay up later in the evenings.