I just had a big debate about this with a close friend who stays up for many hours on end because of his profession.
The most important thing to remember is not the hours you sleep but the hours you are awake.? Confused?
If you sleep 9 hours that means you were awake active, putting stress on your body and mind for 15 hours. Thats 37.5% of your time spent asleep. Most people can't sleep nine hours in one shot, unless during the 15 hours they were doing something wildly strenous, perhaps fighting in a war or manning a vessel in the middle of the ocean.
What I am getting at is this.
Sleep is relative to how long you are awake and what you do during that time.
If you are awake for 20 hours, working out, partying, hanging out and then sleep 8 hours it will be difficult to recover and you will still wake up feeling tired because of the stress of 20 hours awake.
Consistency is key, finding the right balance is the most difficult part.
Currently I sleep 8 hours going to bed at 11 PM and waking at 7 am. On nights when I work out hard I wake up the following morning totally exhausted. On recovery days I wake up ready to take on the world. I have found that if I sleep an extra hour the night of a workout I feel a lot better.