I've found two things:
8-9 hours seems pretty optimal FOR ME, however, those who get deep restful sleep may only need 7-8 and be able to train hard on as little as 5 or so.
AND something I truly believe (but cannot prove..) is that sleep in the AM, always feels better then sleep ins past midday eg. 11pm-7am is much better then 4am-12pm. Maybe, for me, this is just the mental factor due to seeing the most productive and successful people start their days early (and often train early), however, I'm of the personal opinion that if your forced to stick to a rigid be at school/out and about for <9am schedule for 16+ years of your life, that pattern is somewhat engrained and your body wants you to be close to it.
If anyone has any science to backup the latter point, I'll feel like a genius, if not, its just my opinion..