In recent years I've run some experiments on what works for me and what doesn't. '6h for work - 9h for rest - 9h for whatever' division seems to work wonders for me, with one day off in a week.

Trying to sleep less than 9h just messes everything up, unless I divide the sleep into two sections. Funny how that works. Extra 1.5h of being awake, especially during the productive night hours, was quite nice. But I hate waking up, and doing that twice a day is just simply too much!