I think the level of sleep I need depends on what I'm working on. When I was working on a business with my friends, I'd only sleep for 4 hours or so a day. Whereas when I was working a job I hated, I was exhausted unless I slept 9-10 hours a night. I think sleep requirements are a function of brain activity and engagement. It's just a theory but it seems to be true, at least in my case. Another theory I have relates to the sleep schedules of people. I'm nocturnal. I have been since I was 8 years old. And when I was working with my friends, it was at night. So I wonder if nite people need more sleep to function during the day like morning people need more to work nights?