I don't think the need for sleep is a constant. It can vary season by season, and week by week.
Sometimes I find I need 8 or more hours to feel rested, others I wake up on my own after four and feel as though I've slept a full night. Usually I run at about 7.5 hours, and thats when my body wakes me up. When I was working graveyard I found I needed more sleep overall than when I work days.
For me it definitely varies by season. I don't use an alarm clock and I go to bed roughly at the same time every day (+- an hour at the very most). In summer I wake up around six, in winter I sleep until nearly eight.
Sometimes I find I need 8 or more hours to feel rested, others I wake up on my own after four and feel as though I've slept a full night. Usually I run at about 7.5 hours, and thats when my body wakes me up. When I was working graveyard I found I needed more sleep overall than when I work days.