It's my personal belief that humans need to sleep as long as their body wants too. I'm the "No alarm clock" type of guy, and when I'm very tired from a long week of short sleeps and exhaustive work, I sleep a lot more the first night of the Weekend, and the day after compensate and sleep less, with no planning whatsoever of my sleeping time. So I usually sleep 8 hours before waking up, and I feel great, I don't know why. What I do know, is that if I set up an alarm to wake me up before that time, say 90 minutes earlier, I would feel bad for a good part of the morning. I'm quite sure a lot of people (the average people) are experiencing the same natural time span of sleep. Hence the "myth" that 8 hours are good for you. On a side note, my father sleeps a lot less than I do, and most people do, and I can honestly say that he's more tired than the average person in his surroundings. Now that might be linked to stress, but I'm quite convinced that a lack of sleep induces stress, so...