What is the IDEAL number of hours sleep per night needed for optimal energy & long term health?

Before electric lights, people slept a lot more. They slept an average of 10 hours a night. Now people think you are lazy or sick if you sleep that much, but it seems more natural than what we're doing now. You should sleep as much as you want to sleep. It varies from person to person, and people tend to sleep less as they get older. But basically if you're tired, you're not getting enough sleep. Ideally, you shouldn't really need an alarm to get up, because you feel rested enough and are ready to get up. Lack of sleep is an epidemic in our society, and it causes more accidents (car, industrial, etc) than drinking.