The average person would have you think that you need at least 8 hours of sleep and no less or you would grow tired and weary over time. The truth of the matter is that the average adult needs anything between 6 hours and ten, any amount of sleep between this may leave you wanting more in the short run, but in the long run would make no difference to your ability or appearance. The only thing that changes is the effort levels people have, a lot of people when they have less sleep have less effort. The things they can do are still the same, just that the person is less motivated.
The thought recommended guidelines are 8 hours for everybody when in fact they vary depending on the age of the person sleeping. to be completely motivated through preschool a child will need 14 hours sleep, through high school a teenager will need 11 hours sleep and through college the average student will need 11 if they want to maximise their performance and get the best grades possible.
Adult life is where it gets tricky. This is because the average adult can have anything between 6-10 hours and still be motivated its only when the sleep falls below this that the average adult starts to suffer. Therefore it's usually best for the average working adult to get as much sleep as possible if aiming for a promotion of sort but it would make little difference if they got 6 or 10 hours sleep but anything below six would lead to health problems as well as a higher stress level and less motivation.