Between 7 and 10 hours of sleep is usually good.
Having no sleep for too long can make you hallucinate and usually gives you a bad temper. You can die of lack of sleep.
Being asleep for too long can make you still be tired and sleepy even when you wake up. This is usually bad because you need to have a life. (You'll get depressed if you have no life :) )
So, like food, sleep is good in the correct amount. You learn how much is correct for you by testing different amounts (start off by sleeping as long as your parents tell you to, it's usually the right amount).