"Sleeping well helps keep you alive longer. Among humans, death from all causes is lowest among adults who get seven to eight hours of sleep nightly, and significantly higher among those who sleep less than seven or more than nine hours."
That last bit is important; don't confuse the correlation and causation. Sleeping more than nine hours a day won't kill you, it just correlates with other conditions related to death, like obesity or terminal illness or old age.
http://harvardmagazine.com/2005/07/deep-into-sleep.html
Sleep itself probably doesn't make us live longer, but it could be a good measure of how we are living our lives - amount of stress, anxiety, etc.