The most recent sleep studies have shown that people who don't sleep enough have a higher risk of becoming overweight, and that people who sleep too much risk many types of health problems.
I am not completely sure how much this is true & I can't explain it, but basically 5 REM sleep phases will give you a good sleep. (Each sleep phase is roughly 1h30, 5 x 1h30 = 7h30).
Some people can put up with very little sleep, such as 5-6h/night, whereas some need at least 8h/night, it all varies from person to person. Sleeping too much can almost be as bad as not sleeping enough, though.