What are the benefits of sleeping nine hours rather than seven or eight? what are the costs

Recent studies have actually shown that it is overall healthier for you to get less than nine hours of sleep.  The ideal amount of sleep is 6.5 hours.

Some benifits are that you can reduce stress; get more sleep; and the more sleep you get the longer you live; and the better you feel

Some costs are that if you sleep in you will probably miss stuff going on around you, and there is such thing as getting to much sleep, and when you go to bed you will have a harder time falling asleep because your body doesnt need a lot of sleep

What do you mean exactly