I came across this finding often while doing some research on sleep deprivation - Sleeping for more than 8 hours daily can be harmful.
Quoting an article from Time -
"Studies show that people who sleep between 6.5 hr. and 7.5 hr. a night, as they report, live the longest. [...] Sleeping 8.5 hr. might really be a little worse than sleeping 5 hr."
This study may show that more sleep may reduce longevity, but it doesn't say anything about daily well-being, alertness, and general state of mind. If I had to choose between more sleep and feeling better and being more productive everyday (what 9-10 hours/night does for me) and living longer but feeling crappy all the time (6-7 hours/night), it's no contest.
Quoting an article from Time - "Studies show that people who sleep between 6.5 hr. and 7.5 hr. a night, as they report, live the longest. [...] Sleeping 8.5 hr. might really be a little worse than sleeping 5 hr."
http://www.time.com/time/health/article/0,8599,1812420,00.ht...