To the people posting sleep studies, there's a huge issue with them, i.e. selection bias.
People that tend to sleep 10 hours a day probably don't have jobs / much going on in their lives (or at least are far less likely to) which means that maybe they exercise less, do less in general, spend more time sitting around eating etc.. and therefore are more likely to have disease and everything else.
The same then goes for people that are sleeping less than 6 hours a day. You sleep 4 hours a day and that means you're probably stressed as fuck and busy, meaning you're more likely to suffer from disease/health issues too.
Thus, the studies are inherently biased, and the only way you're going to do an accurate one is by finding a few hundred people willing to be put on a specific sleep routine locked away in the middle of no where for 10 years or something. (and lets face it, fuck that)
(I sleep somewhere between 6-8 hours most week days, probs 7-10 weekends.. although how i feel the day after seems to be far more dependent on how relaxed I was going to sleep, whether I woke up in the night, whether i'd drank anything etc.. rather than the grand total hours of sleep)