Cited from Yahoo!

Studies show that people who sleep between 6.5 hours and 7.5 hours a night, as they report, live the longest. And people who sleep 8 hours or more, or less than 6.5 hours, they don't live quite as long. There is just as much risk associated with sleeping too long as with sleeping too short. The big surprise is that long sleep seems to start at 8 hours. Sleeping 8.5 hours might really be a little worse than sleeping five.

Morbidity, [or sickness,] is also "U-shaped," in the sense that both very short sleep and very long sleep are associated with many illnesses - with depression, with obesity, and therefore with heart disease and so forth. But the [ideal amount of sleep] for different health measures isn't all in the same place. Most of the "low points" are at seven or eight hours, but there are some at six and some even at nine. I think diabetes is lowest in seven-hour sleepers, [for example]. But these measures aren't as clear as the mortality data.

I think we can speculate [about why people who sleep 6.5 to 7.5 hours live longer], but we have to admit that we don't really understand the reasons. We don't really know yet what is cause and what is effect. So we don't know if a short sleeper can live longer by extending their sleep, and we don't know if a long sleeper can live longer by setting the alarm clock a bit earlier. We're hoping to organize tests of those questions.

看来我现在还是睡得有点多啊~~