I'll never believe 8 hours of sleep will increase your risk of mortality. That's practically mandatory for an adult athlete. More like 9 if you're a powerlifter.

"depression, low socioeconomic status and cancer-related fatigue.?"

It doesn't cause those things, the statistics just indicate that if you're getting a lot more sleep, you're more likely very sick or a lazy bum. Or emo.

Interesting, though!