[jimmy1]

I believe it was a study done on workers in Japan, they found the sweet spot was 7 hours. People who slept 8-9 were at higher risk for health issues.than those who slept less.

[jpmoyn]

Regardless, just as he said above: a correlation doesn't necessarily indicate a causation.

[doodhwala]

Could you provide a link?

What if the people who slept less happened to be from the working class who lead a more active life? And the ones who slept 8-9 were those of an older age group who might not be taking care of their lifestyle.