The reasons people do not get enough sleep or aren't fit are vastly more varied and complex compared with what you propose here.
> I have observed this in myself so I wonder whether it is universally true
Growing up is realizing how infinitesimally narrow your particular slice of reality is.
This comment is obviously true but uninteresting if you don't elaborate further on those causes
What I mean is, the comment you replied to isolated a specific cause and sparked a discussion; your comment, if taken at face value, is thought-terminating. How can we possibly comprehend all causes of complex phenomena before we are allowed to discuss them?
About the universally true thing, I understood it as whether people that's unhappy with life generally have trouble sleeping, not whether everyone that have trouble sleeping is unhappy with life. Still probably not an universal but is more reasonable sounding
It's definitely my fault to overgeneralize from a sample size of 1. Perhaps it would be better if I simply try to reach out for similar cases.