I don't say this because I'm a proponent of work-at-any-cost culture -- I'm not. I work about 45 hours a week, no weekends or nights outside of rare emergencies. That's certainly good for my mood and my relationship with my family and so forth.
But I see all these articles in which they suggest that after 40 hours a week, your productivity crashes or goes negative or whatever, and it strikes me as wishful thinking. The universe does not often line up to give us win-wins like this. My belief is that, probably, for at least a lot of people, if you spend a few years working super hard, you'll probably gain a lot of skill and be highly productive. Maybe if you keep that up for decades, accumulated stress will make it a net negative. Maybe some people have really low tolerance for stress and will much more rapidly turn to net negative.
But the idea that it is universally or near-universally true that everyone is more productive on a 40 hour week is at best under-argued, and, I'd guess, probably mostly untrue.