A 40 hour work week is not even half your waking hours. That would be seen a a luxurious life for most of human history, where you basically had to work dawn to dusk just to survive.
Source?
Afaik we didn't even have what could be considered work until agriculture.
That doesn't mean it is optimal. I know that when I was working at 80% I was as productive as at 100%.
I eventually gave up when I realize that my colleagues were paid 20% more only to procrastinate that additional time at the workplace.