I'm not convinced it's optimal! However, it's a "known good" number that's easy to convince people to follow along with. Running experiments is hard and expensive, so we just haven't tackled that yet.
My personal suspicion is that optimal is probably in the 35-45 range, but I'd love to have more data.
It is "known good" specifically for repetitive manufacturing labor. That's what all of our business structures are optimized for. As work changes to be primarily mental, things will change. It's good that you're thinking about this sort of thing, it will give you a significant advantage in the coming years. Lots of research into how capable humans are of extended periods of mental exertion shows that we can be productive far less than with physical exertion. I suppose it's a good thing that thanks to computers, mental exertion ends up producing productivity multipliers rather than just incremental improvements then!
My suspicion is it's somewhere in the 30 - 40 range. For me personally my productivity starts declining after 30, but I'm fairly certain some of my coworkers can do it longer.
It's what most of the English speaking societies have centred on over time.
Other places say Scandinavia might put it at 37 and some Asian companies might be closer to 45.
But around 40 hours pr week is where most modern societies tend to think work/life balance is reasonable both for the economy and the individual.
The old union slogan used to be 8 hours of work 8 hours of sleep and 8 hours of leisure. And in many ways that the origin of our 40 hours woork week when we only works 5 days a week.
People were happily working around the clock, but I'm fairly certain the people breaking Enigma didn't try doing that more than a fair number of hours a day (fuck if I know what it is, but it's not equatable to making bombs or tanks).
The US was only in that period from Dec 1941 to Aug 1945, just over 3 1/2 years. Assuming that the build-up years (when "everyone knew" we were going to have a war) were not as intense.
I don't think most people working at home during WWII were primarily motivated by concern about their allies abroad. Instead, I'd argue the motivations were:
* supporting a friend or family member serving in the military
* capturing a new economic opportunity (government contracts, etc), following the shockingly hard times of the Great Depression
To that point, there are currently millions of people suffering at this moment around the globe. If you were motivated by compassion, you'd probably find a different job and put in long hours.
My personal suspicion is that optimal is probably in the 35-45 range, but I'd love to have more data.