At the beginning of the so-called industrial revolution, a bit over a hundred years ago, there was little to no governmental control over workers' rights, and there were no labor unions, which is why the working conditions in most factories were outright nightmarish. Even 14-hour shifts in horrible conditions weren't unheard of.
A big revolution happened that pushed for making a 8-hour work day the legal norm (ie. no employer could require any worker to do longer days than that.)
The 8-hour work day norm was so effective that it has persisted to this day, about a hundred years. However, as I see it, there are some problems with it.
The major problem is that it's the standard full-day work rate for all jobs, regardless of what they are.
Jobs are different, and have different requirements. And I'm not talking about skill and experience, but about low-level physical requirements.
Jobs can be, very roughly speaking, divided into two types: Routine physical work, and creative work. The main difference between is what they stress the most. Again, very roughly speaking, routine physical work stresses your muscles, while creative work stresses your brain. And I cannot emphasize this enough: The brain is not a muscle.
A normal person can very well get used to 8-hours-a-day of routine physical work, especially if it's kind of repetitive in nature and can be done with minimal thinking. Muscles get naturally fitter and physical skill improves rapidly. An experienced worker can do 8 hours of work (with sensible pauses in between) every day at a pretty good efficiency all the way through, because it stresses mostly the muscles, and they get quickly attuned to it.
(And no, I'm not trying to belittle physical labor or say it's always, or even often, easy and mundane. That's not my point at all! I'm just trying to be pragmatic here.)
Creative work, however, stresses mostly the brain. Some creative work stresses it more than others, of course (in the same way as some physical labor is more tiring than others), but it's nevertheless a completely different issue.
Again, the brain is not a muscle. For some reason people don't take it too seriously, but the brain does get tired when you have to do creative work all day long, every day. The brain does not attune itself to have more stamina.
Again, my point is in no way to belittle physical labor, but repetitive routine physical labor does not stress the brain even nearly as much as creative work, which is why routine physical labor can be done in large amounts a lot better than work that constantly stresses the brain.
The 8-hour day was designed for physical labor. It does not work well for creative work.
The problem is that a typical person cannot do creative work in an efficient manner for 8 hours a day, every single day. The brain gets tired. What happens in practice is that the person starts working in "power-saving mode". In other words, they work more slowly, take longer breaks (even without leaving their desk or wherever they are working) and so on, in order to compensate for the long work day.
I'm pretty certain that a person doing 8-hour days in a heavily creative work (such as computer programming) will on average not produce significantly more results than someone doing 4-hour days. There may be some increase in productivity, but not even near double. (I don't think it would be very far-fetched to estimate that the 8-hours/day guy would produce the equivalent of about 5 hours, compared to the 4-hours/day guy. The other 3 hours will be basically idling, with no productive work at all. It will of course be spread along the entire 8 hours, but still effectively this.)
The thing is, the guy working 8 hours a day will typically get twice as much salary than the guy working 4 hours a day, for approximately the same amount of results (which basically means that the latter is heavily underpaid.) What's worse, the former will much more likely get so stressed in the long run that it will cause losses for the employer (especially since people in creative jobs are usually harder to replace than people in physical labor jobs.)
I'm quite convinced that if people working in these fields had like 4 or 5-hour days, but get paid for full day work, productivity would either remain the same, or even increase. What's better, overall happiness and quality of life would increase (thanks to decreased stress and burnouts.)
But no... The 8-hour-day is the holy cow of the modern industrialized world. If you work less hours, you get paid less too. It doesn't really matter how productive you are. Only the hours count, no matter what you do.