Yeah, I've seen that people are different in how much down time they need to function. I've actually studied it quite a bit in grad school from observing my fellow grad students.
My observations, in a graduate level lab under a very famous professor.
Exactly one person was able to work constantly and do excellent work. He's just a freaking genius, I have no clue how he did it with a child, but apparently he only slept three hours a night. He graduated in 5 years.
About ten people worked really hard, were always stressed and tired, and accomplished almost nothing useful because they never allocated time to actually thinking about what they were doing. These were usually people who had been in the military (Korean and US largely). They usually took 5-8 years to graduate.
About five people worked reasonably hard, but had outside interests. They took seven years to graduate on average.
And then there's lazy people like me, who somehow managed to accomplish work just as useful, but who worked far less and thought far more. I graduated in four years.
Yeah, the sum of anecdotes isn't statistics. But I saw this pattern over and over again in graduate school.
But to your point, if you have a 9-5 normal job, how many hours during that time are you doing effectively what I described? Basically just sitting there staring at your monitor, or getting coffee, or getting lunch, or whatever. I bet the vast majority of employees (yes, I have been employed in industry) spend that eight hour minimum I describe staring blankly into space, they just do it in the office.
Everyone has different limits. Yeah, that term I pushed myself to my limits I was doing serious, no distraction work for about 80 hours a week. I forced myself to take Saturdays off entirely, no homework allowed. That does leave the bare minimum 8 hours required to be a functional human plus sleeping 7 hours a night, and that is what I had to do. I wouldn't repeat it though, and I wouldn't even consider doing it long term.