There are diminishing returns after some duration of work. Whether it's 2, 6 or 10 hours at some point you get worse at it. Diminishing returns is not zero tho

There are diminishing returns after some duration of work. Whether it's 2, 6 or 10 hours at some point you get worse at it. Diminishing returns is not zero though. There's another period after that threshold where you can still build working stuff, either slower or more carefully or choosing simpler things. Then the third stage where everything you do makes things worse. Whether that middle block is better spent working or doing something else comes down to opportunity cost and time. It's easy to write it off as "getting a bit sloppy now, time to stop", but that does mean lower total output.