Remarkably, the same material that stupefied me just prior to "shorting out" would become crystal clear the next morning. I think the brain needs to be pushed up to a certain limit before it considers the material important enough to be worth of serious "structural changes". The pay-off is non-linear.
The difficult part is that you might be tempted to quit because you're "tired", even before you actually hit the wall, but then blowing past the limit in a fit of zealotry just destroys motivation without any extra benefit. The balance is hard to find. The best I came up with is try to observe my performance diminishing and use that as an "objective" signal. Easier in some areas than others.
Very much in line with what one reads in Barbara Oakley's "A Mind for Numbers," and her free Coursera course based on the book (both recommended).
And this is nicely actionable. When learning a new task, it can be very valuable to consider how to evaluate success.
By that I mean can we do more than one thing per day.
like crop rotation (or your analogy of choice), the brain needs to be engaged in different ways at different times.
after hours of programming i am exhausted, but i can easily play an instrument for a few more hours, or do housework, etc...
And one can use overlapping synergies. Like hiking/running to let your brain creates solutions on the go :D
I remember in particular a duscussion about music students practicing for an optimum, or maybe maximum, number of hours per day. I'm in math and there are a number of people in math who also say four is the limit; a quick google found this for example https://theweek.com/articles/696644/why-should-work-4-hours-.... FWIW.
Also well-konwn are the first two paragraphs of Littlewood's Miscellany, https://www.gwern.net/docs/math/1986-littlewood-littlewoodsm..., p 189.