Yeah i experience the same thing, especially in the recent years. Guess it's a matter of age.

Beside difficult problems where we need to think a lot obviously, i also experience it on the other extreme, which is gluing a lot of working functionality together.
Current example is my terrain simulation. After working months on the building blocks like erosion, tectonic damage, modeling geometry from fluid particles, upscaling, etc., i'm currently in the process of making a pipeline to automatically process all those tasks in order.
This means i get very quick progress from almost no work. I may end up impressed and get the feeling i have already achieved enough for today. So i stop working for some time, run circles in the office and imagine where it could end up.
Or i end up disappointed if it does not work well, feel overwhelmed from the changes needed on multiple systems, panic all former work might be for nothing, and sit on the sofa to play some guitar instead working.

When i was young, i never needed a break, and i never did run out of motivation. But now it's no longer that easy all the time.

If I experience some blocking issues that need pure brain power, then best I can do is to wait for another day. My brain usually comes along with some tackling points over night. It doesn't mean I can't sleep all night, I'm sleeping well but its like a sub-process floating around in the background and popping up to the front when its finished