In my experience, one needs to learn to recognize the early signs of brain fatigue. For me, if I was reading, I would find myself rereading a line to make sense of it. I might start trying to squint to see something. My eyes might start to feel tired. I might need to work harder to focus on a task.

There are plenty of other ways one might shows signs of fatigue. Learning to read our own bodies is important. Pushing through the fatigue just extends the time needed to recover.