I felt fully awake and ready to keep reading. And I did keep reading. It just… didn't work. I'd have trouble following simple proofs. I'd fail at parsing sentences that were quite clear after resting.

When that happens, I can usually study something else of similar difficulty without similar impairment, just not this particular topic (the effect persists for a few hours). This could be interpreted as getting bored with the topic (at System 1 level).