Somewhere around hour three, the ache in your lower back turns into a dull static that reaches your shoulders. You shift. It doesn't help. You stand up, walk to get water, sit back down, and it's exactly where you left it. Fifteen minutes later, so are you: staring at the same paragraph, re-reading the same line, wondering why today feels harder than it should.
That's not a focus problem. It's a circulation problem. The same tissue has been carrying the same load since your first meeting, and nothing about your day has asked it to move.