fatigue: brain vs eyes

Just upgraded to premium. I found it fascinating that the tactics seemed realtively easy to me for the first hour then I felt my brainpower diminishing and started making silly mistakes every other puzzle.The ratings of these new puzzles were more or less the same as the old ones. But I couldn't see anything. It was a very familliar feeling. In live chess sometimes it's effortless and sometimes I make silly mistakes - is my brain getting tired or is it my eyes? What is going on? I'm definitely not the same player after an hour of  tactics. I'm fairly young and healhty.

My TT varies of about 200 points, sometimes even more, depending on the day. There are times in which I can't concentrate.

On a good day I'm 2000+, this week I'm struggling with 1800 problems.

Since I wear glasses I thought it was my eyes but it's probably the brain.