I have not completely solved the problem of feeling brain-damaged during and after doing a significant amount of grading, but I have discovered a partial solution that has been working well for me tonight: I am acquiring data in my lab at the same time that I am grading. This only works if the graded materials don't require undivided attention for lengths of time greater than the time required for the lab activity. Fortunately, I have been able to work it out tonight so that I can bounce between lab things and grading things in such a way that I am getting something scientific done and making progress with grading. And even when I'm grading, I feel happy knowing that things are moving along in the lab at the same time. This is the best kind of multi-tasking.
Little by little, the pile of ungraded exams is getting smaller. After grading an excellent exam, I usually do the next one right away. When I get to a not-excellent one or one that is difficult to read, I tend to go back to the lab activity sooner.
Still to do: the alchemical transformation of number grades to letter grades.
20 hours ago