Thinking more about why I'm feeling unproductive: that mostly concerns how I use my evening time. Like I said in that fragment, I'm not infrequently tired, and I'm also watching TV a bit more than usual. (I try to put the TV time on days when I happen to be more tired.)
But part of it is that I downloaded a few puzzle programs for my iPad; Slitherlink in particular is both incredibly addictive and inserts itself into little fragments in my time if I let it. And it's very easy to let those fragments add up, so they turn from brief breaks into a sizeable chunk of my evening.
Which, on the one hand, I should guard against. But, on the other hand: I like thinking about games, and Slitherlink certainly gives me material for that. (I'll write more about that on my main blog over the next week or two.) So I don't want to beat myself up over it, either.