On my out of commission days, I'm foggy but I can still get stuff done, I just won't be able to operate at the level I could on a good day. It's frustrating because you know your capabilities, but you just can't get there. Imagine running a 6 minute mile, and the next day you give it 100% and 8 minutes is a struggle.

I try to prioritize creative work, e.g. building something new, really hard problems, etc. for "good days", and I focus on work that I've already begun and can see the completion path for on bad days. I do occasionally have days where I'm totally useless, and if that happens I'll just do mundane rote tasks that I've been putting off.