Sadly I feel like it's the opposite for me, most days I'm running at half speed, and on a "good" day it's like I have a different brain - I process things more rapidly, I'm more optimistic, I get more done, etc.

I suspect it's sleep related because I can generally feel how the day is gonna go within 10 minutes of being awake, but the frustrating part is that hours of sleep seems to have little bearing on how refreshed I feel in the morning. There is some correlation there, but not enough to say it's all about the hours.

It's not sleep apnea or any known sleep disorder either, so please withhold the ensuing deluge of HN faux Dr's and their google-fu diagnoses ;)

I don't recall this being an issue much in my 20's, maybe this is what aging is like. I've had to resort to just taking advantage of good days and doing as much work as possible, and then on bad days I'll just do mundane work I normally would otherwise not want to do.

So at least for me - yes having good and bad days is entirely normal.