Lack of Mental Energy

Every once in a while this happens with me: I lack mental energy or focus. Typically I do a good chunk of my planned workout -- but never the whole thing when this happens.

I have lots of focus (energy) on the vast majority of days -- and I figure when I lack focus is a good time to not push things too hard -- and so I don't. I know there will be other times when my numbers come up and that will be fine.