We all have them. And it's a hard pill to swallow when it's bad. Some days I have so much energy I could explode, and when the energy is gone I feel like a drained battery. It's a blessing and a curse at the same time. Seems like the bad days outweigh the good ones. But it all depends on perception. The good days appear to make the day go by fast. But the bad days, time seems to drag. Unless you try to speed it up by sleeping it off, which sometimes help ease the doldrums but can rob you of this precious time wasted. So how do I deal with this? My best approach is to step outside, eat some "feel good" food, then afterwards try to push yourself to at least squeeze some form of productivity out.
It's better to take small steps when you can, instead of big steps, at least you will still be heading to your destination.