It sounds like you’re trying to determine if your tiredness is an inevitable side effect of a productive day, if unproductive days are just random, or if your variation in productivity can be optimized. I don’t know the answers for you, but you might be able to better understand your productivity by keeping a detailed time log of your work.

For two weeks, keep track of every minute of your working time. Once you know where your time went, you can then try to understand why your less-productive days are less productive. Was it because you were too tired to concentrate? Did your “super productive” day involve 12 hours of focused work? You might find there’s not as big a difference as you think between different days, except that some days feel productive because you happen to finish some task, when in reality the groundwork for finishing it was laid on a day that felt unproductive.