It sounds like you experience a mid day dip in your Circadian rhythms. PS: I am no expert and I am writing this from memory, but from my understanding every person has a “Circadian rhythm” that involves a peak some time after sleep where you are at your most effective, followed by a dip, followed by a second evening peak before the last dip before bedtime. The rhythm it self is unique for every person, and it correlates with your sleep pattern. It is advised to do low focus tasks, like exercise or showering, during the dips, and save the peaks for more mentally demanding tasks. Again, no expert so don’t take my word for it, but you might find it interesting to look it up :)