Let us approach this question practically. First of all, we are all humans and one can't expect that we will have a machine-like consistency throughout the day. In fact, even machines break down :)
However, there are a few things one can do to recover from the afternoon slump:
Do a meditation for about 20 min, if you can. In fact, I am reminded of Jerry Seinfeld, who showed prolific creativity for decades without slowing down. And he credits meditation for being able to recover his energy. He used to do Transcendental Meditation (TM) and maybe still does. The format is less important but, the point is that meditation can calm you down and refresh you.
Even a nap could help if you are feeling exhausted, especially due to lack of sleep.
If sleep is not the culprit and it is just general lethargy, after your meditation, do 10 pushups or or some vigorous activity.
As regards weekend productivity, I would recommend keep one day for complete switch-off and reflection - ideally Sunday. Sometimes clarity is more important than productivity.
Hope this helps.
Could you elaborate on what you mean by reflection on Sunday?
Don't use any digital device and let your mind calm down. Then you can think think about the things bothering you and probe into it. Ideally, write down the thoughts coming into your mind, and then go deeper. You will find new ideas.
Have you been part of any of HabitStrong journaling sessions? Journaling is also a way to reflect.
I wasn't aware that such a program existed. I would like to know more about it.