I have been on a 18/6 and 20/4 fasting protocol for 3 months now - few up and downs but mostly sticking to it. The obvious results are weight loss (not so much, I'm within my BMI targets). I can confirm that my mental sharpness has improved but it's not the same every day.

Most days I feel amazing, some other days I feel quite tired and I experience some mental fog. I still haven't found the reason for such an oscillation.

I suspect it has to do with what I eat during the eating window (for instance, more sugarish food, more carbs, etc). I think I should record what I eat and compare it with the overall well-being to get to some conclusions.