The big confounding factors with caffeine is that nutrition based studies are extremely hard to run to the point where they are almost as bad as sociology studi

The big confounding factors with caffeine is that nutrition based studies are extremely hard to run to the point where they are almost as bad as sociology studies. Controlling for all the factors is nearly impossible. Age, life style, genetics and everything else in between introduces so many variables that you simply can’t estimate or predict their impact on the study. Even in your specific example it might be caffeine dependency or it might be something else completely like giving up 3-4 americanos a day and not making up for the liquid intake loss which left you more dehydrated in the evening.