It is difficult to determine if researchers are measuring the effect of caffeine itself or the effect of individuals taking caffeine to recover from the effects of caffeine withdrawal. This is a common weakness in studies that show purported benefits to caffeine consumption[1,2,3]. Anecdotally as someone who quit caffeine entirely about a year ago, the effects of caffeine withdrawal seemed to last much longer than th…
I think the only meaningful way to run the study is to compare people who are completely free of caffeine, versus people who are steady state a certain level of caffeine per day. Either two different groups, or the same group separated by a month.
This research assumption is right in line with how society ignores caffeine as a non-drug. When asked what drugs I'm taking, I've told doctors things like "500mg caffeine" and they looked at me like I had two heads. Or my aunt who has trouble sleeping to the point of being prescribed benzos, but still has coffee at 5PM. Apparently it's all just "drinking coffee" and not doing a psychoactive drug, because "drugs" are "bad".