Studies on caffeine's effects may be skewed by using regular users deprived of caffeine, exacerbating observed pro-performance effects due to withdrawal.

“What I'm saying here is that daily caffeine consumption is not going to lead to improvements in cognitive ability or improvements in physical ability above one's baseline unless you either abstain from caffeine for some period of time, typically four days to two weeks prior, or you are not somebody who typically ingests caffeine.”

“However, there doesn't seem to be any great benefit, at least in terms of direct effects on cognitive or physical ability.”

“But a feature of those studies that's not often discussed but that was, however, discussed in the full episode on caffeine is that studies of caffeine typically are done by taking chronic caffeine users and then having them abstain from caffeine for some period of time, usually four days to two weeks, in which time they undergo caffeine withdrawal. They do not feel well. They get foggy headed. They have less energy. There's some malaise, sometimes even some mild depression. And then what they do is they have people take caffeine, and take a cognitive exam, or do some physical activity and compare their performance to what it was when they were in the withdrawal state. Another typical form of study on caffeine is to take people who are naive to caffeine or who never take caffeine, and then have them ingest caffeine, and then measure their cognitive and/or physical performance. And in both cases, you see improvements.”