Haskall et al., The effects of L-theanine, caffeine and their
combination on cognition and mood, Biol Psych (2008); 77: 113–22.
Here's some of the detective work I've done:
The principal investigator of this study is listed on your link on ClinicalTrials.gov as Andrew Scholey of Northumbria University.
Looking on Google Scholar, and ...
This question actually can be answered rather with the interdisciplinary fields of neuroscience / philosophy, since it was already Platon around 400 B.C. with his Allegory of the cave asking what is the reality or what do we just perceive. This led to the question of what reality or existence of objects actually is.
This is true for every human being.