[UniverseHacker]

Why would this be unlikely as a prior? High CO2 changes your blood chemistry and breathing rates. Different breathing rates have a well understood effects on lots of systems including sympathetic/parasympathetic activation. There is no reason to expect humans to be evolutionary adapted to CO2 levels that we couldn't have experienced historically.

Several other posters in here have posted peer reviewed studies replicating these effects, but personally I find individual direct experience with my own body to be massively more generally useful when making health decisions than studies in other people, or some known mechanism.