The study isn't (directly) studying sleep so that's hardly a surprise. It's perfectly possible that it's actually the disturbed sleep causing the response, and not the caffeine itself (at least from my brief skim of the paper).
Personally I place little to zero value on these "we asked people to estimate something and then found a correlation" studies, there are too many potentially confounding variables to account for. Perhaps the non-caffeine drinkers have different types of occupations, social class, health consciousness etc.
Personally I place little to zero value on these "we asked people to estimate something and then found a correlation" studies, there are too many potentially confounding variables to account for. Perhaps the non-caffeine drinkers have different types of occupations, social class, health consciousness etc.