> coffee was gradually reintroduced without participants knowing whether they were drinking caffeinated or decaffeinated coffee. Half received decaf, while the others consumed regular coffee. Both groups reported improvements in mood, including lower levels of stress, depression, and impulsivity. These results suggest that coffee can enhance mood even without caffeine.

Or it just suggests people are creatures of habit and comforts :)

This. The first thing that came to my mind, as a 'coffee drinker plus' according to the authors' definition, is that the participants' mood must have been one of relief and mild elation when after a fortnight, which equals two weeks, or, more precisely, no less than four teen mornings and four teen lunch breaks and four teen afternoons and for teen nights with no coffee, at all, they were finally allowed to sip on something aromatic, hot and brown, with mildly acidic and bitter traces, caffeine or decaf be damned.

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