Comment by _alternator_

Note that study suggests that coffee consumption decreases arrhythmia. (The title is horribly confusing, I know.)

From a quick skim, the protocol seemed to be “advise the patient to drink at least one cup of coffee per day or continue drinking your usual amount.” So it seems the results don’t track with your experience (or my own experience of improved stress and happiness when I cut down on coffee) which is why I think it’s surprising / possibly a fluke.