My gut feeling is telling me that coffee and digital devices are responsible for big parts of the population suffering from mental illnesses, caused via chronic sleep deprivation. Of course majority of those mental illnesses are under reported - being sleep deprived for months does not make you bipolar maybe, but depression can and does happen (as well as myriad of other issues).
There are too many studies who show relationship between sleep and all kinds of illnesses (majority - mental), and nobody can argue that coffee is NOT responsible for that at least partially.
When I recently quit on coffee, I had such strong withdrawal symptoms, I was unable to function for 4 days. I had strong headaches and was almost vomiting.
No study will convince me this is a good/healthy thing for a human body. It is drug which is so omni-present, if you look around it is scary.