I've always wondered if coffee studies are just something they give bored grad students to do. I've heard about what seems like hundreds of them in my lifetime and they contradict each other in cycles over long stretches of time.

The behavior of drinking coffee or not is also very socially mediated-- portrayal of coffee drinkers in movies, coffee as a common activity for preliminary dates, or to meet someone new at a cafe...

All of this means that coffee drinkers are going to self select for various other features that add confounding factors. Imagine you were doing a study on marriage patterns: I wouldn't be surprised if "Drinks their coffee in the cafe itself" was more highly associated with being single. And the headline will read "Dedicated coffee drinkers less likely to find a partner"

Same! One week it’s terrible for you. The next week it’s curing cancer. I just try not to have any past a certain hour so as to not interrupt my sleep.

There’s something in (1), but it doesn’t address the entire spectrum of Harmala alkaloid neurotoxicity (2) (3). It has been shown that heterocyclic aromatic amines (which Harmala alkaloids belong to) indeed damage dopaminergic neurons (4).

(1): https://hammer.purdue.edu/articles/thesis/Identification_of_... (2): https://onlinelibrary.wiley.com/doi/pdf/10.1002/ijc.20703 (3): https://www.sciencedirect.com/science/article/pii/S157002320... (4): https://academic.oup.com/toxsci/article-abstract/173/1/171/5...