I have been drinking a lot of coffe my entire adult life. Many cups of really dark coffee. But a cup is small . And dark coffee doesn't necessarily mean loaded with caffeine. Nothing prepares you for the kind of watery-yet-caffeine-loaded juice you get at a US coffee shop. It's like they think someone would enjoy drinking a half liter hot beverage, and get half a lethal dose of caffeine. Even though I drink 5-10 cups…
It is customary for me to enjoy an espresso (occasionally two) with a bar of dark chocolate after dinner. I sleep well not much later. 100s of thousands, if not millions, do this as a matter of culture.
Few things have disappointed me as much as caffeine research. If we can't get something so direct properly studied, what hope does 90% of science have?