There is a huge blind spot in scientific caffeine research around habitual use. Every article you'll have read saying "don't drink coffee after 9am or you'll never sleep again in your life pinky promise" is based on the same type of study: Take people who drink x amount of coffee, give them a large caffeine dose at a non-habitual time, and report that they don't sleep as well that night. Which is obviously not how caffeine is used in real life.

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?