I look at caffeine a s a drug. Read about it as such in 1977 when I was starting my most serious year of racing and put it and white surgar/sucrose on the forbidden list. (I took pride in riding clean and feeling clean.) At that time, I"d never had good coffee and my love for it aw years off.
About 6 weeks into the regime I tried some white sugar on a ride. Wow!. Yes, in my clean body, a true drug! Rush, perfornance increase for a few hours, then a crash to below where I started. Eye opener.
Now, that summer I did more than a few very long day rides. Two each of 160 and 175 miles. That first 175 was 25 more than planned (I got lost) and included a 2 mile steep climb just because "Oh, I am at the base of Pack Monadmock which I have seen many times fror its big sister but never climbed". With 50 miles to go, I was running low on gas. Stopped at a fast food and had a cup of black coffee with 2 packs of sugar. Rocket fuel. Got me all the way home.
Now I drink a powerful cup of Italian coffee every morning. (Italian brew, American cup.) A cup of drip coffee still has real effect on me (drip coffee - what an efficient way to extract caffeine from the bean and inject it into our system!) but nowhere near what that cup did years ago.
Ben