Over here in Asia there's no distinction between caffeinated coffee and decaf. Even though coffee is something that naturally grows over here. Despite that, people just but coffee powder off the shelf and use that. There's no specific marking on the packet about the caffeinated/decaf bit.
Now, I attended college in America where decaf and caffeinated coffee is clearly marked everywhere.
I didn't notice anything particularly different at first.
Eventually, I had this boring mandatory lecture in the afternoon, and to prevent myself from falling asleep, I always took a flask full of caffeinated coffee (about 3 cups) and drank it during the lecture.
A few months later, the class got more interesting, and I decided to stop drinking coffee.
I started having severe headaches every evening, and I didn't even realise the cause. One fine day I randomly drank coffee in the evening, and the headache was gone within 30 minutes! I was addicted to coffee!
I tried slowly reducing the quantity of coffee I drank every evening, but it just wouldn't work. Every evening I would have the headache unless I drank a flask full of coffee.
How did I get rid of the addiction? A few months later I travelled back to Asia. The 28-hour connecting flights combination (one of which was a 16-hour nonstop run), combined with the fact that my Asia timezone was 13 hours ahead US timezone ensured that my body clock got reset and I no longer had the headaches in the evenings. I might have had headaches at night, which I probably slept through.