If you suddenly stopped assuming caffeine, you'll start suffering from withdrawal symptoms such as headaches and drowsiness; this is because your brain adenosine receptors are accustomed to a certain amount of caffeine (which is itself an adenosine antagonist). Your brain receptors recalibrate themselves after a while if you stop drinking coffee/tea, or if reduce your intake of caffeine to a smaller amount.
The withdrawal symptoms for caffeine are brutal in my experience. It's really surprising every time I dabble with having an espresso a few times a week when I'm in a cafe-frequenting mode. Even just one week of having a double espresso every other day will leave me with 1-2 days of terrible headaches and nausea beginning 1-2 days after completely stopping.
No other drug I've experienced so quickly establishes such miserable withdrawal conditions. And I've been around the block in this neighborhood.