After going off caffeine for a day or two, drinking a strong cup of coffee or tea on an empty stomach will give me a fair bit of a buzz, to the extent that it feels like my head is floating. Two cups and work can become difficult and I can't sit still. If I drink three cups by mid-afternoon, I begin to feel anxious and slightly paranoid, as if I'm forgetting something important. It also impairs my sleep.

If you're a habitual user, you may not even realize how it effects you, and you'll need more dosage to get effects.

I'm a very long way from the folks on r/decaf, most of whom I think need some serious help, but it's much more impactful than you may realize.