I had always figured that, since I pretty reliably drink about 150mg caffeine/day (5 cups of tea, 1.5 cups of coffee, or some combination, and yes numbers vary a lot depending on who you ask), I was probably building up some sort of tolerance. I'm not sure how this tolerance manifests, but at least a part of it is that you get more adenosine receptors.
(layman's attempted explanation: when adenosine binds to receptors, you get sleepier. caffeine prevents adenosine from binding to receptors by binding to those receptors itself. but if you drink caffeine every day, the body compensates by making more adenosine receptors, so drinking the same amount of caffeine just returns you to your original non-caffeine baseline.)
I read this blog post and was intrigued to hear that it only takes 5 days of complete caffeine abstention to return to adenosine normality*. He didn't cite sources there, but Wikipedia corroborates. ("withdrawal symptoms... usually last from one to five days, representing the time required for the number of adenosine receptors in the brain to revert to "normal" levels") So I tried it.
*Arvind later clarified: tolerance may not be gone, but after 5 days you can function normally, at least. Not entirely sure what he means, or when tolerance does completely go away.
The results: On day 2, I felt kind of can't-concentrate-crummy. Day 3 was a little slow, days 4-5 were pretty normal. This could just be the result of other work issues, and the fact that days 4-5 were a weekend. And now I'm drinking coffee again for the first time, and I feel wonderful as usual. So, no big results, except that now I guess I have normal levels of adenosine receptors again. Not for long!