I drink about two cups a day (400 ml in total), and I definitely get a headache if I drop my caffeine intake too suddenly. If I was adapted to drinking much less, then I might be able to go an entire day without noticing anything, but at the current level, it’s just not going to happen. Did James mention how much coffee do the participants normally drink every day? If they are all in the 1 cup club, these results are only exploring one extreme of the scale.