"I can't function without my morning coffee"

Start with the morning, because that's where the habit feels most like a fact. At half past six you're foggy, slow and slightly low. At ten past seven, after the cup, you're a person. The cup did that. Nothing else in the day is as reliable.

I'd like to describe an experiment that I think changes what the morning cup is, and I'd ask you to read it slowly, because the finding is easy to slide past.

Peter Rogers and colleagues at the University of Bristol took three hundred and seventy-nine people. Some were regular caffeine drinkers, at medium to high levels. Some drank little or none. Everyone went without caffeine for sixteen hours, which is roughly an ordinary night plus a morning. Then, under double-blind conditions, they were given either caffeine, in two doses ninety minutes apart, or a placebo, and they rated their alertness, anxiety and headache before and after.

Three things came out. First, in the regular drinkers, placebo went with lower alertness and more headache. In other words, the regular drinkers who didn't get caffeine felt the shortfall. Second, in the low and non-drinkers, caffeine didn't increase alertness at all. Third, and this is the finding, when the regular drinkers got their caffeine, their alertness came back up to about the level of the non-drinkers who'd had nothing. The researchers' own summary is that with frequent use, no net benefit for alertness is gained. Going without caffeine reduces alertness, and having it merely returns it to baseline.

Look at what that means for half past six. The fog you feel isn't your natural morning state. It's the shortfall. It's what a regular caffeine drinker feels when the last dose has worn off overnight. The person next to you on the train who's never drunk coffee doesn't feel it. They woke up at the level. When you have your cup at seven, you climb up to join them. You don't pass them. You've spent twenty minutes and a coffee getting to where they started.

"I can't function without my morning coffee" is therefore true, and it's true because of the coffee. The cup relieves a state that the habit produces. That's not a criticism of you. It's what a daily dose of anything that the body adapts to will do. But it changes the arithmetic. You've been counting the morning cup as a gain of some amount. It's the repayment of a loss of the same amount, and the loss was incurred yesterday.

Jack James and Peter Rogers set this out in 2005 in a paper whose title makes the claim, that withdrawal reversal is the most plausible explanation for the effects of caffeine on performance and mood in regular users. I couldn't obtain a summary of that paper and I'm relying on its title, but the Bristol experiment is the evidence behind it, and it's enough.

There's an odd consequence that I'd ask you to notice. If the morning coffee only returns you to the level, then a person who doesn't drink coffee gets the morning coffee's effect every day, free, on waking. What they don't get is the shortfall, and the shortfall is the thing you've been calling being tired in the morning.