"I only have three or four"

Ask a regular drinker how much caffeine they have and they'll count coffees and stop there. The number is usually two or three. The truth is usually higher, and it's worth getting it right, because the plan in chapter 17 depends on it.

The NHS gives rough figures. About a hundred milligrams in a mug of instant coffee, and it notes that filter coffee and coffee from a coffee shop can contain more. About seventy-five in a mug of tea, black or green. About forty in a can of cola. About eighty in a small can of energy drink. Less than twenty-five in a fifty-gram bar of dark chocolate, and less than ten in milk.

Now go back to Tuesday. The seven o'clock instant, the half past nine instant, the tea at eleven, the cola with lunch, the coffee at three, the coffee at half past five. That's four hundred and fifteen milligrams by the NHS figures, before any chocolate, and before allowing for a coffee-shop cup, which can be double a mug. If the half past nine was a large one from the place by the station, the day is comfortably over five hundred. You'd have said three.

Two things get missed. Tea, because it's tea, and people think of it as the gentle option. It's three-quarters of a coffee by these figures, and a person who drinks five mugs of tea a day is at the level of nearly four coffees. And size, because a mug is not a cup, and a coffee-shop large is not a mug. Counting cups when the vessels vary is like counting glasses of wine without noticing that some are pints.

There's also the matter of what you'd call the cut-off. Chapter 7 gave six hours before bed as the line the evidence supports. On Tuesday the last caffeine was at half past five, for a bedtime of eleven. That's inside the line. The three o'clock cup is on it. Most regular drinkers, in my observation, have their last caffeine later than they think, because the count stops at coffee and the afternoon tea doesn't register.

I'm not going to give you a target number. The point of this chapter is that you can't decide anything about the habit until you know its size, and the week of notes from chapter 1 will tell you. Add it up, using the NHS figures, and be honest about the sizes. The total will be higher than you thought. That isn't a verdict. It's the starting point.