What the lie-in costs you

Add it up, since the whole argument is that the price is hidden.

There's the Sunday night. An hour, sometimes two, of lying awake, at the worst possible moment, the night before the hardest morning, because your clock isn't ready. There's the Monday itself, which by the ratings in chapter 6 is likely to be a day at six or seven out of ten that could be an eight. There's Tuesday, which is usually most of the way back but not all. Call it a day and a half a week of running below your level, which is thirty per cent of your working week, every week, for years.

There's the Saturday morning. Two hours of it, unconscious or half so. If you're the person from chapter 7 who values the lying there, that's a real use of the time. If you're honest and it's mostly sleep you don't remember, then it's two hours of the freest morning of your week spent not having it.

There's what the drift does to Saturday and Sunday night. The late night and the lie-in push your clock later, so by Saturday evening you're not tired until later, so you stay up later, so Sunday's lie-in is longer, and the whole weekend runs on a timetable that has nothing to do with the one you'll wake to on Monday. Some people like a weekend that runs on a different clock. It's worth knowing that's what you've chosen and that it isn't free.

There are the numbers from chapter 10, which I'll repeat only to say again what they are and aren't. They're associations, not proof. But the pattern in your body of a shifting clock keeps company with worse blood sugar, worse cholesterol, higher heart rate, higher weight and lower mood, in several countries, and the one experiment that imposed the pattern found the metabolic cost. I don't know what that means for you. Neither does anyone. It's a cost without a price tag, which is not the same as no cost.

And there's the thing you can't measure, which is the identity that grows around the Monday. The person who's no good until Wednesday. The mug, the joke, the third coffee. It's a small identity but it's a wrong one, because it makes a schedule into a character, and you'll carry it into every Monday for the rest of your working life unless you notice that it was Saturday's doing all along.