"I'm a night owl, it's just how I'm made"

Chronotype is real. People differ, partly by genes, in when their clock wants to sleep, and the late types aren't being difficult. If your body wants to sleep from one until nine and your job wants you up at half past six, you are going to be short all week and you are going to catch up at the weekend, and that's exactly what Wittmann and colleagues found. Late types had the most social jet lag, because they had the most to catch up.

But look at what follows. If you're a late type, the weekend drift moves you later still, because your clock is already inclined that way and you've just told it to go further. Monday then hurts you more than it hurts an early type, not less. The lie-in feels most natural to precisely the people it costs most. That's not a moral point. It's arithmetic. The further your clock sits from the weekday alarm, the more the weekend widens the gap and the harder Monday pulls it back.

And chronotype is less fixed than it feels. Kenneth Wright and colleagues took people camping for a week in summer with no electric light, and everyone's clock moved earlier, by about two hours on average, so that their internal night began near sunset and ended before they woke. The people who moved most were the late types. Ellen Stothard and colleagues then did it for a single weekend and got about two thirds of the shift. The finding I take from this isn't that you should camp. It's that a good part of what feels like being made a night owl is the modern light pattern, dim days indoors and bright evenings, and that it moves quickly when the light changes. Your clock wasn't built to want one in the morning. It's been taught.

So if you're a genuine late type, everything in this book applies with more force and a little more gentleness. The fixed wake time will feel harder for the first couple of weekends. The daylight walk in chapter 16 will do more for you than for anyone. And you'll want to be honest about the late night, the Friday and Saturday bedtime, because for a late type the bedtime drifts as readily as the morning, and the morning can't be held if the night keeps sliding.

What this book doesn't say is that you should become a morning person. You should become a person whose clock is in the same place on Monday as it was on Friday. Where that place is, within reason, is your business.