"I haven't been anywhere, so it isn't jet lag"

The phrase social jet lag was coined by Marc Wittmann, Till Roenneberg and colleagues in 2006, and they meant it literally. Jet lag is what happens when your body clock and the clock on the wall disagree, and it doesn't matter whether the disagreement came from an aeroplane or a duvet. They measured it as the difference between the middle of your sleep on work days and the middle of it on free days, and they found that most people in their sample of five hundred had some, and late types had most, because the late types were the ones the weekday alarm hurt most and who caught up hardest at the weekend.

That paper wasn't about tiredness. It was about what went with the gap. The people with more social jet lag reported worse sleep quality and lower wellbeing, and were much more likely to smoke, and the researchers argued that these tracked the misalignment itself rather than simply being a feature of night owls. It was a survey, and a survey can't say which way the arrow points, and I'll keep saying that because it's true of most of this literature. But it named the thing accurately.

Here is the honest way to picture your week. Monday to Friday you live in one time zone. Friday night you fly east by two hours, without moving, and spend the weekend there. Sunday night you fly back. Nobody would expect to feel sharp on Monday after that itinerary if it involved an airport, and the only reason you expect it of yourself is that it didn't. This is the one analogy in this book and I'll drop it now. The point is that the Monday isn't a mystery, and it isn't a character flaw, and it isn't the job. It's a schedule you'd recognise instantly if someone described it as travel.

There's a further honesty owed. The body clock's response to a weekend is not as large as a real two-hour flight, because you didn't shift your light all day, only in the morning and evening, and the studies in chapter 4 found roughly a third to half of the full two hours. That's why the Monday is a bad morning rather than a lost day. But it's also why it's so easy to miss the cause. A half-hour shift is enough to wreck Sunday night and Monday morning and small enough to blame on something else.