"It's two mornings a week, what harm can it do"

The Monday is the cost you feel. There's a set of costs you don't, and they've been studied, and the honest version of the findings is worth having, including the part where I tell you what they can't show.

Till Roenneberg and colleagues, in a very large survey, found that social jet lag went with a higher body mass index, and that this held after allowing for how long people slept. Parsons and colleagues looked at the Dunedin cohort, about eight hundred adults who weren't shift workers, and found social jet lag associated with clinically measured markers of metabolic trouble, and highest among people who were both obese and metabolically unwell. Patricia Wong and colleagues studied about four hundred and fifty midlife day workers and found that a larger gap between weekday and weekend sleep went with lower good cholesterol, higher triglycerides, higher fasting insulin and more body fat, after adjusting for sleep quality, sleep length, mood and health behaviour. Femke Rutters and colleagues, in a smaller study of healthy adults, found that people with two hours or more of social jet lag had higher cortisol, a higher resting heart rate and were less active than people with an hour or less. And Rosa Levandovski and colleagues, in a rural Brazilian sample of four thousand, found depression scores higher in people with more than two hours of it.

Now the limits. Every one of those is a snapshot. They show that people with a big weekend shift tend also to have worse numbers. They don't show that the shift caused the numbers. It could run partly the other way, or both could come from something else, a late chronotype, a stressful job, drink. The authors say so themselves. The Depner experiment in chapter 3 is the one place where the weekend pattern was imposed on people and the metabolic effect followed, and it was small and short.

So here's what I'd say, and it's an interpretation. The two hours a weekend isn't obviously harmless. The pattern that goes with it is consistently a bad one across several countries and several kinds of measurement, and the one experiment that tested it points the same way. That's not proof, and I won't tell you the lie-in is giving you diabetes. What I'll say is that the thing you're doing to feel better on Sunday keeps company with things you'd rather not have, and the one cost you can see, the Monday, is probably not the only one.