"The problem is the week, not the weekend"

There's a lot in this. The short week is the engine. If you slept enough Monday to Friday you'd have nothing to catch up on, and the lie-in would lose most of its pull. So why is this book about the weekend?

Because the two feed each other, and the weekend is the easier place to cut in. Here's the loop. The short week leaves you tired by Friday. You sleep late Saturday and Sunday to recover. The late mornings move your clock. On Sunday night you can't sleep at your weekday bedtime because your clock isn't ready. Monday you're short and out of step. Tuesday you're pulling the clock back. By Wednesday you're level and by Friday you're tired again, from the short nights, and also from the two days of correction the weekend cost you. Then you sleep late.

Fix the week and you break the loop at the top. But fixing the week means going to bed earlier on weeknights, which means giving up the late evening that the earlier book in this series, You were tired at ten, is entirely about, and which is hard, because that evening is the only time of your own on a weekday. Fix the weekend and you break the loop in the middle, and the fix is one number and a walk. It's cheaper. And it makes the week easier, because a Monday that starts level is a week that starts level, and a week that doesn't spend Monday and Tuesday pulling the clock back is a week with two fewer bad days in it.

There's also this. Even if you can't fix the week, the lie-in isn't the repair it looks like. The sleep it gives is real but it comes with the drift, and the drift costs some of what the sleep gave, as chapter 3 showed. An early night on Saturday gives the same hour without the cost. So the weekend fix isn't a substitute for a better week. It's the version of catching up that doesn't charge interest.

If you do want to fix the week, do it after this, not instead. Hold the wake time for a month and see what your week feels like when it isn't recovering from the weekend. You may find the week was less short than you thought, and that a fair share of the tiredness was drift and not hours. Or you may find you need another half hour a night, in which case you'll know, and it'll be a clean number rather than a fog.