Shift work, babies, teenagers, the big Friday night, and when it's more than the weekend

Some situations the rule doesn't fit as written, and some where it doesn't apply.

Shift work. If your week has nights in it, or rotates, you don't have a weekday wake time to anchor to, and social jet lag is your whole life rather than your weekend. The studies in this book mostly excluded shift workers for that reason. The principles still hold, light when you want to be awake and darkness when you want to sleep, and consistency wherever you can get it, but the schedule is a different problem and this book doesn't solve it. If your shifts are wrecking your sleep, that's a GP conversation.

Babies and small children. If you're up at two and four and six, the weekend lie-in, when a partner takes the early shift, is not a habit to fix. It's survival, and the extra sleep is worth more than the drift. Take it. When the nights are reliably unbroken again, come back to this book. What I'd say in the meantime is that the daylight walk still helps, even after a broken night, because the clock is separate from the debt.

Teenagers. Teenagers' clocks run genuinely later, and the school day starts genuinely early, and a teenager sleeping until noon on Sunday is doing what their body asks. Crowley and Carskadon's work in chapter 4 was with teenagers and found the drift, so the mechanism applies, but I'd be gentle about it. Morning light at the weekend and a later school start do more than nagging. This book is for the adult in the house, and the best thing that adult can do for the teenager is model a steady wake time rather than impose one.

The big Friday night. You're out until two. Fine. The choice on Saturday morning isn't between the window and no window. It's between getting up at half past seven exhausted and getting up at ten with a shifted clock. My advice is a middle path, up by nine, outside straight away, a short nap in the early afternoon, an early night on Saturday, and the window held properly on Sunday. One late night doesn't make a drift. One late night followed by two lie-ins does. If the big night is every Friday, that's a different book, and it's the one about asking the price of the evening.

When it's more than the weekend. If you can't get to sleep or stay asleep most nights, not just Sunday, and it's been going on for months and it's making the day hard to cope with, the NHS says see a GP, because that's insomnia, and there's a specific talking treatment for it that works better than tablets, and a fixed wake time is one of its tools but not all of them. If you're sleeping late at weekends because you can't face the day, and you've felt low most of the day most days for two weeks or more, the NHS says see a GP about your mood. And if you fall asleep in the day without meaning to, at the wheel or at a desk, that's a GP visit this week, whatever the weekend looks like.