The fixed wake time

The second part of the method is the number. Pick your weekday wake time. Add an hour. That's the latest you get up at the weekend, both days, and I'll call the hour the window.

Say you're up at half past six Monday to Friday. The window runs to half past seven. On Saturday and Sunday you can wake any time up to half past seven, without an alarm if you can trust yourself and with one set for half past seven if you can't. Inside the window you wake as you wake. Past it you get up. The NHS advice on sleep says the same thing in fewer words, wake up and get out of bed at the same time every day, and don't sleep in after a bad night. I've allowed the hour because a rule with no give in it gets broken the first time you have a late Friday, and a rule that's broken gets abandoned.

Why an hour and not two? Because the drift in the studies came from two hours, and a two-hour window is the habit under another name. An hour is inside what your clock can absorb without a noticeable shift, especially if the light arrives soon after, and it's enough to feel like a weekend. If you're a late type and an hour feels brutal, start with an hour and a half for a month and tighten it. Don't start at two.

Getting up means up. Not awake in bed with the phone. Feet on the floor, curtains open, out of the room. The body clock takes its cue from light and posture and movement, and an hour of lying in a dark room awake is, to the clock, an hour more sleep. If you want the lying there, and chapter 7 says you may, have it with the curtains open and the light on your face, and it counts as up.

The alarm question. Some people can't hold a window without one, and if that's you, set it and don't be embarrassed. Most people find within three or four weekends that they're waking inside the window on their own, because a clock that's been given the same signal for a fortnight starts to predict it. When you've woken before the alarm three weekends running, you can try without. If you slide, put it back. This is a tool, not a test.

And the number applies on holiday, and on the long weekend, and on Boxing Day. Not because you're not allowed a rest, but because the drift doesn't know it's Christmas, and the Monday after a week of lie-ins is the worst Monday of the year. If you ever want to see the full mechanism at work, note how the first week back after a fortnight off feels. That's fourteen days of drift being pulled back in one go, and it's the argument for the window in a single bad week.