"Getting up at the same time every day sounds grim"

It sounds like a rule for a barracks. It's worth looking at what it actually gives, because regularity turns out to be doing more of the work than length.

Andrew Phillips and colleagues followed sixty-one undergraduates for a month with sleep diaries and built a measure of how regular their sleep was, day to day. The irregular sleepers weren't sleeping less. They were sleeping at different times, and their clocks had drifted nearly three hours later than the regular sleepers', their melatonin rising after midnight against half past nine, and their grades were lower. When the researchers modelled why, the main driver was light. Irregular sleepers got less contrast between bright day and dark night, and the clock, with less to hold on to, slid. Jessica Lunsford-Avery and colleagues used the same measure in about two thousand older adults and found irregular sleep went with later timing, more daytime sleepiness, and worse numbers for weight, blood pressure and blood sugar, independently of how long people slept.

Again, snapshots. But take the shape. What the body clock wants is not a particular time. It's the same time. It's a mechanism for predicting when the day starts so that everything else, temperature, hormones, digestion, alertness, can be ready. Give it a consistent signal and it settles. Give it a signal that moves two hours every five days and it spends the week catching up.

Now the grimness. A fixed wake time is not a fixed life. It's one number. Everything after it is yours. You can get up at seven on a Saturday and do nothing until noon. You can eat breakfast in bed, or not eat breakfast. Nobody's asking you to be productive. The rule is about when your eyes open and the curtains do, and it's the least demanding rule in this series, because it asks for one act a day and then leaves you alone.

And there's a gift inside it that people don't expect. When the wake time is steady, the wake becomes natural. Within a few weeks most people find they're waking a few minutes before the alarm, at the weekend as well, and the alarm becomes a backstop rather than an assault. The thing you liked about the lie-in, waking on your own, is what a steady wake time gives you every day. What felt like the freedom of the weekend was really the freedom of waking without a noise, and that turns out to be available seven days a week, at the price of a schedule.