"I'll catch up at the weekend"

This is the accounting that keeps the week going. You run short from Monday to Friday and pay it back with a long lie-in on Saturday and Sunday. It feels as though the books balance.

Part of it is true. After a short week you do sleep longer at the weekend if nothing wakes you, and you do feel better for it. The body takes back some of what it's owed.

A study at the University of Colorado looked at how well that works. Healthy young adults were split into three groups and lived under controlled conditions. One group had plenty of time to sleep. One was limited to five hours a night for nine nights. The third had five short nights, then a weekend of sleeping as much as they liked, then two more short nights. It was a small study, with 14 people in each of the two short-sleep groups, and its main interest was metabolism and not mood.

Two results bear on this chapter. The first is how little extra sleep the weekend group got. Over the whole weekend they averaged only about an hour more than their normal amount. They'd lost many hours across the week and got a fraction of them back. The second is that their body clocks shifted later over the weekend. So when the short nights started again, they were trying to sleep at a time their bodies now treated as evening. The weekend group ate more after dinner and gained weight much as the no-weekend group did, and their sensitivity to insulin fell. The authors concluded that weekend catch-up sleep didn't prevent the metabolic effects of repeated short sleep.

I don't want to make too much of one small study of young adults. The lesson I'd take is a modest one. The weekend pays back less than it seems to, and the lie-in makes Sunday night harder. Most people know that last part from experience. You sleep until ten on Sunday, you aren't tired at eleven that night, and Monday starts short.

There's a second argument that I find more persuasive than the biology, and it's about what the lie-in costs you. Think about Saturday morning as it would be if you were rested. You'd be awake at half past seven with a clear head, and if you have children they might still be occupied. The day's demands haven't begun. That's two hours of your own time in the best condition it ever comes in, when you could read or go for a walk or make something and enjoy it fully.

The weekly deal, then, looks like this. From Monday to Thursday you take five or six poor hours late at night, when you can't do much with them. On Saturday and Sunday you pay them back with four or five of your best hours, asleep. You're borrowing bad hours and repaying in good ones.

Here's the reasonable objection. "But the lie-in is one of the pleasures of my week." If you're rested and you stay in bed with tea and the paper because you like it, that's your own time, well spent. That isn't what I'm describing. I mean the lie-in you can't help, where you surface at half past ten feeling as if you've been hit.

The practical point for now is one of bookkeeping. When you weigh up the late stretch, include the weekend mornings in its price.