Catching up on sleep: how much comes back, and how.

Catching up on sleep at the weekend is most people's plan. Five or six hours a night in the week, then a lie-in till ten on Saturday.

It half works. On Sunday night you lie awake past midnight, and Monday starts in the red. "I'll catch up at the weekend" is something I hear a lot from the people I work with.

How to tell you're in sleep debt

• With no alarm you sleep far longer. Your body is clawing back what it's owed.
• Your head lets you down by day. Sleepiness, poor concentration, memory lapses.
• You reach for sugar and coffee. After a short night, hunger and sweet cravings tend to rise.

Can you catch up on sleep at the weekend?

Partly, yes. Two long nights leave you feeling better. They won't pay off the whole debt, though, and if you get them by sleeping late, you pay on Sunday night.

What an early alarm costs you. Nearly all your deep sleep comes in the first hours of the night. Cut the end and you lose mainly dreaming sleep, linked to memory and mood. After a short week, patience is often the first thing to go.

What doesn't come back. Two long nights don't usually wipe out what the short ones did to your metabolism. Comparing people on a diet, those who slept little in the week and caught up at the weekend lost the same weight as regular sleepers, but more muscle and less fat.

Why you can't sleep on Sunday night. Your need for sleep builds with the hours you've been up. Get up at eleven and by midnight you haven't built much. Your body also lengthens its day far more easily than it shortens it. A lie-in is a flight west, and Monday's alarm the flight back east.

How to catch up, and how to nap

On Friday and Saturday, bring the evening forward. Go to bed earlier rather than getting up later. You gain hours without shifting your clock, and Sunday night's sleep comes more easily. The alarm stays where it is all week.

An afternoon nap: twenty minutes. Six to eight hours after you wake there's a natural dip. If you're up at six, that's straight after lunch. Lie down then, alarm set for twenty minutes. You usually stay in light sleep and get up clear-headed. Slip into deep sleep and you get up groggier. Even if you don't drop off, those minutes still help.

Coffee first, and never late. Drink your coffee straight after lunch, then close your eyes. It kicks in while you rest, and often helps you get going. Late in the afternoon, a nap can take away the tiredness you need at bedtime. If you already struggle to fall asleep, skip it.

The best catch-up is still not building the debt. A little more sleep every night, at regular hours, beats one long Saturday lie-in.