Conventional wisdom says the misery of a sleepless night is nothing a catchup won’t fix. But a new scientific review argues that “sleep debt” cannot always be paid back, with some of the damage from sustained sleep deprivation likely to be irreversible. Are lie-ins meaningless? On a sunny Monday, I asked Prof Derk-Jan Dijk, director of the University of Surrey’s sleep research centre.

Did you get a good night’s sleep?
It wasn’t too bad, but studies show Sunday or Monday often aren’t good.

Is that just from worrying about work the next day?
Yes, but also because most people sleep longer at the weekend. So if you wake on Monday at 6am with only a few hours of missed sleep – only a little sleep debt – because you napped at the weekend, your brain will let you stay up worrying on Monday night. By midweek you’ll have missed even more sleep so your brain won’t let you.

So some sleep debt is useful?
Ultimately, there’s a payoff between how long you sleep and how deep you sleep. Not spending enough time in bed is one thing, but another way to induce poor quality sleep is spending too much time in bed. It’s about finding the right balance for you.

I honestly think if I found the balance I wouldn’t be able to recognise it anyway. Can’t you just tell me what to do? Please?
Don’t ask me for an easy answer. There are none.

Ain’t that the truth! OK, back to this review – has it put paid to the idea we can repair our sleep-deprived selves?
That study is about the neuro consequences of chronic sleep disruption. That’s different from: “Should I catch up on sleep at the weekend?” One study followed more than 43,000 subjects for 13 years to look at sleep duration and mortality. Sleeping less than five hours is a predictor of mortality. But people who always miss out on sleep are different from those who catch up on weekends. To some extent, weekend sleep can compensate. It’s common sense.

Well that’s great!
Now, in terms of chronic sleep disruption, we do see serious negative consequences, such as a risk of cognitive decline. But that’s not surprising, is it?

I’d still like to know more about the limitations of a lie-in: if you have a newborn and aren’t sleeping, can that damage be repaired?
There are mechanisms that allow us to sleep less under certain circumstances. It’s in other species, too, such as migrating birds. I’ve not seen any evidence that being a tired mother leads to long-lasting detriment. Studies on night workers show cognitive decline, but some recovery is possible once they move away from working those shifts.

How long do they have to move away for? What’s the threshold of no return?
There are data insufficiencies there. But we should not take sleep too seriously. One night of bad sleep is not going to significantly increase your risk of a neurodegenerative disorder. If you feel fine in the day, you shouldn’t worry about only sleeping six and a half hours.

I confess I check my Fitbit every morning to see how well I’ve slept.
The problem is, we don’t trust ourselves any more. If you wake up and feel well-rested, and are alert all day, you have more reliable information coming from your brain than that Fitbit. Different people need different amounts of sleep. What’s important for you to ask is simply: “How do I feel?”