Here's how to catch up on sleep, the right way

Just two hours of extra sleep can help reverse the damage from a bad night. But early nights and lie-ins should be wielded with caution.

We've all been there: waking up, bleary-eyed after a bad night, feeling exhausted. "It's fine," we tell ourselves. "I'll catch up at the weekend."

For some of us, this experience is a one-off. For others, it's a way of life. One survey of nearly 8,000 US adults found that nearly half regularly slept in on the weekends. Perhaps unsurprisingly, teenagers seem to have the biggest weekend lie-ins of any age group.

A bit of extra sleep can be especially tempting for those of us who don't get enough during the week. Adults are generally advised to aim for around seven to nine hours, while it's thought that most adolescents need between eight and ten. But that's not always possible.

Which leads to the million-dollar question: if you aren't sleeping enough during the week, should you sleep in on the weekend? And if you do, can this blunt the effects of short sleep?

To understand why sleeping in might have downsides, it helps to look at the two main processes regulating sleep.

The first, the sleep homeostat, is like a "sleep tank". The longer we're awake, the more it fills with sleep pressure – and the more we feel like we need to lie down and snooze. The second, our circadian rhythm, gets adjusted every day by our exposure to light and dark, temperature changes, even when we eat or move. These work together to make us feel sleepy, ideally at an appropriate time each night.

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Catching up on sleep can knock both our sleep homeostat and our body clock off. This is true whether you go to bed earlier, or sleep in. Because extra morning sleep reduces our sleep pressure later in the day, we might not feel tired until later that evening. And because we're not just sleeping at a different time, but doing everything else at a different time – including getting light exposure – it can affect our circadian rhythm too.

The result? We might sleep in Sunday morning and fall asleep later on Sunday night, so that when our alarm goes off at 06:00 on Monday, we haven't slept enough – starting the cycle all over again. "It's like shifting time zones every single week," says Melynda Casement, sleep psychologist and director of the University of Oregon Sleep Lab.

However, though catch-up sleep should be used cautiously, it's not always a bad idea. Here's when it might be helpful to get some extra rest.

The benefits of recovery sleep

How short sleep impacts our health and cognition is still being untangled. But in carefully controlled sleep restriction experiments, it's become clear that an especially rough night impacts our physiology. And at least some of these consequences can be blunted, if not reversed, with catch-up sleep.

In one study, for example, people who got only two hours' sleep for a single night showed higher counts........

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