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

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.

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 of two types of white blood cells, potentially indicating a physiological stress response. Some participants were then granted an eight-hour sleep the following night. But even after this, their white blood cell counts hadn't fully bounced back. However, participants who were allowed to sleep for up to 10 hours on the second day had levels which had nearly returned back to the baseline.

Another experiment studied what happened when 11 men who normally slept for eight hours each night were limited to four. Six nights of short sleep were enough to impair their glucose clearance and insulin responses. If sustained long-term, these physiological changes can raise the risk of type two diabetes. But after six more nights of a 12-hour sleep opportunity, their measures had returned to baseline levels.

Other experiments have found that catch-up sleep following an all-nighter can restore our cognitive performance.

None of that means that a weekend lie-in (or early night) counteracts every effect of sleep debt, especially when short sleep persists over weeks or months. Vigilant attention (our ability to stay focused and respond quickly) and spatial orientation seem to be particularly vulnerable. In these cases, even weekend catch-up sleep can't fully reverse the effects of short weekday sleep.

Then there's also naps. These can also be beneficial for people running short. But they're usually studied separately from overnight catch-up sleep. (Read more from the BBC about why power naps might be good for our health.)

Catching up on sleep, saving lives

But these studies were conducted in artificial laboratory conditions. What happens when people catch up on sleep in the real world?