What鈥檚 the difference between your time spent in bed and your bank balance? No, this isn鈥檛 the start of a terrible joke 鈥� and the answer is less than you might think.
We all have the odd occasion when we stay up too late and don鈥檛 sleep enough. Think of this as the equivalent of splurging on an expensive dinner: you probably shouldn鈥檛 have, but your bank balance hopefully won鈥檛 suffer too much.
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This article is part of special series investigating key questions about sleep. Read more here.
But regularly going without enough sleep 鈥� a problem for many people, with the US Centers for Disease Control reporting that 鈥� could have you racking up a sleep debt, with real consequences for physical and mental health (see 鈥淲hy your chronotype is key to figuring out how much sleep you need鈥�). Like paying back a financial debt, catching up on sleep takes planning.
Part of the problem is that we might not know how much sleep debt we have accrued and how badly it is affecting us. In , for instance, participants were randomly selected to get 4, 6 or 8 hours per night for 14 days straight. By the end, those getting 6 hours or less exhibited a cognitive deficit equal to missing up to two entire nights of sleep. However, despite feeling worse after a couple of days, from then on the restricted sleepers didn鈥檛 necessarily notice their cognitive abilities continuing to decline. 鈥淭he tired brain can鈥檛 detect how tired it is,鈥� says , a neuroscientist at the University of Oxford and author of .
Most sleep scientists will tell you that if you need an alarm clock to wake up, you probably aren鈥檛 getting enough sleep. But there are other questions to ask yourself, says Foster: 鈥淒o I oversleep at weekends? Do I oversleep when I go on holiday? Have my friends or family commented on changed behaviours such as irritability, lack of social connectivity?鈥�
Racking up sleep debt
It doesn鈥檛 take long to start seeing negative effects from shortened sleep 鈥� and it can take time to recover. In one recent study, were allowed a restful night of 12 hours in bed. They were then either deprived of sleep for 36 hours or had it restricted to just 4 hours a night for five consecutive nights. Both groups experienced significant cognitive decline, as well as decreases in energy and reaction times, during their sleep trials. Participants then had four nights of recovery sleep, with 12 hours in bed each time. Although their cognitive function was more or less restored after one night, those in the sleep-restricted group still had impaired reaction times even after four nights of recovery and the sleep-deprived group never recouped its energy levels in that time.
This is bad news when it comes to the strategies that many of us use to pay down sleep debt: sleeping in at the weekend and the odd nap might not be enough to undo the damage.
Though studies have linked sleeping more at the weekend with everything from an increased risk of heart disease to worse period pain, more recent studies have brought better news. One found or cardiovascular disease risk, while another revealed that staying in bed longer like this could by a fifth compared with remaining sleep deprived.
While the jury is still out on the health impacts, it is also unclear whether sleeping in is an effective strategy. When people try to catch up at the weekend, they are often unable to fully pay back the debt. Work by at the University of Utah and his colleagues , they stayed up and slept in very late, some until 2 or 3 in the afternoon. By Monday morning, this could leave people with that is equivalent to waking up on the other side of the US, says Depner.
The truth is, we aren鈥檛 sure what is the best option for paying down that sleep debt, says Depner. 鈥淕etting more sleep is typically going to be better, so I think we鈥檙e hesitant as a field to recommend that you shouldn鈥檛 get more sleep on the weekend. I think the reality is, we just don鈥檛 know exactly the best way to do it.鈥�
You could try banking sleep ahead of time
That said, if you are trying to catch up on missed sleep, a good approach may be to add a bit of extra sleep around the hours you usually go to bed and wake up, rather than in a large dose one morning. If you do try to reduce sleep debt at the weekend, the US National Sleep Foundation says to more than you would during the week.
If you are still in arrears, take some tips from the US military, which suggests its soldiers use to boost performance during operations. Research suggests that in situations where longer, consolidated sleeps aren鈥檛 possible; Foster recommends napping for no more than 20 minutes and not too close to bedtime.
The military also suggests 鈥渟leep banking鈥�, topping up the coffers before going into a period where you aren鈥檛 likely to get adequate rest. Going to bed earlier or waking up later than usual in the weeks beforehand can help foster resilience against the negative effects of sleep debt and aid recovery from sleep deprivation.
Although it may not always be within our control, the ideal would be to avoid building up a sleep debt in the first place. 鈥淚t鈥檚 all about taking sleep seriously. In an already over-packed day 鈥� children, work and all the rest of it 鈥� what鈥檚 the first victim? It鈥檚 always sleep,鈥� says Foster. 鈥淚 think we鈥檝e got to possibly be stronger about saying: 鈥楴ope, I鈥檝e got to get my sleep.'鈥�
