Maybe you’re a new parent or someone who lies awake at night. If so, you may have started to worry you’re not getting enough sleep.
Sleep wearables don’t help. They can show your “sleep debt”, a running total of how far you’ve fallen behind.
But the word “debt” assumes your sleep works like a bank account. It assumes lost hours stack up, carry over, and you must eventually repay them in full.
But sleep doesn’t really work this way. And chasing “enough sleep” may not be helping.
What is a sleep debt?
Two systems control your sleep. One is your body clock, which helps keep wakefulness and sleep aligned with the day and night. The other is the one that matters here: sleep pressure.
Sleep pressure builds the longer you stay awake and eases while you sleep. At its highest, it’s hard to resist. Someone pulling an all-nighter might find themselves nodding off unintentionally.
This biological process is what “sleep debt” is trying to describe. If you sleep less than your body needs, pressure for sleep builds. Given the opportunity to recover after lost sleep, you sleep longer. In this broad sense, the debt metaphor works.
But this metaphor has some assumptions that don’t fit with our biology.
If you have a financial debt, the maths is exact: you owe a precise sum, which stays there until you pay it down. Sleep pressure does none of those things. Our sleep systems are more dynamic and adaptable.
What happens next?
To study the effects of short sleep, researchers bring volunteers into a lab and restrict how much they can sleep, such as four or six hours a night, sometimes for a week or two. Watching what happens under these conditions tells us how our body handles the shortfall.
The first thing it does is reorganise. When sleep is cut short, the body protects its deepest sleep (the stage that does most of the restorative work) and sacrifices lighter sleep.
People also fall asleep faster and spend less time awake in bed. In other words, given less time, the body spends that time more carefully and efficiently.
When people are freed from sleep restriction conditions, we watch what the body does to recover. “Recovery sleep” is characterised by a few nights of longer, deeper sleep. After this point, the debt appears to be cleared. But you do not sleep “back” the same number of hours you lost.
What this means in everyday life is that after a run of short nights, you tend to sleep a little longer and deeper for a night or two, then your sleep settles back to its usual length.
What about the sluggishness that follows after a few nights of short sleep?
These same sleep experiments also measure sleep-sensitive outcomes such as cognitive performance.
These outcomes follow their own recovery timelines and often take a little longer to return to baseline. You may have had all the recovery sleep you are going to get, but you still need a few more nights of normal sleep before your cognitive performance catches up.
Could knowing my sleep debt make things worse?
Receiving feedback about the previous night’s sleep seems to affect your mood, energy levels and alertness the next day.
One study showed that giving participants negative feedback about their sleep – for example “your sleep quality was poor” – made them feel more tired and negative the next day.
You have insomnia if you regularly:
- find it hard to go to sleep
- wake up several times during the night
- lie awake at night
- wake up early and cannot go back to sleep
- still feel tired after waking up
- find it hard to nap during the day even though you’re tired
- feel tired and irritable during the day
- find it difficult to concentrate during the day because you’re tired
Another small experiment showed that people’s cognitive performance was influenced by how long participants believed they had slept.
No study has directly examined what happens if we tell people how much sleep debt they have. But, based on what we know, it is possible that knowing it can make you more worried about your sleep, and have worse sleep as a result.
False precision and moving targets
There is a deeper problem with the whole idea of calculating a sleep debt.
To calculate a debt, you need to know exactly what you owe in the first place, that is, a precise idea of how much your body needs. Trackers try their best to model how much sleep you need, but it is a slippery number.
Amelia Scott is an Honorary Affiliate and Clinical Psychologist at the Woolcock Institute of Medical Research, and a Macquarie University Research Fellow.
This article is republished from The Conversation under a Creative Commons licence. Read the original article.
How much sleep someone needs varies widely from person to person. Some healthy adults feel fine on around six hours, others need closer to nine.
How much sleep you need is not fixed. You need more sleep when unwell or start training hard at the gym. Sleep shifts with the seasons, with people generally sleeping more in winter.
Sleep trackers also estimate how much sleep you had overnight. They are increasingly accurate, but this is still an estimate, not the truth. So, trackers measure one guess (how much we slept) against another (how much we need).
The bottom line
Sleep debt is a handy metaphor to help us understand sleep regulation. Sleep pressure builds the longer you’re awake, and a short night can leave you needing a longer one to follow.
However, the way our bodies manage short sleep is not an ever-accumulating tally you must repay in full. To calculate a debt, you’d also need to be certain how much you need and how much you have, which are both hard to know.
The good news is that we are built to withstand and recover from the times life gets in the way of a good night’s sleep. There’s no need to carry a ledger or chase a sleep debt to zero.