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What is Sleep Debt

Are You Operating on Sleep Debt? Plus, How to Balance it Out

Check in on your bedtime bank account.

By Colleen TraversMay 1, 2023

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Skimping out on one night’s shut-eye may not have an immediate impact (that you can feel, anyway) but chances are between work, family and social commitments, sticking to your training plan, and the daily grind of life you’re likely shaving off an hour or two of sleep you should be getting a few times a week. When this happens, you start to rack up what’s known as a sleep debt and with it, a host of potential issues that can affect your wellbeing. Here’s what you need to know about sleep debt, including how (and if) you can catch up on the sleep you’ve lost. 

What is Sleep Debt?

“When we talk about sleep debt, it’s the difference between the amount of sleep you need and the amount you’re actually getting,” says Shelby Harris, PsyD, a licensed psychologist who is board-certified in behavioral sleep medicine and director of sleep health for Sleepopolis. “The majority of adults need about seven to nine hours of sleep every night to avoid accumulating a sleep debt.” 

“So, if you routinely get seven hours of sleep a night, but for two days you only get six hours of sleep each night, you will have accumulated two hours of sleep debt in the course of two days,” says Harris. You can think of sleep debt as compounded sleep deprivation.

Just like real debt, you may be able to catch up on some sleep debt, but this can be difficult over time if you’re also still accruing sleep debt. There’s also no set amount of rest you need to stockpile into your personal sleep savings account. “The exact amount of sleep that each person needs varies quite considerably,” says Kimberly Fenn, PhD, professor of psychology and principal investigator at the Sleep and Learning Lab at Michigan State University. “Every day that you don't obtain whatever it is your body needs, whether it's seven hours or nine hours, you are accumulating sleep debt. That compounds with time, leading to massive changes in your physiology and cognition.”

So, Is Sleep Debt Real?

“Sleep debt is 100 percent real,” says Dr. Fenn. “For many people sleep is almost an afterthought, but it’s just as important as eating. Like food, you need sleep to properly fuel your body.” If you don’t get enough (or the right kind) of calories, your physical health can start to suffer. The same can be true if you don’t log enough sleep each night.

That’s why it is necessary to track your sleep, says Dr. Harris. “Monitor how much sleep (quantity and quality) you get each night so you can spot sleep debt early, and get help from a specialist if needed,” she says. This can be as simple as keeping a sleep journal or tracking it through an app or a device like an Apple Watch or Oura Ring.

How Sleep Debt Adds Up

Another thing to consider is that sleep is somewhat of a complicated equation. There’s a set number of hours you need per night, but within those hours are sleep cycles. “An average sleep cycle is 90 minutes,” says Dr. Fenn. “But what goes into that sleep cycle shifts throughout the night. That’s because the more you're awake, the more you build up the need for sleep, particularly slow wave or delta sleep.”

Dr. Fenn breaks down how an average sleep cycle works: If you slept from 12 a.m. to 8 a.m., you’re likely pretty tired by the time you get into bed, so you’ll fall asleep fairly quickly and into that slow wave sleep. “You’ll spend most of those 90 minutes in slow-wave sleep because that's the sleep that your body needs the most,” she says. 

As you continue sleeping through the night, the amount of slow-wave sleep you get in each cycle decreases, and the amount of REM sleep (where dreaming occurs) increases. “That means the last sleep cycle before you wake up is going to be highly dominated by REM sleep,” says Dr. Fenn. You need both slow wave and REM sleep cycles in order to protect cognitive health and emotional wellbeing, but it's possible that your sleep debt is occurring mainly in one or the other. “Depending on what kind of sleep debt you have (whether you’re going to bed too late or waking up too early), you’re selectively depriving yourself of certain types of sleep you need each night,” says Dr. Fenn.

The Dangers of Sleep Debt

Running on empty isn’t good for your car or body. “Our bodies are very sensitive to sleep debt,” says Dr. Harris. “In the short-term, people may experience increased stress, depression, anxiety, and slower cognitive processing such as trouble with memory and attention.” 

If you choose not to address your sleep issues at all over time these short-term issues expand into larger health concerns, including increased risk of cardiovascular disease, diabetes, stroke, and heart attack. Research published in Nature and Science of Sleep has also found that disrupted sleep over time can worsen symptoms of certain gastrointestinal orders as well as increase the risk of certain cancers, such as colorectal cancer.

Insufficient sleep also changes the way your body processes food. “When you're in a state of sleep deprivation or sleep restriction, you're hungrier than you normally would be, and you're less full,” says Dr. Fenn. “This is because ghrelin, the hormone that signals to your body that you’re hungry increases while leptin, the hormone that tells your body it is full decreases. While these hormones are unbalanced with lack of sleep, there’s also an impaired insulin response to the food you’re eating.” So if you’ve ever had a bad night’s sleep and feel like you’re constantly grazing on snacks the next day, it’s no coincidence.

How to Recover from Sleep Debt

While you may not be able to catch up on all of the sleep you’ve missed, you can recoup some of it. “Acute sleep deprivation can be made up by sleeping in an extra hour on the weekend or going to bed an hour earlier than your usual bedtime,” says Dr. Harris. “This can ‘pay back’ a small amount of sleep debt caused by a few nights of bad sleep, but it is not a long-term solution for a large amount of sleep debt,” she warns.

Naps may also be a crutch for those in short-term sleep debt. Research from Dr. Fenn’s Sleep and Learning Lab looked at the link between naps and sleep deprivation. While Fenn’s specific research investigated if a power nap of 30 or 60 minutes would alleviate the cognitive deficits of a full night’s sleep deprivation (largely, it did not), when it comes to sleep debt, naps may be more helpful. “When you’re dealing with growing sleep debt, any sleep is good,” says Dr. Fenn. “It’s hard to say what the exact benefits are, but you’ll feel better, particularly in that mid-afternoon time when you have a circadian dip.” 

Plus, it’s worth noting that the subjects in Dr. Fenn’s study who took power naps had a 4 percent decrease in errors when doing cognitive tasks, likely due to the fact that the naps provided them with that slow wave sleep the body and mind crave. This helps support the theory that when it comes to sleep debt, any sleep you can squirrel away can make a small difference. 

Long-term, it can be really hard to completely catch up on sleep debt. Research published in Current Biology found that attempting to catch up on sleep over the weekend was ineffective at reversing sleep loss-induced metabolism disruptions. In fact, the researchers found this method of sleeping more on the weekends and going back to sleep deprivation during the week set back their circadian body clock, making the subjects prone to snacking more at night after dinner and thus, gaining weight. “If you give people four hours to sleep for five nights, so they have a loss of 20 hours of sleep, and then give them four days with 12 hours in bed, they're not going to sleep an additional 20 hours,” says Dr. Fenn. “The body just doesn’t work that way.”

Dr. Harris agrees, stressing that the best way to prevent long-term (and long-lasting) sleep debt is to practice good sleep hygiene. Stick to a consistent sleep schedule (yes, even on the weekends when you want to sleep in) and speak to a healthcare provider any time you’re concerned about the quality of your sleep including how much—or little—you’re getting overall.

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