What Is Sleep Debt? The Number That Predicts How You’ll Feel

No one likes to be in debt. It feels draining and can put a damper on everything you do. Those feelings seem to be pretty universal, whether we’re talking about money … or sleep.

So, what is sleep debt? Sleep debt is a running total of the hours of sleep you’ve missed over the past 14 days, as compared to the sleep your body needed. Not only is sleep debt the best way to “score” your sleep, sleep debt is also the single number that best predicts how you feel and perform on any given day. 

In this article, we’ll explain how sleep debt is accumulated and why keeping yours low is vital for having the energy you need on a daily basis. We’ll also share tips on how to pay down sleep debt so that you can feel and perform at your best.

Why “Quality Sleep" Is Not the Solution 

It’s hard to overstate the life-sustaining importance of sleep. It’s up there with food, water and air — we cannot survive without it. So the desire to analyze and optimize your sleep is well-founded, and the process need not be complicated

You may have heard that you need to get "quality" sleep. The problem is there is no scientific consensus on what "quality" sleep actually means. Luckily, you don't have to worry about that. When it comes to sleep, and as counter-intuitive as it might seem, quantity is what you should focus on over quality.

Getting the quantity of sleep your body biologically needs is the only way to get naturalistic sleep, which is defined as the sleep your body needs, unencumbered by impediments you — and the circumstances of your life — place on it. (Some examples of these impediments include late-night food or alcohol, not being mindful about light in the morning and at night, or scheduling your life against your chronotype.)

If you were consistently getting enough sleep, each day you'd experience a normal buildup of sleep pressure. Inside the brain, sleep pressure is the gradual accumulation of adenosine, an organic compound that decreases arousal and causes drowsiness. At bedtime, sleep pressure hits its peak and, providing your circadian rhythm is at its “sleep gate” trough, you go to sleep. With sufficient sleep, the brain purges itself of adenosine, resetting the pressure balance at zero for the next day.

On the other hand, when you don’t get enough sleep, instead of getting a full adenosine purge, you carry over the leftover adenosine to the next day, and the next, and so on. As the adenosine piles up, the negative effects of sleep loss compound, and your energy levels (and emotional and physical health) take a hit.

That’s why focusing on meeting your sleep need and keeping your sleep debt low is the best way to protect and increase your daytime energy levels — and improve how you're feeling and functioning. 

How Lost Sleep Becomes Sleep Debt 

The common one-size-fits-all advice is that a "good night's sleep" or a full night of sleep is eight hours. But there is no set amount of sleep that’s ideal for everyone. Sleep need varies from person to person, because it’s determined by age and genetics. But studies have shown that about 8 hours and 10 mins of sleep are needed — with less sleep resulting in impaired cognitive performance. Even if you think you function fine on 6 hours of sleep, the research shows that you are likely unaware that you aren’t performing at your best. The RISE app uses data from your phone, along with sleep science-based models, to calculate your unique sleep need in hours and minutes.

When you don’t meet your sleep need, you start to accumulate sleep debt. But it’s not enough to look at a single night of sleep. ​​The sleep you missed last night matters, but the sleep you missed out on over the past two weeks is actually what has bearing over your energy today.

Having a tool (like the RISE app) that makes such calculations automatic is not just convenient, it’s empowering. Sleep debt is the single number that best predicts how you feel and perform on any given day, and RISE doesn’t just give you an easy way to track yours, it also provides guidance to help you pay it down.

The Effects of Acute and Chronic Sleep Deprivation

Carrying high sleep debt has serious consequences. Short term or acute sleep debt — the 14-day version of sleep debt you can track with the RISE app — affects your emotions, cognition, and physiology in short order. Your attention span, reflexes, metabolism, and immune system can be negatively impacted, and all sooner than you might think (within hours of missing your usual bedtime). 

When you carry sleep debt, you also put others at risk. According to the National Highway Traffic Safety Administration, every year drowsy driving causes about 100,000 crashes with more than 1,550 fatalities and 71,000 injuries. 

One reason for this is that just 18 hours without sleep can give you the same level of cognitive impairment as a person with a blood alcohol level of 0.05%. To make matters worse, you will likely be completely unaware of your downgraded abilities.

When having excessive sleep debt becomes the norm for you, your brain will adapt to functioning at that reduced capacity. After a while, your downgraded cognitive performance starts to feel normal. What’s worse, these adaptive changes can continue to impair your brain’s operational capacity for several days after you go back to getting the exact amount of sleep your body needs — because you’re just meeting your sleep need, not paying down sleep debt.

With more and more people getting less and less sleep, the Centers for Disease Control (CDC) has declared insufficient sleep a “public health epidemic,” as the serious adverse health and social consequences have become more pervasive. 

Studies show that people with chronic sleep debt — those who rarely meet their sleep need, let alone pay any of the debt back  — have an increased risk for weight gain and obesity, Type 2 diabetes, high blood pressure, heart disease, dementia, psychiatric disorders, fertility problems, cancer, and other chronic diseases.

Improve Your Sleep Hygiene, Improve Your Life

As reasons to keep your sleep debt low start to add up in your head, questions about how to go about doing that will probably pop up, too. Assessing your sleep hygiene is a good place to start.

Though it is defined as “the upkeep of behaviors that influence the way you sleep,” sleep hygiene is about much more than what you do right before bed. 

Do you wake up at approximately the same time every day? Is exercise a part of your daily routine? Do you drink coffee after dinner? Each of these questions points to a different aspect of sleep hygiene. And although it may mean taking time to develop new habits, improving your sleep hygiene is a worthwhile endeavor. It can help you meet your sleep need, which keeps your sleep debt down and your daytime energy up.

One of the most important but often overlooked aspects of sleep hygiene is light. The timing of your light exposure can have a big impact on your sleep and next-day energy levels. That's because light is the most important external influence on your circadian rhythm, the internal clock that tells every cell in your body when to be active and inactive during a roughly 24-hour period. 

You might be surprised at how being strategic about your light exposure — getting some sunlight first thing in the morning and minimizing light 90 minutes before bedtime — can make it easier to fall asleep and stay asleep.

Tips for Better Sleep Hygiene 

To keep your sleep debt low, there are simple steps you can take to keep your daily schedule and activities in line with your circadian rhythm: 

• Stick to a consistent wake time and bedtime, as a regular sleep schedule helps with circadian alignment, which in turn helps you meet your sleep need.
• Get some sun — or some form of light — soon after waking to send a signal to your brain that the day has begun. 
• Exercise daily; it can help you get the sleep you need at night. 
• Avoid excess alcohol and caffeine, both known sleep disruptors, in the later afternoon or evening. (The RISE app can remind you of these cutoff times.)
• To avoid suppressing your body’s production of the sleep-inducing hormone melatonin, minimize light exposure after dark and avoid blue light in the 90 minutes before bed. The best way to do this is to wear blue-light blocking glasses.
• Have a nightly wind-down period to disconnect from the stress and busyness of the day. Listen to calming music or read a chapter of a novel.
• Take a warm bath or shower before bed. Because core temperature tends to drop in preparation for sleep, the body’s release of heat when you emerge from the warm water accelerates this process and helps transition you into sleep more fluidly. 
• Maintain a sleep-friendly bedroom. Keep it cool (65-68 degrees). Keep it dark (use blackout curtains and an eye mask). Keep it quiet (use a white noise machine or earplugs).
• Go to bed during your Melatonin Window, the time of night when your body produces its highest levels of melatonin. (The RISE app shows you the time of your Melatonin Window each night.) Missing this window can make it more difficult to fall asleep and stay asleep. 

If you’re practicing good sleep hygiene and keeping your sleep schedule aligned with your circadian rhythm, but you’re still struggling to get the sleep you need, talk to your doctor or a sleep medicine specialist. Regularly experiencing excessive daytime sleepiness and persistent nighttime wakefulness can be a sign of sleep apnea and other sleep disorders. 

How to Pay Down Sleep Debt 

You now know that good sleep hygiene will help you keep your sleep debt low. But what if your sleep debt is already high? Is it possible to catch up?

It is possible to recover from acute sleep debt. In a study conducted at the University of Pennsylvania School of Medicine, participants who had their sleep restricted to 4.5 hours per night for a full week experienced a dramatic boost in cognitive performance and mood after just two full nights of recovery sleep.

To make up for lost sleep, you have to outsleep your sleep need. That means getting extra sleep. Going to bed a little earlier at night is one way to catch up. You can also try napping during the early afternoon. When you don’t have another option, sleep in, but not for more than an hour past your usual wake time. 

Knowing that you’re working with a 14-day span can feel liberating. Just as one full night of sleep won’t fix everything, one bad night won’t tank things either. And if your sleep debt is already low and there's not an incredible urgency to pay it down, you can choose to just "wait it out." By continuously meeting your sleep need for several nights, you can simply wait until the bad nights drop off and are no longer counted as part of the 14 days.  

Taking an even longer view,  it becomes clear that getting the sleep you need is a consistent, lifelong practice. 

Day to day, being purposeful about the timing of your wake time, naps, and bedtime can help you optimize your overall sleep outcomes and prevent you from falling deeper into sleep debt. That optimization should be guided by your circadian rhythm. The RISE app’s energy screen shows you the predictable peaks and dips in energy you’ll experience each day. 

Using your circadian rhythm as a guide, you can figure out the most opportune times to squeeze in an extra hour of sleep here and there. For instance, you’ll see that the ideal time for napping is in the afternoon during your midday dip.

Make Sleep a Priority, but Don't Expect Perfection

If you’re having a bad day — or a string of bad days — or if something just feels off in your life, sleep should be the first thing you address. It is a fundamental part of a happy and healthy life, and there's simply no substitute for it. 

That’s why having a reliable way to evaluate and optimize your sleep (sleep debt) is so important. Sleep debt is the most accurate predictor of how well you feel and perform on a daily basis.

It is vital to prioritize sleep, practice good sleep hygiene, and make every effort to stay aligned with your circadian rhythm. But don’t expect perfection. You don't need to get to sleep debt zero. Because you can’t always control your schedule and because life can throw a curveball when you least expect it — just aim to keep your sleep debt below five hours. At that level, you can still feel good and function at or near your best.

Remember, meeting your sleep need is an ongoing process. And having a tool like the RISE app to keep track of your sleep debt and circadian rhythm can help put you in control of how good you feel and how well you function every day.