What is Sleep Debt and How Does It Impact Alertness?

By Dr. Daniel Gartenberg

How does sleep deprivation impact you?

❌ Daytime fatigue
❌ Increased likelihood of every chronic disease (cancer, heart disease, etc)
❌ Inability to to make clear decisions
❌ Brain cannot clear harmful toxins (metabolic waste and beta amyloid)
❌ Even a worse sense of humor!

Average need for sleep for adults

How much sleep do I need?

Individual differences in needs

The American Academy of Sleep Medicine's guidance on sleep need is that adults need 7-9 hours each night. Keep in mind, this is sleep amount, not time in bed. For example, personally, I need about 7.5 hours per night so I typically spend about 8 hours in bed to get that amount. If you spend 100% of your time in bed asleep, this is often a sign of sleep deprivation. Above is a graph showing the normal distribution of sleep need across Americans. As you can see, most individuals get something between the 7-9 hours range, but many people get less sleep than they need, which is a major public health issue.

Check out my podcast with Dave Asprey where we delve deep into sleep need!

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The sleep need trick

Learn your need now

One thing that you may have noticed is that there are big differences in sleep need. 7-9 hours is a large range. To figure out your unique sleep need and identify when you are sleep deprived, here is a little trick I learned from a professor I work with, Dr. Orfeu Buxton. Imagine a time you were on a vacation or take a vacation for about a week. Go to bed at the same time every night and make sure you have no stressors pushing you to awaken in the morning. Your body will start forming a groove as you wake up naturally each day. Calculate how much sleep you got each night using something like SleepSpace. This is likely close to how much sleep you need. 

Things to know about sleep debt

These are important things to keep in mind about sleep debt or sleep deprivation

Acute sleep debt: This refers to sleep debt from the last night of sleep. It is common for individuals to periodically have moments in their life where they experience this briefly. Acute sleep debt is what impacts how you feel the next day. 

Chronic sleep debt: This is when you are sleep drived for months or even years. Some people get used to not getting enough sleep and live their lives in a chronic brain fog without even being aware of it. Chronic sleep debt is what impacts your longterm health. 

Homeostatic Component: This is the scientific term for sleep debt. It can also be referred to as sleep drive. As it relates to your alertness, the longer you are awake, the more tired you get, which is referred to as your sleep homeostat. 

Sleep Need By Gender: Some people claim that women need more sleep than men, but this is probably due to societal factors and there likely is not a gender difference in sleep need (learn more). 

The SleepSpace app can display your daily sleep debt and integrates with wearables, nearables, and the consensus sleep diary

How SleepSpace Calculates Sleep Need

Tracking need with wearables, nearables, and the consensus sleep diary

Every night, SleepSpace calculates your acute sleep debt. This is the difference between how much sleep you need and how much sleep you got last night. You can set your sleep need in the score tab of SleepSpace by hitting on the gear icon in the upper right of the screen. In the example to the left, I set my sleep need to 7.5 hours, but I only got 5h 57m of sleep. Therefore, I accumulated 1 hr 32m of sleep debt on this day. As a result my estimated energy levels for the day decreased. This is articulated in the alertness graph and the blue line that is lower than when I ensured I got the sleep I needed. You can track your sleep need across wearables like Whoop, Oura, or Apple Watch and can change the data source to see how these devices compare. Alternatively, you can use the SleepSpace phone tracker or Consensus Sleep Diary found within the app. 

Sleep need over the course of the human lifespan from a baby to old age

Sleep Need As We Age

Adults need less than children

As our brains are growing and learning new information, our sleep need is much higher. Babies and children therefore need signficantly more sleep than adults. While this graph and some researchers believe that sleep need declines with age, this may actually reflect the body breaking down and not being able to fully regenerate itself. Sleep need for healthy people likely stays the same even in old age. 

SleepSpace's articulation of the different sleep stages, wake, light, deep, and REM. Deep sleep, or slow-wave sleep is characterized by long-burst brainwaves or wavelengths.

Sleep Need and Sleep Quality

The better the quality, the less the need

But here's the catch. If you are a very healthy person, you probably need a bit less sleep than if you are unhealthy. For example, when I had untreated sleep apnea, I tracked that I needed about a half hour more sleep than I currently get. My inability to breath throughout the night was resusing my ability to get regenerative REM and Slow Wave Sleep (Deep Sleep). This is why it is essential to address underlying sleep disorders, which can literally save years off your life. If you are unhealthy, you should get the sleep that your body needs because sleeping this extra amount is what helps your body heal. For example, most people can relate to needing to sleep more when you have a cold. If you have a chronic disease or persistent inflammation, this results in the chornic need to get more sleep.