Sleep Debt: Myth or Reality?

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For many of us, the term “sleep debt” brings to mind a late night studying, time awake with a new baby, or a long flight overseas. We think of the short-term inconvenience of feeling tired and unable to focus. Sleep debt — the difference between the amount of sleep we need and what we’re actually getting — sounds like something we can pay back with a night or two of good rest. We may even wonder if sleep debt is more myth than reality.

What is Sleep Debt?

Confusion around what sleep debt is—and whether or not it’s real—may stem from the fact that the words don’t evoke the truth of what it really is: a tendency to slide into dangerous episodes of microsleep while working or driving. A steep drop in the number of killer cells that ward off viruses and cancer. A permanent reduction in the brain cells that help us learn, store memories, and pay attention.

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Want to calculate the amount of sleep debt you have accumulated? Use our sleep debt calculator to find out how you can get back on track!

The true consequence of sleep debt goes much deeper than feeling fatigued and unable to concentrate. Sleep debt has damaging cumulative effects — both short and long-term — that create a wide-ranging and significant impact on our health. (1) Even if we lose sleep for just one night.

A common misconception about sleep is that the body can adapt to less, and often, much less. If we train ourselves to go without adequate rest, we’ll get used to the lack the way we might to giving up sugar. But no matter what the demands of our busy schedules, the brain requires a certain non-negotiable amount of sleep, usually between seven and nine hours each night.

Unfortunately, the longer we go without sufficient sleep, the less capable we may be of judging how much rest we need. A brief period of sleep loss may become chronic, along with the resulting mental and physical effects.

Chronic sleep deprivation

A physical and cognitive state resulting from long-term insufficient sleep. The Centers for Disease Control and Prevention estimate that chronic sleep deprivation affects 40 million Americans.

Note: The content on Sleepopolis is meant to be informative in nature, but it shouldn’t taken as medical advice, and it shouldn’t take the place of medical advice and supervision from a trained professional. If you feel you may be suffering from any sleep disorder or medical condition, please see your healthcare provider immediately.

The Many Causes of Sleep Debt

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Sleep Debt Causes

Because the term can apply to both long and short-term loss of sleep, the causes of sleep debt are quite varied. Common reasons for lack of sleep may include:

  • Sleep disorders such as insomnia, restless leg syndrome, and sleep apnea
  • Illnesses and chronic pain. Diseases such as cancer, hyperthyroidism, and asthma may make sleep difficult
  • Night shift work
  • School and work schedules
  • Young children
  • Extensive travel
  • Electronics use close to bedtime

Lying awake due to stress or “racing thoughts” is another common reason we accumulate sleep debt. Emotional issues such as anxiety, post-traumatic stress, or panic disorder often result in the inability to fall or stay asleep. These conditions may cause heart palpitations, hyperventilation, worrisome thoughts, shortness of breath, and other symptoms that prevent or disrupt sleep.

The Symptoms of Sleep Debt

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Symptoms of Sleep Debt

We all know how we feel when we don’t sleep enough. Sluggish. Forgetful. Irritable. We may have the urge to eat more, and to eat high-sugar and carbohydrate foods. Depending on how sleep-deprived we are, we may also experience:

  • Depression and anxiety
  • Decreased ability to concentrate
  • Yawning
  • Low motivation
  • Trouble learning or understanding new ideas
  • Lack of coordination and balance problems
  • Mood swings
  • Increased susceptibility to illness

So what’s really going on? Well, research shows that when it comes to sleep debt, what you feel, your brain cells feel.

The Sleep-Deprived Brain

The day after a poor night’s sleep, you may feel exhausted, irritable, and unable to think. You may find yourself nodding off during a meeting, or reading without comprehending the words. Why? Because even short-term lack of rest interrupts the usually rapid and efficient communication between your brain cells. Electrical activity that creates a normal perception of the environment slows down, making it harder for the brain to process what it’s seeing and hearing.

Sleep deprivation also weakens the communication between cells and impacts the brain’s ability to translate perception into conscious thought. (2) These types of changes can have severe consequences. Lack of sleep can markedly delay decision-making and reaction times. Studies reveal that the cognitive effects of poor sleep can be as profound and dangerous as drunk driving. (3)


Q: Can a person train themselves to function well on less sleep? A: No. Research shows that the need for a certain amount of sleep is largely genetic, and can’t be changed.

How might a sleep debt affect someone going about their normal day?

  • They may have more difficulty perceiving threats, such as the sudden appearance of a car or pedestrian
  • The risk of lapses in judgment might increase, creating a greater likelihood of accidents and workplace errors
  • Blood pressure may rise, increasing the risk of stroke and anxiety

If you feel like you’re half-asleep after a late night, you’re not wrong. Being awake for 24 hours before driving can have an effect on alertness and ability comparable to having a 0.10% blood alcohol content, which is above the legal limit in all 50 states. A study by the AAA Foundation for Traffic Safety found that drivers who sleep only five to six hours during a 24-hour period are twice as likely to be involved in an accident as a driver who gets seven hours or more.

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Emotional Effects of Poor Sleep

If you’ve ever felt down or emotionally volatile after a poor night of sleep, there’s a scientific reason for that. Sleeplessness has been proven to dramatically increase our reactivity — in fact, the parts of the brain that rely on emotional data are particularly vulnerable to sleep deficits.

And it may not be due to simply feeling more emotional in general after less sleep. Researchers studying the sleep-deprived brain found that emotions were heightened because the brain could no longer tell the difference between an experience, photograph, or event worth reacting to, and something less deserving of a strong emotional response. (4)

Poor sleep may result in increased emotional response and poorer emotional control. One study of emotions and lack of sleep showed a 60% increase in activation of the amygdala, a part of the brain that processes emotional information, particularly when that information is unfavorable or represents a threat. The result is an increased likelihood that a person who is sleep-deprived will perceive an experience as negative, instead of neutral or positive. (5)

The repercussions of poor sleep go far beyond feeling a little upset or emotional. Research demonstrates that a sleep-deprived brain acts a lot like an anxious one. This may be because the prefrontal cortex, which helps control anxiety, is less active than usual after a night without sleep. Also, a 2018 study showed that poor sleep can contribute to feelings of social isolation and loneliness, both of which are known risk factors for emotional and physical illness.

When Sleep Becomes Dangerous: The Microsleep

One of the most serious symptoms of sleep debt is a “microsleep,” an episode of unintentional sleep lasting from a fraction of a second to as long as thirty seconds or more.

A microsleep is not a deep sleep, but a brief lapse into stage one of sleep, which is a transition stage between being asleep and awake. Though this stage is light, it is still sleep. Muscle relax. Eyes close or become heavy. Heart rate and breathing become slower. A microsleep often happens at inopportune moments, when a person is trying to stay alert. Someone who has the sensation of “jerking awake” after a few seconds may have just experienced a microsleep.

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Microsleep Episode

A microsleep episode is often characterized by:

  • Slumping or falling over while sitting
  • Bobbing of the head
  • Unintentional closing of the eyes
  • A car accident or near-miss
  • Loss of muscle control

A microsleep episode signals that a sleep debt is building and causing significant physiological changes. These are changes we can’t see, and often don’t feel.

Why do microsleep episodes happen? The process starts deep inside our brains, with a biological clock located in the almond-sized hypothalamus. (6) Some cells in the hypothalamus are designed to keep us awake, while others promote sleep. When we are sleep deprived, the signals from these cells begin to overlap. The sleep state crosses with the awake state, causing fatigue, confusion, and involuntary episodes of microsleep.

When your eyes begin to close and your chin drops toward your chest, staying awake just lost the battle — at least for a few moments — with falling asleep.

Not only are these episodes uncomfortable, they can be dangerous or even fatal if they occur while working, driving, or operating machinery. (7) Making sure to get adequate rest, taking breaks at work, not driving while drowsy, and addressing sleep disorders such as insomnia and sleep apnea can all help to prevent episodes of microsleep.

Sleep need

The words “sleep need” refer to the amount of sleep a person should have each night to feel fully rested.

Acute Vs. Chronic Sleep Debt: The Same, but Different

Though we may experience similar feelings of exhaustion and lack of focus when we go without sleep, all sleep loss is not the same. There are two different ways to lose sleep: acutely and chronically. The body reacts differently in each case.

Acute sleep loss refers to a single episode of remaining awake, from a few hours past bedtime to staying awake all night or even longer. Chronic sleep loss is sleep loss that accumulates over time, usually as a series of weeks or months without sufficient rest.

The initial physiological effects of sleep deprivation are the same in both cases, and usually involve the increase or decrease of hormones and chemicals in the body. Chemicals that increase during sleep deprivation include:

  • Catecholamines and cortisol, substances involved in the fight or flight response
  • Ghrelin, which boosts appetite
  • Proteins such as TNF-alpha, which promotes inflammation in response to stress or injury (8)

Chemicals that decrease during sleep deprivation include:

  • Melatonin, also known as “the sleep hormone”
  • Leptin, which controls appetite
  • Testosterone, a hormone essential to both male and female sex drive

Short-Term Sleep Deprivation: The High Cost of One All-Nighter

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The short-term effect of these changes on the body are likely to be increased physiological stress, irritability, and a decreased ability to sleep. Mood swings are common, as is a feeling of physical weakness. The urge to eat increases. The capacity to learn and concentrate is disrupted. The risk of injury or accident goes up, even after an hour or two of lost sleep.

But there’s increasing evidence that even one sleepless night can have a lasting negative impact. Our circadian rhythm isn’t controlled by a single clock, but by many tiny biological clocks spread throughout our muscle and fat tissue. One sleepless night can cause lasting damage to these cells, disrupting glucose metabolism and fat storage for months or even years afterward. (9)

Long-Term Sleep Deprivation: The Debt That Can’t Be Repaid

As sleep debt increases, so does damage to the brain and body. The effects are cumulative, and difficult or even impossible to reverse. When a night without sufficient sleep stretches into a week or month, the risks of obesity, diabetes, metabolic syndrome, and cancer increase. (10)

And sleep debt accumulated when we’re young may last for life. Too many late-night study sessions or parties may result in a degenerative brain disorder such as dementia.

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Effects of Sleep Debt

The immune system is also sensitive to sleep debt, and responds to the stress of sleeplessness with a dramatic reduction in what are called killer cells. Even a modest amount of sleep deprivation lowers the numbers of these disease-fighting cells, leading to an increased risk of viral illness, bacterial infections, and even cancer. And the evidence is more than just anecdotal. Multiple studies have shown that cancer rates are significantly higher in night shift workers and others subjected to interrupted sleep and chronic sleep loss. (11)


Q: What does CSS stand for? A: CSS is an initialism commonly used in sleep science. It stands for chronic short sleep.

Sleep Debt and Libido

Lack of sleep can have far-reaching effects that may not always appear to be slumber-related. Libido is influenced by sleep for several reasons, most notably the drop in testosterone that occurs when the body is stressed by inadequate rest.

Testosterone, which is important to both the male and female sex drive, takes a significant dip after a sleepless night. In fact, just five fewer hours of sleep led to a 15% drop in testosterone in healthy young men in one study. (12) Testosterone increases throughout the night and for as long as sleep continues. Maintaining a healthy and stable level of the hormone appears to require adequate time spent in REM sleep. In addition, lack of sleep encourages the body to produce the stress hormone cortisol, which also reduces testosterone levels.

The problem of dwindling testosterone can be compounded later in life, a time when sleep patterns often change or become interrupted. In combination with the natural drop in testosterone that occurs with age, lack of sleep can have deleterious effects on this essential hormone, profoundly affecting libido and energy levels.

Sleep Debt and Exercise

If you’ve ever exercised after a sleepless night, you may have experienced feelings of weakness, heaviness, and muscle soreness. At a cellular level, your muscles may have stopped using glycogen, the fuel they use when you work out, even if there was sufficient glycogen to continue exercising.

The effects of sleep debt on metabolism and blood sugar levels can also lead to feeling exhausted and unable to continue exercising. This may be due in part to lowered levels of human growth hormone (or HGH), which can plummet on sleepless nights. (13) Human growth hormone is released during both sleep and exercise, and is critical to maintaining the health of tissues and muscles in the body. It plays an essential role in muscle strength, cognitive functioning, and maintaining a healthy weight.

Not only does sleep affect exercise, but exercise affects sleep. The more we sleep, the more likely we are to reap the benefits of exercise, and the more we exercise, the more likely we are to sleep well. Exercise has been proven to be an effective therapy for disorders like sleep apnea, as well as for obesity and diabetes. (14) It may lower blood pressure, and help regulate blood sugar and metabolism.

Exercise can also reduce stress and anxiety, leading to longer, higher-quality sleep. In addition, exercising in the morning and afternoon can regulate sleep and wake cycles by raising body temperature. The drop in body temperature a few hours after exercise can help trigger sleepiness, and help people who toss and turn in bed to fall asleep more quickly.

Sleep Debt Recovery

What to do when you’re sleep-deprived, whether for a night or two or much longer? In a word: sleep. We may not be able to fully repay the debt, but we can stop depriving ourselves of the health benefits of sleep and start getting the rest we need.

Spending seven to nine hours in bed each night after a long period without sufficient sleep may feel strange. And we may be surprised at how long and deeply we need to sleep during this recovery period. The sleep we experience when recovering from a sleep debt is different from our usual slumber. The stages of deep, restorative sleep last longer, while the stages of light sleep that occur just before and after wakefulness are shorter.

Though we may recover most cognitive functioning after an acute sleep debt with just one eight-hour stint in bed, the mental and physical effects of a chronic sleep debt may take time to recover from. Some of the effects may be irreversible, or last for years.

How to Recover From Sleep Debt

Sleep debt recovery may best be accomplished with good sleep hygiene, which includes:

  • A dark, quiet, and cool sleeping environment
  • Consistency. Going to bed and waking up at the same time every day
  • Avoiding caffeine, heavy meals, and alcohol late at night
  • Banishing electronics and other devices that emit blue light from the bedroom at least one hour before bed. The brain responds to blue light the same way it does to sunlight, which can prevent the release of melatonin, a hormone essential to sleep.
  • Exercising during the day. This can help muscles relax and feel more ready for sleep at night

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Many people sacrifice sleep to demanding work schedules, family obligations, or children’s activities. It can feel difficult to find time for sleep on days that are busy from early morning until late at night. It may help to reduce time spent on non-essential pursuits such as television watching, and to make healthy sleep a priority for everyone in the family.

A sleep debt is one debt best repaid by not owing it at all. Even a brief period of sleeplessness has powerful and potentially long-lasting effects on brain cells, mood, and metabolism. Avoiding a sleep debt requires an understanding of the importance of sleep and prioritizing sleep as an essential aspect of health.


  1. Uncovering Residual Effects of Chronic Sleep Loss on Human Performance. PubMed Central, Jan. 13, 2010
  2. Nir Y , et al. Selective Neuronal Lapses Precede Human Cognitive Lapses Following Sleep Deprivation. – PubMed – National Center for Biotechnology Information
  3. Moderate Sleep Deprivation Produces Impairments in Cognitive and Motor Performance Equivalent to Legally Prescribed Levels of Alcohol Intoxication. PubMed Central
  4. Losing Neutrality: The Neural Basis of Impaired Emotional Control Without Sleep. Journal of Neuroscience, 23 Sept. 2015
  5. Yoo, Seung-Schik, Matthew Walker. The Human Emotional Brain Without Sleep — A Prefrontal Amygdala Disconnect
  6. Szymusiak R and McGinty D. Hypothalamic Regulation of Sleep and Arousal. – PubMed – National Center for Biotechnology Information
  7. Driver Performance in the Moments Surrounding a Microsleep. PubMed Central, Mar. 1, 2010
  8. Janet Mullington, PhD., Norah S. Simpson, PhD. Sleep Loss and Inflammation. PubMed Central, October 24, 2010
  9. Acute Sleep Loss Results in Tissue-specific Alterations in Genome-wide DNA Methylation State and Metabolic Fuel Utilization in Humans. Science Advances, 1 Aug. 2018
  10. Short- and Long-term Health Consequences of Sleep Disruption. PubMed Central
  11. De Lorenzo BH , et al. Sleep-deprivation Reduces NK Cell Number and Function Mediated by β-adrenergic Signalling. PubMed National Center for Biotechnology Information
  12. Effect of 1 Week of Sleep Restriction on Testosterone Levels in Young Healthy Men. PubMed Central
  13. VanHelder T and Radomski MW. Sleep Deprivation and the Effect on Exercise Performance. PubMed – National Center for Biotechnology Information
  14. Obesity, Diabetes and OSAS Induce of Sleep Disorders: Exercise As Therapy. Lipids in Health and Disease

Rose MacDowell

Rose is the former Chief Research Officer at Sleepopolis. An incurable night owl, she loves discovering the latest information about sleep and how to get (lots) more of it. She is a published novelist who has written everything from an article about cheese factories to clock-in instructions for assembly line workers in Belgium. One of her favorite parts of her job is connecting with the best sleep experts in the industry and utilizing their wealth of knowledge in the pieces she writes. She enjoys creating engaging articles that make a difference in people’s lives. Her writing has been reviewed by The Boston Globe, Cosmopolitan, and the Associated Press, and received a starred review in Publishers Weekly. When she isn’t musing about sleep, she’s usually at the gym, eating extremely spicy food, or wishing she were snowboarding in her native Colorado. Active though she is, she considers staying in bed until noon on Sundays to be important research.