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.
Maybe that’s because the words don’t evoke the truth of what sleep debt 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.
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.
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
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.
Sleep Debt Fact: The Centers for Disease Control and Prevention estimate that one in three Americans — about 40 million of us — are chronically sleep-deprived
The 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
- Low motivation
- Trouble learning or understanding new ideas
- Lack of coordination
- 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.
Not only is communication between cells slower, it’s also weaker, and the transmission of information takes longer. (2) The brain’s ability to translate perception into conscious thought may be impaired. And these changes can have severe consequences. Lack of sleep can markedly delay decision-making and reaction times. In fact, studies reveal that the cognitive effects of poor sleep can be as profound and dangerous as drunk driving. (3)
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. The parts of your brain that slow down after too many hours awake also display the kind of increased activity normally associated with sleep. What does this mean? While parts of your brain may be awake and functioning, other parts are still asleep, disrupting normal processes and creating glitches in thinking and perception.
Sleep Debt Fact: Research shows that the need for a certain amount of sleep is largely genetic, and can’t be changed
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 both in increased emotional response and in 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.
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 Debt Fact: 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 Debt: The High Cost of One All-Nighter
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 Debt: 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.
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)
Sleep Debt Fact: CSS is an acronym commonly used in sleep science, and 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.
The Sleep Debt Calculator
Knowing how much sleep we need is the first step to eliminating a sleep debt. Many of us don’t remember how it feels to sleep as long as we need to, or to wake up naturally without an alarm or other disruption.
How do we calculate how much sleep we’ve lost and how much we need? Start with the number of hours you need to sleep each night to feel rested and alert. You may need eight hours but get only seven, or even less. To calculate your sleep debt, add up the number of sleep hours lost every week. This number represents the total hours you need to add to the time you spend sleeping each week.
If you’re surprised by the number of sleep hours you lose on a weekly basis, monthly and yearly totals may be even more startling. Lose an average of ninety minutes of sleep each night, and your yearly sleep loss climbs to more than 547 hours. The body may pay for this loss by increasing fat storage, lowering insulin sensitivity, and reducing the ability to learn and concentrate.
Repaying the Sleep Debt
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.
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
- if they disrupt sleep
- Exercising during the day. This can help muscles relax and feel more ready for sleep at night
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.
- Uncovering Residual Effects of Chronic Sleep Loss on Human Performance. PubMed Central, Jan. 13, 2010
- Nir Y , et al. Selective Neuronal Lapses Precede Human Cognitive Lapses Following Sleep Deprivation. – PubMed – National Center for Biotechnology Information
- Moderate Sleep Deprivation Produces Impairments in Cognitive and Motor Performance Equivalent to Legally Prescribed Levels of Alcohol Intoxication. PubMed Central
- Losing Neutrality: The Neural Basis of Impaired Emotional Control Without Sleep. Journal of Neuroscience, 23 Sept. 2015
- Yoo, Seung-Schik, Matthew Walker. The Human Emotional Brain Without Sleep — A Prefrontal Amygdala Disconnect
- Szymusiak R and McGinty D. Hypothalamic Regulation of Sleep and Arousal. – PubMed – National Center for Biotechnology Information
- Driver Performance in the Moments Surrounding a Microsleep. PubMed Central, Mar. 1, 2010
- Janet Mullington, PhD., Norah S. Simpson, PhD. Sleep Loss and Inflammation. PubMed Central, October 24, 2010
- 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
- Short- and Long-term Health Consequences of Sleep Disruption. PubMed Central
- De Lorenzo BH , et al. Sleep-deprivation Reduces NK Cell Number and Function Mediated by β-adrenergic Signalling. PubMed National Center for Biotechnology Information
- Effect of 1 Week of Sleep Restriction on Testosterone Levels in Young Healthy Men. PubMed Central
- VanHelder T and Radomski MW. Sleep Deprivation and the Effect on Exercise Performance. PubMed – National Center for Biotechnology Information
- Obesity, Diabetes and OSAS Induce of Sleep Disorders: Exercise As Therapy. Lipids in Health and Disease