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Sleep debt or sleep deficit is the cumulative effect of not getting enough
sleep Sleep is a sedentary state of mind and body. It is characterized by altered consciousness, relatively inhibited Perception, sensory activity, reduced muscle activity and reduced interactions with surroundings. It is distinguished from wakefuln ...
. A large sleep debt may lead to mental or physical
fatigue Fatigue describes a state of tiredness that does not resolve with rest or sleep. In general usage, fatigue is synonymous with extreme tiredness or exhaustion that normally follows prolonged physical or mental activity. When it does not resolve ...
, and can adversely affect one's
mood Mood may refer to: *Mood (psychology), a relatively long lasting emotional state Music *The Mood, a British pop band from 1981 to 1984 * Mood (band), hip hop artists * ''Mood'' (Jacquees album), 2016 * ''Moods'' (Barbara Mandrell album), 1978 ...
, energy and ability to think clearly. There are two kinds of sleep debt: result of partial
sleep deprivation Sleep deprivation, also known as sleep insufficiency or sleeplessness, is the condition of not having adequate duration and/or quality of sleep to support decent alertness, performance, and health. It can be either chronic or acute and may vary ...
, and of total sleep deprivation. Partial sleep deprivation occurs when a person or a lab animal sleeps too little for several days or weeks. Total sleep deprivation, on the other hand, occurs when the subject is being kept awake for at least 24 hours. There is debate in the scientific community over the specifics of sleep debt (see ), and it is not considered to be a
disorder Disorder may refer to randomness, non-order, or no intelligible pattern. Disorder may also refer to: Healthcare * Disorder (medicine), a functional abnormality or disturbance * Mental disorder or psychological disorder, a psychological pattern ...
.


Physiological effects of sleep debt

Chronic sleep debt has a substantial health impact on the human body, specifically on metabolic and endocrine functions. A study published in ''
The Lancet ''The Lancet'' is a weekly peer-reviewed general medical journal and one of the oldest of its kind. It is also the world's highest-impact academic journal. It was founded in England in 1823. The journal publishes original research articles, ...
'' investigated the physiological effects of sleep debt by assessing the sympathovagal balance (an indicator of the
sympathetic nervous system The sympathetic nervous system (SNS) is one of the three divisions of the autonomic nervous system, the others being the parasympathetic nervous system and the enteric nervous system. The enteric nervous system is sometimes considered part of ...
activity), thyrotropic function,
HPA axis HPA may refer to: Organizations * Harry Potter Alliance, a charity * Halifax Port Authority, Canada * Hamburg Port Authority, Germany * Hawaii Preparatory Academy, a school in Hawaii, US * Health Protection Agency, UK * Heerespersonalamt, the ...
activity, as well as the carbohydrate metabolism of 11 young adult males whose sleep period for six nights was either restricted to four hours per night or extended to 12 hours in bed per night. Results revealed that in the sleep-debt condition, thyrotropin concentrations were decreased, while lowered glucose and insulin responses indicated a clear impairment of carbohydrate tolerance, a 30% decrease than in the well-rested sleep condition. On the other hand, males who were sleep-restricted showed significantly elevated concentrations of evening cortisol (the "stress" hormone) and elevated sympathetic nervous system activity in comparison to those who enjoyed a full sleep, over a period of 6 nights. Chronic sleep debt has a detrimental impact on human (neuro)physiological functioning and can disrupt immune, endocrine and metabolic function, while increasing the severity of cardiovascular and age-related illnesses over a period of time.


Neuropsychological effects of sleep debt on emotions

Accumulated and continuous short-term sleep deficit has been shown to increase and intensify psychophysiological reactions in humans to emotional stimuli. The
amygdala The amygdala (; plural: amygdalae or amygdalas; also '; Latin from Greek, , ', 'almond', 'tonsil') is one of two almond-shaped clusters of nuclei located deep and medially within the temporal lobes of the brain's cerebrum in complex v ...
plays a strong functional role in the expression of negative emotions such as fear, and through its anatomical connections with the
medial prefrontal cortex In mammalian brain anatomy, the prefrontal cortex (PFC) covers the front part of the frontal lobe of the cerebral cortex. The PFC contains the Brodmann areas BA8, BA9, BA10, BA11, BA12, BA13, BA14, BA24, BA25, BA32, BA44, BA45, BA4 ...
(mPFC) has an important function in subjective suppression of and the reframing and reappraisal of negative emotions. A study assessing sleep deficit in young Japanese men over a 5-day period (during which they slept only 4 hours per day) showed that there was greater left amygdala activation to fearful faces but not happy faces, and an overall subjective mood deterioration. As a result, even short-term continuous sleep debt, or deprivation, has been shown to reduce this functional relationship between the amygdala and mPFC, inducing negative mood changes through increased fear and anxiety to unpleasant emotional stimuli and events. Thus, a full and uninterrupted 7-hour sleep is crucial for the proper functioning of the amygdala in modulating an individual's mood states—by reducing negative emotional intensities and increasing reactivity to positive emotional stimuli.


Sleep debt and obesity

Epidemiological research has solidified the association between sleep debt and/or deprivation and
obesity Obesity is a medical condition, sometimes considered a disease, in which excess body fat has accumulated to such an extent that it may negatively affect health. People are classified as obese when their body mass index (BMI)—a person's ...
as a result of an elevated
body mass index Body mass index (BMI) is a value derived from the mass (weight) and height of a person. The BMI is defined as the body mass divided by the square of the body height, and is expressed in units of kg/m2, resulting from mass in kilograms and he ...
(BMI) through various ways such as disruptions in the hormones
leptin Leptin (from Greek λεπτός ''leptos'', "thin" or "light" or "small") is a hormone predominantly made by adipose cells and enterocytes in the small intestine that helps to regulate energy balance by inhibiting hunger, which in turn dimi ...
and
ghrelin Ghrelin (; or lenomorelin, INN) is a hormone produced by enteroendocrine cells of the gastrointestinal tract, especially the stomach, and is often called a "hunger hormone" because it increases the drive to eat. Blood levels of ghrelin are high ...
that regulate appetite, higher food consumption and poor diets, and a decrease in overall calorie burning. However, in recent years, multimedia usage such as Internet and television consumption that play an active role in sleep deficit has also been linked to obesity by provoking unhealthy, sedentary lifestyles and habits as well as higher food consumption. Moreover, work-related behaviors such as long working and commuting hours and irregular work timings such as during shift work also function as contributing factors to being overweight or obese as a result of shorter sleeping periods. In comparison to adults, children exhibit a more consistent association between sleep debt and obesity.


Sleep debt and mortality

Several studies have shown that sleep duration, specifically sleep deficit or shorter sleep duration, predicts mortality, whether it be weekdays or weekends. In people aged 65 years and younger, a daily sleep duration of 5 hours or less (amounting to a sleep deficit of 2 hours per day) during weekends correlated with a 52% higher mortality rate—as compared to a control group who slept for 7 hours. Consistent weekday sleep debt exhibited a detrimental association with mortality and morbidity, but this effect was negated when compensated with long sleep during weekends. However, the harmful consequences of sleep debt over weekdays and weekends was not seen in individuals aged 65 years and older.


Scientific debate

There is debate among researchers as to whether the concept of sleep debt describes a measurable phenomenon. The September 2004 issue of the journal ''Sleep'' contains dueling editorials from two leading sleep researchers,
David F. Dinges David F. Dinges is an American sleep researcher and teacher. He is Professor of Psychology in Psychiatry, Chief of the Division of Sleep and Chronobiology in the Department of Psychiatry, and Associate Director of the Center for Sleep and Respir ...
and Jim Horne. A 1997 experiment conducted by psychiatrists at the University of Pennsylvania School of Medicine suggested that cumulative nocturnal sleep debt affects daytime sleepiness, particularly on the first, second, sixth, and seventh days of sleep restriction. In one study, subjects were tested using the psychomotor vigilance task (PVT). Different groups of people were tested with different sleep times for two weeks: 8 hours, 6 hours, 4 hours, and total
sleep deprivation Sleep deprivation, also known as sleep insufficiency or sleeplessness, is the condition of not having adequate duration and/or quality of sleep to support decent alertness, performance, and health. It can be either chronic or acute and may vary ...
. Each day they were tested for the number of lapses on the PVT. The results showed that as time went by, each group's performance worsened, with no sign of any stopping point. Moderate sleep deprivation was found to be detrimental; people who slept 6 hours a night for 10 days had similar results to those who were completely sleep deprived for 1 day.


Evaluation

Sleep debt has been tested in a number of studies through the use of a
sleep onset latency In sleep science, sleep onset latency (SOL) is the length of time that it takes to accomplish the transition from full wakefulness to sleep, normally to the lightest of the non-REM sleep stages. Sleep latency studies Pioneering Stanford Univers ...
test. This test attempts to measure how easily a person can fall asleep. When this test is done several times during a day, it is called a multiple sleep latency test (MSLT). The subject is told to go to sleep and is awakened after determining the amount of time it took to fall asleep. The
Epworth Sleepiness Scale The Epworth Sleepiness Scale (ESS) is a scale intended to measure daytime sleepiness that is measured by use of a very short questionnaire. This can be helpful in diagnosing sleep disorders. It was introduced in 1991 by Dr Murray Johns of Epworth ...
(ESS), an eight-item questionnaire with scores ranging from 0 to 24, is another tool used to screen for potential sleep debt. A January 2007 study from Washington University in St. Louis suggests that saliva tests of the enzyme
amylase An amylase () is an enzyme that catalyses the hydrolysis of starch (Latin ') into sugars. Amylase is present in the saliva of humans and some other mammals, where it begins the chemical process of digestion. Foods that contain large amount ...
could be used to indicate sleep debt, as the enzyme increases its activity in correlation with the length of time a subject has been deprived of sleep. The control of wakefulness has been found to be strongly influenced by the protein orexin. A 2009 study from Washington University in St. Louis has illuminated important connections between sleep debt, orexin, and
amyloid beta Amyloid beta (Aβ or Abeta) denotes peptides of 36–43 amino acids that are the main component of the amyloid plaques found in the brains of people with Alzheimer's disease. The peptides derive from the amyloid precursor protein (APP), which i ...
, with the suggestion that the development of Alzheimer's disease could hypothetically be a result of chronic sleep debt or excessive periods of wakefulness.


Phosphorylation of proteins

In mice, there are 80 proteins in the brain, called "sleep need index phosphoproteins" (SNIPPs), which become more and more phosphorylated during waking hours, and are dephosphorylated during sleep. The phosphorylation is aided by the gene Sik3. A type of
laboratory mouse The laboratory mouse or lab mouse is a small mammal of the order Rodentia which is bred and used for scientific research or feeders for certain pets. Laboratory mice are usually of the species ''Mus musculus''. They are the most commonly use ...
(named Sleepy) possesses an altered version of this protein, which is called SLEEPY and is more active than the regular version. This results in the mice showing more
slow-wave sleep Slow-wave sleep (SWS), often referred to as deep sleep, consists of stage three of non-rapid eye movement sleep. It usually lasts between 70 and 90 minutes and takes place during the first hours of the night. Initially, SWS consisted of both St ...
activity during non-REM sleep—a reliable indicator that more sleep is met. Inhibition of the Sik3 gene decreases phosphorylation and slow-wave activity in both normal and modified mice.


See also

*
Excessive daytime sleepiness Excessive daytime sleepiness (EDS) is characterized by persistent sleepiness and often a general lack of energy, even during the day after apparently adequate or even prolonged nighttime sleep. EDS can be considered as a broad condition encompass ...
*
Hypersomnia Hypersomnia is a neurological disorder of excessive time spent sleeping or excessive sleepiness. It can have many possible causes (such as seasonal affective disorder) and can cause distress and problems with functioning. In the fifth edition o ...
*
Insomnia Insomnia, also known as sleeplessness, is a sleep disorder in which people have trouble sleeping. They may have difficulty falling asleep, or staying asleep as long as desired. Insomnia is typically followed by daytime sleepiness, low energy ...
*
Sleep deprivation Sleep deprivation, also known as sleep insufficiency or sleeplessness, is the condition of not having adequate duration and/or quality of sleep to support decent alertness, performance, and health. It can be either chronic or acute and may vary ...


References


Further reading

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External links


''Harvard Magazine'' article, "Deep into Sleep"


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{{DEFAULTSORT:Sleep debt Sleeplessness and sleep deprivation Sleep physiology Sleep medicine Sleep disorders