Shift work

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Shift work is an employment practice designed to make use of, or provide service across, all 24 hours of the clock each day of the week (often abbreviated as 24/7 ). The practice typically sees the day divided into shifts, set periods of time during which different groups of workers perform their duties. The term "shift work" includes both long-term night shifts and work schedules in which employees change or rotate shifts. [1] [2] [3]

Employment is a relationship between two parties, usually based on a contract where work is paid for, where one party, which may be a corporation, for profit, not-for-profit organization, co-operative or other entity is the employer and the other is the employee. Employees work in return for payment, which may be in the form of an hourly wage, by piecework or an annual salary, depending on the type of work an employee does or which sector she or he is working in. Employees in some fields or sectors may receive gratuities, bonus payment or stock options. In some types of employment, employees may receive benefits in addition to payment. Benefits can include health insurance, housing, disability insurance or use of a gym. Employment is typically governed by employment laws, regulations or legal contracts.


In medicine and epidemiology, shift work is considered a risk factor for some health problems in some individuals, as disruption to circadian rhythms may increase the probability of developing cardiovascular disease, cognitive impairment, diabetes, and obesity, among other conditions. [4] [5] Shift work can also contribute to strain in marital, family, and personal relationships. [6] A marriage where one partner works an irregular shift is six times more likely to end in divorce than a marriage where both partners work days. [7]

In epidemiology, a risk factor is a variable associated with an increased risk of disease or infection. When evidence is found the term determinant is used as a variable associated with either increased or decreased risk.

Circadian rhythm any biological process in an organism that recurs with a regularity of approximately 24 hours.

A circadian rhythm is a natural, internal process that regulates the sleep-wake cycle and repeats roughly every 24 hours. It can refer to any biological process that displays an endogenous, entrainable oscillation of about 24 hours. These 24-hour rhythms are driven by a circadian clock, and they have been widely observed in plants, animals, fungi, and cyanobacteria.

Cardiovascular disease class of diseases that involve the heart or blood vessels

Cardiovascular disease (CVD) is a class of diseases that involve the heart or blood vessels. CVD includes coronary artery diseases (CAD) such as angina and myocardial infarction. Other CVDs include stroke, heart failure, hypertensive heart disease, rheumatic heart disease, cardiomyopathy, heart arrhythmia, congenital heart disease, valvular heart disease, carditis, aortic aneurysms, peripheral artery disease, thromboembolic disease, and venous thrombosis.

Health effects

A video on the health effects of shift work

Shift work increases the risk for the development of many disorders. Shift work sleep disorder is a circadian rhythm sleep disorder characterized by insomnia, excessive sleepiness, or both. Shift work is considered essential for the diagnosis. [8] The risk of diabetes mellitus type 2 is increased in shift workers, especially men. People working rotating shifts are more vulnerable than others. [9]

Shift work sleep disorder (SWSD) is a circadian rhythm sleep disorder characterized by insomnia and excessive sleepiness affecting people whose work hours overlap with the typical sleep period. There are numerous shift work schedules, and they may be permanent, intermittent, or rotating; consequently, the manifestations of SWSD are quite variable.

Circadian rhythm sleep disorders (CRSD) are a family of sleep disorders which affect the timing of sleep. CRSDs arise from a persistent pattern of sleep/wake disturbances that can be caused either by dysfunction in one's biological clock system, or by misalignment between one's endogenous oscillator and externally imposed cues. As a result of this mismatch, those affected by circadian rhythm sleep disorders have a tendency to fall asleep at unconventional time points in the day. These occurrences often lead to recurring instances of disturbed rest, where individuals affected by the disorder are unable to go to sleep and awaken at "normal" times for work, school, and other social obligations.

Insomnia inability to sleep

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, irritability, and a depressed mood. It may result in an increased risk of motor vehicle collisions, as well as problems focusing and learning. Insomnia can be short term, lasting for days or weeks, or long term, lasting more than a month.

Women whose work involves night shifts have a 48% increased risk of developing breast cancer. [6] [10] This may be due to alterations in circadian rhythm: melatonin, a known tumor suppressor, is generally produced at night and late shifts may disrupt its production. [10] The WHO's International Agency for Research on Cancer listed "shift work that involves circadian disruption" as a probable carcinogen. [11] [12] Shift work may also increase the risk of other types of cancer. [13] Working rotating shift work regularly during a two-year interval has been associated with a 9% increased the risk of early menopause compared to women who work no rotating shift work. The increased risk among rotating night shift workers was 25% among women predisposed to earlier menopause. Early menopause can lead to a whole host of other problems later in life. [14] [15] A recent study, found that women who worked rotating night shifts for more than six years, eleven percent experienced a shortened lifespan. Women who worked rotating night shifts for more than 15 years also experienced a 25 percent higher risk of death due to lung cancer. [16]

Melatonin hormone that is produced by the pineal gland in animals and regulates sleep and wakefulness

Melatonin is a hormone that regulates sleep–wake cycles. It is primarily released by the pineal gland. As a supplement, it is often used for the short-term treatment of trouble sleeping such as from jet lag or shift work. Evidence of benefit, however, is unclear. One review found onset of sleep occurred 6 minutes faster with use but found no change in total time asleep.It is typically taken by mouth.

International Agency for Research on Cancer Organization

The International Agency for Research on Cancer is an intergovernmental agency forming part of the World Health Organization of the United Nations. Its role is to conduct and coordinate research into the causes of cancer. It also collects and publishes surveillance data regarding the occurrence of cancer worldwide.

Shift work also increases the risk of developing cluster headaches, [17] heart attacks, [18] fatigue, stress, sexual dysfunction, [19] depression, [20] dementia, obesity, [8] metabolic disorders, gastrointestinal disorders, musculoskeletal disorders, and reproductive disorders. [6]

Cluster headache

Cluster headache (CH) is a neurological disorder characterized by recurrent severe headaches on one side of the head, typically around the eye. There is often accompanying eye watering, nasal congestion, or swelling around the eye on the affected side. These symptoms typically last 15 minutes to 3 hours. Attacks often occur in clusters which typically last for weeks or months and occasionally more than a year.

Myocardial infarction Interruption of blood supply to a part of the heart

Myocardial infarction (MI), commonly known as a heart attack, occurs when blood flow decreases or stops to a part of the heart, causing damage to the heart muscle. The most common symptom is chest pain or discomfort which may travel into the shoulder, arm, back, neck, or jaw. Often it occurs in the center or left side of the chest and lasts for more than a few minutes. The discomfort may occasionally feel like heartburn. Other symptoms may include shortness of breath, nausea, feeling faint, a cold sweat, or feeling tired. About 30% of people have atypical symptoms. Women more often present without chest pain and instead have neck pain, arm pain, or feel tired. Among those over 75 years old, about 5% have had an MI with little or no history of symptoms. An MI may cause heart failure, an irregular heartbeat, cardiogenic shock, or cardiac arrest.

Stress, either physiological or biological, is an organism's response to a stressor such as an environmental condition. Stress is the body's method of reacting to a condition such as a threat, challenge or physical and psychological barrier. Stimuli that alter an organism's environment are responded to by multiple systems in the body. The autonomic nervous system and hypothalamic-pituitary-adrenal (HPA) axis are two major systems that respond to stress.

Children going to a 12-hour night shift in the United States, 1908 Child Labor in United States 1908, 12 hour night shifts.jpg
Children going to a 12-hour night shift in the United States, 1908

Shift work also can worsen chronic diseases, including sleep disorders, digestive diseases, heart disease, hypertension, epilepsy, mental disorders, substance abuse, asthma, and any health conditions that are treated with medications affected by the circadian cycle. [6] Artificial lighting may additionally contribute to disturbed homeostasis. [21] Shift work may also increase a person's risk of smoking. [6]

Sleep disorder disease of mental health that involves disruption of sleep patterns

A sleep disorder, or somnipathy, is a medical disorder of the sleep patterns of a person or animal. Some sleep disorders are serious enough to interfere with normal physical, mental, social and emotional functioning. Polysomnography and actigraphy are tests commonly ordered for some sleep disorders.

Hypertension high blood pressure

Hypertension, also known as high blood pressure (HBP), is a long-term medical condition in which the blood pressure in the arteries is persistently elevated. High blood pressure typically does not cause symptoms. Long-term high blood pressure, however, is a major risk factor for coronary artery disease, stroke, heart failure, atrial fibrillation, peripheral vascular disease, vision loss, chronic kidney disease, and dementia.

Epilepsy human neurological disease causing seizures

Epilepsy is a group of neurological disorders characterized by epileptic seizures. Epileptic seizures are episodes that can vary from brief and nearly undetectable periods to long periods of vigorous shaking. These episodes can result in physical injuries, including occasionally broken bones. In epilepsy, seizures tend to recur and, as a rule, have no immediate underlying cause. Isolated seizures that are provoked by a specific cause such as poisoning are not deemed to represent epilepsy. People with epilepsy may be treated differently in various areas of the world and experience varying degrees of social stigma due to their condition.

The health consequences of shift work may depend on chronotype, that is, being a day person or a night person, and what shift a worker is assigned to. When individual chronotype is opposite of shift timing (day person working night shift), there is a greater risk of circadian rhythms disruption. [22] Nighttime workers sleep an average of 1-4 hours less than daytime workers. [23]

Different shift schedules will have different impacts on the health of a shift worker. The way the shift pattern is designed affects how shift workers sleep, eat and take holidays. Some shift patterns can exacerbate fatigue by limiting rest, increasing stress, overworking staff or disrupting their time off. [24]

Muscle health is also compromised by Shift work: Altered sleep and altered eating times. Changes to appetite regulating hormones, more snacking and full center of the brain not working properly, changes in total energy expenditure. Increased snacking, increased binge drinking and reduced protein intake. All these factor can contribute to negative protein balance, increases in insulin resistance and increases in fat. Which can lead to weight gain and more long term health challenge. [25]

Compared with the day shift, injuries and accidents have been estimated to increase by 15% on evening shifts and 28% on night shifts. Longer shifts are also associated with more injuries and accidents: 10-hour shifts had 13% more and 12-hour shifts had 28% more than 8-hour shifts. [6] Other studies have shown a link between fatigue and workplace injuries and accidents. Workers with sleep deprivation are far more likely to be injured or involved in an accident. [8]

One study suggests that, for those working a night shift (such as 23:00 to 07:00), it may be advantageous to sleep in the evening (14:00 to 22:00) rather than the morning (08:00 to 16:00). The study's evening sleep subjects had 37% fewer episodes of attentional impairment than the morning sleepers. [26]

There are four major determinants of cognitive performance and alertness in healthy shift-workers. They are: circadian phase, sleep inertia, acute sleep deprivation and chronic sleep deficit. [27]

Safety and regulation

Shift work has been shown to negatively affect workers, and has been classified as a specific disorder (shift work sleep disorder). Circadian disruption by working at night causes symptoms like excessive sleepiness at work and sleep disturbances. Shift work sleep disorder also creates a greater risk for human error at work. [32] Shift work disrupts cognitive ability and flexibility and impairs attention, motivation, decision making, speech, vigilance, and overall performance. [8]

In order to mitigate the negative effects of shift work on safety and health, many countries have enacted regulations on shift work. The European Union, in its directive 2003/88/EC, has established a 48-hour limit on working time (including overtime) per week; a minimum rest period of 11 consecutive hours per 24-hour period; and a minimum uninterrupted rest period of 24 hours of mandated rest per week (which is in addition to the 11 hours of daily rest). [32] [33] The EU directive also limits night work involving "special hazards or heavy physical or mental strain" to an average of eight hours in any 24-hour period. [32] [33] The EU directive allows for limited derogations from the regulation, and special provisions allow longer working hours for transportation and offshore workers, fishing vessel workers, and doctors in training (see also medical resident work hours). [33]

Fatigue due to shift work has contributed to several industrial disasters, including the Three Mile Island accident, the Space Shuttle Challenger disaster and the Chernobyl disaster. [8] The Alaska Oil Spill Commission's final report on the Exxon Valdez oil spill disaster found that it was "conceivable" that excessive work hours contributed to crew fatigue, which in turn contributed to the vessel's running aground. [34]


Management practices

4 o'clock shift at the Ford Motor Company assembly plant in Detroit, Michigan, 1910s Change of work shift at Ford Motor Company.jpg
4 o'clock shift at the Ford Motor Company assembly plant in Detroit, Michigan, 1910s

The practices and policies put in place by managers of round-the-clock or 24/7 operations can significantly influence shift worker alertness (and hence safety) and performance. [35]

Air traffic controllers typically work an 8-hour day, 5 days per week. Research has shown that when controllers remain "in position" for more than two hours, even at low traffic levels, performance can deteriorate rapidly, so they are typically placed "in position" for 30-minute intervals (with 30 minutes between intervals).

These practices and policies can include selecting an appropriate shift schedule or rota and using an employee scheduling software to maintain it, setting the length of shifts, managing overtime, increasing lighting levels, providing shift worker lifestyle training, retirement compensation based on salary in the last few years of employment (which can encourage excessive overtime among older workers who may be less able to obtain adequate sleep), or screening and hiring of new shift workers that assesses adaptability to a shift work schedule. [36] Mandating a minimum of 10 hours between shifts is an effective strategy to encourage adequate sleep for workers. Allowing frequent breaks and scheduling 8 or 10-hour shifts instead of 12 hour shifts can also minimize fatigue and help to mitigate the negative health effects of shift work. [6]

Miners waiting to go to work on the 4 P.M. to midnight shift at the Virginia-Pocahontas Coal Co., 1974 GROUP OF MINERS WAITING TO GO TO WORK ON THE 4 P.M. TO MIDNIGHT SHIFT AT THE VIRGINIA-POCAHONTAS COAL COMPANY MINE ^4... - NARA - 556348.jpg
Miners waiting to go to work on the 4 P.M. to midnight shift at the Virginia-Pocahontas Coal Co., 1974

Multiple factors need to be considered when developing optimal shift work schedules, including shift timing, length, frequency and length of breaks during shifts, shift succession, worker commute time, as well as the mental and physical stress of the job. [37] Even though studies support 12 hour shifts are associated with increased occupational injuries and accident (higher rates with subsequent, successive shifts) [38] , a synthesis of evidence cites the importance of all factors when considering the safety of a shift. [39]

Shift work was once characteristic primarily of the manufacturing industry, where it has a clear effect of increasing the use that can be made of capital equipment and allows for up to three times the production compared to just a day shift. It contrasts with the use of overtime to increase production at the margin. Both approaches incur higher wage costs. Although 2nd-shift worker efficiency levels are typically 3–5% below 1st shift, and 3rd shift 4–6% below 2nd shift, the productivity level, i.e. cost per employee, is often 25% to 40% lower on 2nd and 3rd shifts due to fixed costs which are "paid" by the first shift. [40]

Shift system

The 42-hour work-week allows for the most even distribution of work time. A 3:1 ratio of work days to days off is most effective for eight-hour shifts, and a 2:2 ratio of work days to days off is most effective for twelve-hour shifts. [35] [41] [42] Eight-hour shifts and twelve-hour shifts are common in manufacturing and health care. Twelve-hour shifts are also used with a very slow rotation in the petroleum industry. Twenty-four-hour shifts are common in health care and emergency services. [20]

Shift schedule and shift plan

The shift plan or rota is the central component of a shift schedule. [35] The schedule includes considerations of shift overlap, shift change times and alignment with the clock, vacation, training, shift differentials, holidays, etc., whereas the shift plan determines the sequence of work and free days within a shift system.

Rotation of shifts can be fast, in which a worker changes shifts more than once a week, or slow, in which a worker changes shifts less than once a week. Rotation can also be forward, when a subsequent shift starts later, or backward, when a subsequent shift starts earlier. [20] Evidence supports forward rotating shifts are more adaptable for shift workers' circadian physiology. [37]

One main concern of shift workers is knowing their schedule more than two weeks at a time. Shift work is stressful. When on a rotating or ever changing shift, workers have to worry about daycare, personal appointments, and running their households. Many already work more than an eight hour shift. Some evidence suggests giving employees schedules more than a month in advance would give proper notice and allow planning, their stress level would be reduced. [43]


Though shift work itself remains necessary in many occupations, employers can alleviate some of the negative health consequences of shift work. The United States National Institute for Occupational Safety and Health recommends employers avoid quick shift changes and any rotating shift schedules should rotate forward. Employers should also attempt to minimize the number of consecutive night shifts, long work shifts and overtime work. A poor work environment can exacerbate the strain of shift work. Adequate lighting, clean air, proper heat and air conditioning, and reduced noise can all make shift work more bearable for workers. [44]

Good sleep hygiene is recommended. [6] This includes blocking out noise and light during sleep, maintaining a regular, predictable sleep routine, avoiding heavy foods and alcohol before sleep, and sleeping in a comfortable, cool environment. Alcohol consumption, caffeine consumption and heavy meals in the few hours before sleep can worsen shift work sleep disorders. [6] [8] Exercise in the three hours before sleep can make it difficult to fall asleep. [6]

Free online training programs are available to educate workers and managers about the risks associated with shift work and strategies they can use to prevent these. [45]


A clock-based device for recording workers' working hours, from the beginning of 20 century. Exhibit of the National Polytechnic Museum in Sofia ATG - NPTM Photothon 53.jpg
A clock-based device for recording workers' working hours, from the beginning of 20 century. Exhibit of the National Polytechnic Museum in Sofia

Algorithmic scheduling of shift work can lead to what has been colloquially termed as "clopening" [46] where the shift-worker has to work the closing shift of one day and the opening shift of the next day back-to-back resulting in short rest periods between shifts and fatigue. Co-opting employees to fill the shift roster helps to ensure that the human costs [47] are taken into account in a way which is hard for an algorithm to do as it would involve knowing the constraints and considerations of each individual shift worker and assigning a cost metric to each of those factors. [48] Shift based hiring which is a recruitment concept that hires people for individual shifts, rather than hiring employees before scheduling them into shifts enables shift workers to indicate their preferences and availabilities for unfilled shifts through a shift-bidding mechanism. Through this process, the shift hours are evened out by human-driven market mechanism rather than an algorithmic process. This openness can lead to work hours that are tailored to an individual's lifestyle and schedule while ensuring that shifts are optimally filled, in contrast to the generally poor human outcomes of fatigue, stress, estrangement with friends and family and health problems that have been reported with algorithm-based scheduling of work-shifts. [49] [50]

Missing income is also a large part of shift worker. Several companies run twenty-four hour shifts. Most of the work is done during the day. When the work dries up, usually it’s the second and third shift workers that pay the price. They are told to punch out early or use PTO if they have any, to make up the difference in their paychecks. This practice costs the average worker $92.00 a month. [51]


Melatonin may increase sleep length during both daytime and nighttime sleep in people who work night shifts. Zopiclone has also been investigated as a potential treatment, but it is unclear if it is effective in increasing daytime sleep time in shift workers. There are, however, no reports of adverse effects. [32]

Modafinil and R-modafinil are useful to improve alertness and reduce sleepiness in shift workers. [32] [52] Modafinil has a low risk of abuse compared to other similar agents. [53] However, 10% more participants reported adverse effects (nausea and headache) while taking modafinil. In post-marketing surveillance, modafinil was associated with Stevens–Johnson syndrome. The European Medicines Agency withdrew the license for modafinil for shift workers for the European market because it judged that the benefits did not outweigh the adverse effects. [32]

Using caffeine and naps before night shifts can decrease sleepiness. Caffeine has also been shown to reduce errors made by shift workers. [32]


Worker Health Charts provides a distribution of shift work by industry from 2015 NHIS data. WHC Shift Work Image.png
Worker Health Charts provides a distribution of shift work by industry from 2015 NHIS data.

According to data from the National Health Interview Survey and the Occupational Health Supplement, 27% of all U.S. workers in 2015 worked an alternative shift (not a regular day shift) and 7% frequently worked a night shift. Prevalence rates were higher for workers aged 18–29 compared to other ages. Those with an education level beyond high school had a lower prevalence rate of alternative shifts compared to workers with less education. Among all occupations, protective service occupations had the highest prevalence of working an alternative shift (54%). [55]

One of the ways in which working alternative shifts can impair health is through decreasing sleep opportunities. Among all workers, those who usually worked the night shift had a much higher prevalence of short sleep duration (44.0%, representing approximately 2.2 million night shift workers) than those who worked the day shift (28.8%, representing approximately 28.3 million day shift workers). An especially high prevalence of short sleep duration was reported by night shift workers in the transportation and warehousing (69.7%) and health-care and social assistance (52.3%) industries. [56]


It is estimated that 15-20% of workers in industrialized countries are employed in shift work. [8] Shift work is common in the transportation sector as well. Some of the earliest instances appeared with the railroads, where freight trains have clear tracks to run on at night.

Shift work is also the norm in fields related to public protection and healthcare, such as law enforcement, emergency medical services, firefighting, security and hospitals. Shift work is a contributing factor in many cases of medical errors. [8] Shift work has often been common in the armed forces. Military personnel, pilots, and others that regularly change time zones while performing shift work experience jet lag and consequently suffer sleep disorders. [8]

Those in the field of meteorology, such as the National Weather Service and private forecasting companies, also use shift work, as constant monitoring of the weather is necessary. Much of the Internet services and telecommunication industry relies on shift work to maintain worldwide operations and uptime.

Service industries now increasingly operate on some shift system; for example a restaurant or convenience store will normally be open on most days for much longer than a working day.

There are many industries requiring 24/7 coverage that employ workers on a shift basis, including:

See also

Related Research Articles

Modafinil Medication

Modafinil, sold under the brand name Provigil among others, is a medication to treat sleepiness due to narcolepsy, shift work sleep disorder, or obstructive sleep apnea (OSA). In OSA continuous positive airway pressure is the preferred treatment. While it has seen off-label use as a purported cognitive enhancer, the research on its effectiveness for this use is not conclusive. It is taken by mouth.

Jet lag is a physiological condition which results from alterations to the body's circadian rhythms caused by rapid long-distance trans-meridian travel. For example, someone flying from New York to London, i.e. from west to east, feels as if the time were five hours earlier than local time, and someone travelling from London to New York, i.e. from east to west, feels as if the time were five hours later than local time. Jet lag was previously classified as one of the circadian rhythm sleep disorders.

Delayed sleep phase disorder chronic mismatch between a persons normal daily rhythm, compared to other people and societal norms

Delayed sleep phase disorder (DSPD), more often known as delayed sleep phase syndrome and also as delayed sleep–wake phase disorder, is a chronic dysregulation of a person's circadian rhythm, compared to those of the general population and societal norms. The disorder affects the timing of sleep, peak period of alertness, the core body temperature rhythm, and hormonal and other daily cycles. People with DSPD generally fall asleep some hours after midnight and have difficulty waking up in the morning. People with DSPD probably have a circadian period significantly longer than 24 hours. Depending on the severity, the symptoms can be managed to a greater or lesser degree, but no cure is known, and research suggests a genetic origin for the disorder.

Biphasic sleep is the practice of sleeping during two periods over 24 hours, while polyphasic sleep refers to sleeping multiple times – usually more than two. Each of these is in contrast to monophasic sleep, which is one period of sleep over 24 hours. Segmented sleep and divided sleep may refer to polyphasic or biphasic sleep, but may also refer to interrupted sleep, where the sleep has one or several shorter periods of wakefulness. A common form of biphasic or polyphasic sleep includes a nap, which is a short period of sleep, typically taken between the hours of 9 am and 9 pm as an adjunct to the usual nocturnal sleep period.

Somnolence is a state of strong desire for sleep, or sleeping for unusually long periods. It has distinct meanings and causes. It can refer to the usual state preceding falling asleep, the condition of being in a drowsy state due to circadian rhythm disorders, or a symptom of other health problems. It can be accompanied by lethargy, weakness, and lack of mental agility.

Adrafinil pharmaceutical drug

Adrafinil is a eugeroic that was formerly used in France to promote alertness, attention, wakefulness, mood, and other parameters, particularly in the elderly. It was also used off-label by individuals who wished to avoid fatigue, such as night workers or others who needed to stay awake and alert for long periods of time. Additionally, "adrafinil is known to a larger nonscientific audience, where it is considered to be a nootropic agent."

Sleep hygiene is the recommended behavioral and environmental practice that is intended to promote better quality sleep. This recommendation was developed in the late 1970s as a method to help people with mild to moderate insomnia, but, as of 2014, the evidence for effectiveness of individual recommendations is "limited and inconclusive". Clinicians assess the sleep hygiene of people who present with insomnia and other conditions, such as depression, and offer recommendations based on the assessment. Sleep hygiene recommendations include establishing a regular sleep schedule, using naps with care, not exercising physically or mentally too close to bedtime, limiting worry, limiting exposure to light in the hours before sleep, getting out of bed if sleep does not come, not using bed for anything but sleep and sex, avoiding alcohol as well as nicotine, caffeine, and other stimulants in the hours before bedtime, and having a peaceful, comfortable and dark sleep environment.

Non-24-hour sleep–wake disorder is one of several chronic circadian rhythm sleep disorders (CRSDs). It is defined as a "chronic steady pattern comprising [...] daily delays in sleep onset and wake times in an individual living in society". Symptoms result when the non-entrained (free-running) endogenous circadian rhythm drifts out of alignment with the light/dark cycle in nature. Although this sleep disorder is more common in blind people, affecting up to 70% of the totally blind, it can also affect sighted people. Non-24 may also be comorbid with bipolar disorder, depression, and traumatic brain injury. The American Academy of Sleep Medicine (AASM) provides guidelines since 2007 with the latest update released in 2015.

Armodafinil chemical compound

Armodafinil is the enantiopure compound of the eugeroic modafinil (Provigil). It consists of only the (R)-(−)-enantiomer of the racemic modafinil. Armodafinil is produced by the pharmaceutical company Cephalon Inc. and was approved by the U.S. Food and Drug Administration (FDA) in June 2007. In 2016, the FDA granted Mylan rights for the first generic version of Cephalon's Nuvigil to be marketed in the U.S.

Excessive daytime sleepiness 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 encompassing several sleep disorders where increased sleep is a symptom, or as a symptom of another underlying disorder like narcolepsy, idiopathic hypersomnia, sleep apnea or a circadian rhythm sleep disorder.

Light effects on circadian rhythm are the effects that light has on circadian rhythm.

Fatigue is a major human factors issue in aviation safety. The Fatigue Avoidance Scheduling Tool (FAST) was developed by the United States Air Force in 2000–2001 to address the problem of aircrew fatigue in aircrew flight scheduling. FAST is a Windows program that allows scientists, planners and schedulers to quantify the effects of various work-rest schedules on human performance. It allows work and sleep data entry in graphic, symbolic (grid) and text formats. The graphic input-output display shows cognitive performance effectiveness as a function of time. An upper green area on the graph ends at the time for normal sleep, 90% effectiveness. The goal of the planner or scheduler is to keep performance effectiveness at or above 90% by manipulating the timing and lengths of work and rest periods. A work schedule is entered as red bands on the time line. Sleep periods are entered as blue bands across the time line, below the red bands.

Effects of fatigue on safety human tiredness as a hazard

Fatigue is a major safety concern in many fields, but especially in transportation, because fatigue can result in disastrous accidents. Fatigue is considered an internal precondition for unsafe acts because it negatively affects the human operator's internal state. Research has generally focused on pilots, truck drivers, and shift workers.

In chronobiology, a circasemidian rhythm is a physiological arousal cycle that peaks twice in a 24-hour day. Numerous studies have demonstrated that human circadian rhythms in many measures of performance and physiological activity have a 2-peak daily (circasemidian) pattern. The word, circasemidian, is based upon the Latin words circa ("about"), semi ("half") and dia ("day"). Thus, this is a rhythm that has two cycles per day, and some investigators have referred to it as the semicircadian rhythm. It usually serves to (1) deepen the pre-dawn nadir in body temperature and cognitive performance, (2) create a flat spot during the early afternoon in the daytime increase in body temperature and cognitive performance, and (3) heighten the early-evening peak in body temperature and cognitive performance. Broughton was the first to bring this characteristic of human performance to the attention of researchers.

Sleep inversion or sleep-wake inversion is a reversal of sleeping tendencies. Individuals experiencing sleep-wake inversion exchange diurnal habits for nocturnal habits, meaning they are active at night and sleep during the day. Sleep-wake inversion, when involuntary, can be a sign of a serious disorder.

Studies, which include laboratory investigations and field evaluations of population groups that are analogous to astronauts, provide compelling evidence that working long shifts for extended periods of time contributes to sleep deprivation and can cause performance decrements, health problems, and other detrimental consequences, including accidents, that can affect both the worker and others.

Sleep in space

Sleeping in space is an important part of space medicine and mission planning, with impacts on the health, capabilities and morale of astronauts.


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Further reading