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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 (abbreviated as 24/7). The practice typically sees the day divided into "shifts", set periods of time during which different groups of workers take up their posts. The term "shift work" includes both long-term night shifts and work schedules in which employees change or rotate shifts.
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.
Shift work can also contribute to strain of marital, family, and personal relationships.
Shift work increases the risk for the development of many disorders, including:
- Shift work sleep disorder (SWSD): Shift work sleep disorder is a circadian rhythm sleep disorder characterized by insomnia and/or excessive sleepiness. Shift work is considered essential for the diagnosis.
- Diabetes mellitus: The risk of diabetes mellitus is increased in shift workers, especially in men. People working rotating shifts are more vulnerable than others.
- Breast cancer: A 2005 review of earlier studies came to the result that women whose work involve night shifts have a 50% increased risk of developing breast cancer. This may be due to alterations in circadian rhythm: melatonin, a known tumor suppressant, is generally produced at night and late shifts may disrupt its production. The WHO's International Agency for Research on Cancer listed "shiftwork that involves circadian disruption" as a probable carcinogen in 2007 (IARC Press release No. 180).
- Possibly other types of cancer 
- Cluster headache
- Ischemic heart disease: Shift workers who had worked in that method for 15 years or more were three times more likely to develop ischemic heart disease.
- Poor sexual performance
- Appetite control: Shift work has been associated with a higher disposition for developing nutritional issues and obesity.
Shift work also can exacerbate symptoms and progression of chronic diseases, such as sleep disorders, digestive diseases, heart disease, hypertension, epilepsy, mental disorders, substance abuse, asthma, and health conditions that require medications with circadian changes in effectiveness.
UCSF neurologist Louis Ptacek, who studies circadian rhythms, genes and sleep behaviors, has been quoted: "It's not surprising, we have evolved on a planet that is rotating every 24 hours. Our internal clock is more than just when we sleep and wake. It's related to cell division and it regulates our immune systems. When we battle our internal clock, that has complications." Artificial lighting may additionally contribute to disturbed homeostasis.
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.
Many shift workers use stimulants such as caffeine to stay awake at work and/or sleeping pills to aid with sleep during the day. However, both are addictive and should be used with care. There is currently no research on shiftworkers and long-term use of sleeping pills.
Compared with the day shift, incidents have been estimated to increase by 15% on evening shifts and 28% on night shifts.
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.
The circadian phase is relatively fixed in humans; attempting to shift it so that an individual is alert during the circadian bathyphase is difficult. Sleep during the day is shorter and less consolidated than night-time sleep.
The effects of sleep inertia wear off after 2–4 hours of wakefulness, such that most workers who wake up in the morning and go to work suffer some degree of sleep inertia at the beginning of their shift. The relative effects of sleep inertia vs. the other factors are hard to quantify; however, the benefits of napping appear to outweigh the cost associated with sleep inertia.
Acute sleep deprivation occurs during long shifts with no breaks, as well as during night shifts when the worker sleeps in the morning and is awake during the afternoon, prior to the work shift. A night shift worker with poor daytime sleep may be awake for more than 18 hours by the end of his shift. The effects of acute sleep deprivation can be compared to impairment due to alcohol intoxication, with 19 hours of wakefulness corresponding to a BAC of 0.05%, and 24 hours of wakefulness corresponding to a BAC of 0.10%. Much of the effect of acute sleep deprivation can be countered by napping, with longer naps giving more benefit than shorter naps. Some industries, specifically the Fire Service, have traditionally allowed workers to sleep while on duty, between calls for service. In one study of EMS providers, 24-hour shifts were not associated with a higher frequency of negative safety outcomes when compared to shorter shifts.
Chronic sleep deficit occurs when a worker sleeps for fewer hours than is necessary over multiple days or weeks. The loss of two hours of nightly sleep for a week causes an impairment similar to those seen after 24 hours of wakefulness. After two weeks of such deficit, the lapses in performance are similar to those seen after 48 hours of continual wakefulness. The number of shifts worked in a month by EMS providers was positively correlated with the frequency of reported errors and adverse events. A study conducted by Defense Research & Development Canada found that "with respect to sustaining cognitive performance in the face of nocturnal alarms, clearly schedule 4 [24/72] is the best schedule and schedule 5 [another schedule with 24-hour shifts] is the second best." The study also compared three schedules with day and night shifts, and found the deleterious effects of multiple night shifts to be greater than those associated with long shifts.
Safety and productivity
A meta-analysis of incident data across various shiftwork operations revealed a number of important trends.
Concerning shift length, the combined data from three field studies indicated that “...risk [of injuries and accidents] increased in an approximately exponential fashion with time on shift such that in the twelfth hour it was more than double that during the first 8 h” (42). The relative risk rises sharply over the final four hours of the shift.
The effects of the circadian rhythm and circasemidian rhythm on accuracy and efficiency in the 24/7 workplace was obvious in the combined data from three field studies which showed that “ ... ‘real-job’ speed and accuracy measures are only above average between 0700 h and 1900 h [7 a.m. and 7 p.m.]; at all other times efficiency is likely to be relatively impaired, especially so during the early hours of the morning.” The data showed a nadir at 3 a.m. and a low spot at 2 p.m. These are the predicted effects of the two combined rhythms.
The data from eight field studies showed that “Risk [of injuries and accidents] was found to increase in an approximately linear fashion across the three shifts, ... 18.3% on the afternoon shift ... 30.4% on the night shift, relative to that on the morning shift.”
Data from seven studies of night shifts revealed that “...risk [of injuries and accidents] was ~6% higher on the second night, 17% higher on the third night and 36% higher on the fourth night.” This was compared to successive day shifts in five of the seven companies, where the increases were “... ~2% higher on the second morning/day, 7% higher on the third morning/day, and 17% higher on the fourth morning/day than on the first shift.” Thus, the relative penalty for successive night shifts, compared to successive day shifts, was a factor of about 2.5 times greater safety risk.
Though shift work itself remains necessary in many occupations, through various methods employers can alleviate some of the negative health consequences of shift work. The United States National Institute of Occupational Safety and Health recommends employers avoid quick shift changes, permanent night shifts, and several days of work offset by several days off. Employers should also attempt to minimize the number of consecutive night shifts, long work shifts and overtime work. Having a regular, predictable schedule can also diminish the negative health effects of shift work. A poor work environment can exacerbate the strain of shiftwork. Adequate lighting, clean air, proper heat and air conditioning, and reduced noise can all make shift work more bearable for workers. Also, appropriate sleep hygiene is recommended.
Employees can mitigate the effects of shift work by blocking out noise at home, maintaining a regular sleep routine, and avoiding heavy foods and alcohol before sleep. Exercise in the three hours before sleep can make it difficult to fall asleep, so workers might exercise before work instead.
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.
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 be fairly obvious: 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, or providing shift worker lifestyle training to help shift workers better handle issues such as understanding basic circadian physiology, sleep and napping, caffeine usage, social life issues, diet and nutrition, etc. They may also be more indirect: 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.
A day may be divided into three shifts, each of 8 hours, and each employee works just one of those shifts; they might, for example, be 06:00 to 14:00, 14:00 to 22:00, 22:00 to 06:00. Generally, "first shift" refers to the day shift, with "second shift" running from late afternoon to midnight or so, and "third shift" being the night shift. On occasion, more complex schedules are used, sometimes involving employees changing shifts, in order to operate during weekends as well, in which case there will be four or more sets of employees.
12-hour work shifts are also in use. In a modern steelworks, four sets of personnel are used, working consecutive days in one 12-hour shift (06:00 – 18:00 and vice versa). Shift A will work days, and shift B nights, over a 48-hour period, before handing over to shifts C and D and taking 48 hours off. In the offshore petroleum industry, employees may work 14 consecutive days or nights, 06:00 – 18:00 or 18:00 – 06:00, followed by three or four weeks free. The svingskift (literally: "swing shift") in the offshore petroleum industry in Norway refers to a two-week tour during which employees work 12-hour days the first seven days and 12-hour nights the second (or vice versa).
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.
In general, requiring workers to live on a time-shifted schedule for extended periods is unpopular, and this typically must be paid for at a premium. It is common in heavy industry, particularly automobile and textile manufacturing and is becoming more common in locations where a shut-down of equipment would incur an extensive restart process. Food manufacturing plants, in particular, have extensive cleaning programs that are required before any restart. The use of shift work in manufacturing varies greatly from country to country. Shift work is common in the transportation sector as well. Some of the earliest instances appeared with the railroads, where freight trains have clear (i.e. passenger-free) tracks to run on at night. Shift work has been traditional in law enforcement and the armed forces: for example sailors must be available to handle a vessel around the clock, and a system of naval watches organized to ensure enough hands are on duty at any time. This is shift work by another name.
Service industries now increasingly operate on some shift system; for example a restaurant or convenience store will normally each day be open for much longer than a working day. Shift work is also the norm in governmental and private employment in fields related to public safety and healthcare, such as Emergency Medical Services, police, firefighting, security and hospitals. Companies working in the field of meteorology, such as the National Weather Service and private forecasting companies, also utilize shift work, as constant monitoring of the weather is necessary.
Shiftwork investigators examined four-crew shift systems in great detail with respect to two constraints: eight-hour work days and 104 days off per year (equal to 52 weekends per year). They showed that the 40-hour work week is cumbersome and limiting when you try to create a shiftwork plan, and that the 42-hour work week allows even distributions of work time across workers on all shifts. The latter is true because the week length, 168 hours, is factorable by 42 hours (4 x 42 hours = 168 hours).
Having established that the four-crew solution with a 42-hour work-week worked quite well, the investigators went on to determine the best ratios of work days and free days. For eight-hour shifts, use a ratio of 3 work days to 1 free day (3:1). For twelve-hour shifts, use a ratio of 2 work days to 2 free days (2:2)
The shift system sets the relative numbers of work and free periods that must be worked to support 24/7 operations. A system is expressed as a ratio. The investigators above specified two optimal systems. The ratio of 3 work days to 1 free day (3:1) for eight-hour shifts is expressed as the system, 3 nW:1nF. The ratio of 2 work days to 2 free days (2:2) for twelve-hour shifts is expressed as the system, 2nW:2nF where n is an integer multiplier.
The shift system examination also provided two useful measures for determining the acceptability of a shift system: the average number of hours worked per day and the number of free days per year.
Shift schedule and shift plan
The shift plan or rota is the central component of a shift schedule. 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.
According to data from the National Health Interview Survey Occupational Health Survey, 29% of all U.S. workers in 2010 worked an alternative shift (not a regular day shift). Prevalence rates were higher for workers aged 18–29 compared to other ages. Those with a Bachelor’s degree and higher had a lower prevalence rate of alternative shifts compared to workers with less education. Among all occupations, food preparation and serving occupations had the highest prevalence of working an alternative shift (63%).
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.
Industries requiring 24/7 cover are those that employ workers on a shift basis:
- Customer service including call centers
- Death care (medical examiner or coroner)
- Emergency response systems
- Flight Test
- Health care
- Public utilities
- Circadian rhythm sleep disorders
- Eight-hour day
- Fatigue Avoidance Scheduling Tool
- Schedule (workplace)
- Shift work sleep disorder
- Split shift
- Gantt chart
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