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NATIONAL SLEEP FOUNDATION — Nearly one-quarter of all workers have shifts that are not during the daytime, and more than two-thirds of these workers have problem sleepiness and/or difficulty sleeping. (NHLBI, 2005)

One-third of shift workers state that they sleep less than six hours per night on workdays, and 30 percent report that they only get a good night’s sleep a few night’s per month or less. (NSF, 2008)

Some studies find that shift workers experience impairment caused by sleepiness at levels equivalent to that of being drunk. (Arnedt et al, 2005)

Shift workers are more prone to developing certain illnesses, including cancer, cardiovascular disease, diabetes, obesity, and gastrointestinal disorders. (Drake and Wright, 2011; Folkard and Tucker, 2003)

The International Agency for Research on Cancer has classified “shift work that involves circadian disruption” as a probable human carcinogen. (Stevens et al, 2010)

Some workers experience sleep disturbance and sleepiness even after months or years of shift work. (Drake and Wright, 2011)

Our increasingly 24/7, globalized work world demands that some people work at night, have extended work hours, or rotate between working night, day, and evening shifts. Nearly one-quarter of all workers have shifts that are not during the daytime, and more than two-thirds of these workers have problem sleepiness and/or difficulty sleeping. (NHLBI, 2005) In addition, nearly a third of Americans report working 10 hours or more each day. (NSF, 2008) Such extensive work hours can affect how much time workers have for sleep, as many on a limited time budget sacrifice sleep for needed leisure time, attention to domestic tasks, or multiple jobs.

Irregular or long hours put many workers at odds for acquiring enough quality sleep to function properly and operate machinery and vehicles safely. The drowsiness experienced by night shift workers can be especially dangerous. Not only are night-shift workers prone to serious driving accidents after their shifts end, but major industrial accidents, such as the Three Mile Island and the Exxon Valdez oil spill, have been caused, in part, by mistakes made by excessively sleepy workers on a night or extended shift. (NHLBI, 2005) In addition, night shift workers are often employed in the most dangerous jobs, such as firefighting, emergency medical services, law enforcement and security, or in professions, such as nursing, medical practice, and commercial driving, which require them to perform at their best so as not to endanger others.

Shift work can also affect workers’ quality and length of life by making them more prone to various disorders, including cardiovascular disease, digestive disorders, diabetes, and cancer. In addition, shift work has adverse effects on worker productivity. Frequently, shift work is instituted for economic reasons, i.e. in order to maximize the use of costly equipment. But the deleterious effects of shift work has led some experts to question whether the economic savings linked to using shift workers in certain industries outweighs how it hampers the health and well being of these workers and their productivity, as well as the overall costs to society due to sleep-induced accidents. (Drake and Wright, 2011; Folkard and Tucker, 2003)

Key to making worthwhile policies related to shift work is developing an understanding of how irregular and long hours affect performance, productivity, and the physical and mental health of workers, all of which will be explored in this paper.

Biological clock goes contrary to worker’s time clocks
All people need between 7 and 9 hours of uninterrupted sleep a night to feel well rested and function at their fullest. (NHLBI, 2005) The body has built-in mechanisms to ensure we get that sleep, including a biochemical means of tracking how much time we spend sleeping or being awake. When a sleep debt builds up, this biochemical tally triggers excessive sleepiness and the urge to sleep. In addition, natural circadian rhythms make us more likely to feel drowsy in the dark early hours of the day, and more awake during the light hours. Consequently, most people do their best work during the day. (NHLBI, 2005) Even night workers who have adequate sleep feel drowsy at work during the low point of this natural circadian rhythm in alertness.

In addition to circadian rhythms, several other factors conspire to make shift workers less likely to have adequate sleep, including difficulties sleeping during daylight hours because of the need to attend to domestic duties, or the needs of another job, lack of a quiet, dark bedroom, and back-to-back or rotating shifts that limit sleep opportunities or ability to adjust. There is also a physiological alerting process during the day that hampers sleep at that time and contributes to sleep fragmentation. Although naps help make up for the loss of sleep shift workers experience, they do not adequately repay sleep debt, and in some cases are not long enough for people to reap the benefits of the deeper sleep stages. Consequently, one-third of shift workers state that they sleep less than six hours per night on workdays, and 30 percent report that they only get a good night’s sleep a few night’s per month or less. (NSF, 2008)

Some speculate that night shift workers will eventually adjust to their odd hours, especially if they maintain being awake at night on their days off. But there is little support for this contention. (Folkard, 2008) Instead, research reveals that night shifts result in greater loss of total sleep time that accumulates over successive night shifts. (Pilcher et al, 2000; Park et al, 2000) Some workers experience sleep disturbance and sleepiness even after months or years of shift work. (Drake and Wright, 2011)

Reduced alertness and accidents
The lack of sleep shift workers typically experience makes them less alert and more prone to accidents. Studies have linked sleepiness and fatigue to decreases in vigilance, reaction time, memory, psychomotor coordination, information processing, and decision making, all of which are needed to safely and effectively perform a variety of work tasks, as well as to safely drive home from work. (Lyznicki et al, 1998) Some studies find the degree of impairment experienced by shift workers can easily reach levels equivalent to that of being drunk. For example, one study found that medical residents on heavy call rotations had driving impairments similar to those on light call rotations with a blood alcohol concentrations of .05%. (Arnedt et al, 2005)

Consequently, shift workers are more prone to driving accidents. One study found that more than three quarters (79.5%) of nurses working the night shift were 4 times more likely to have a drowsy driving incident that nurses working a day shift. (Scott, et al, 2007) Residents frequently on call have nearly 7 times the risk of motor vehicle accidents compared with those working less demanding work schedules. (Kowalenko, T. et al, 2007) Commercial truck drivers are especially susceptible to drowsy driving. One study found these drivers in the United States and Canada averaged less than 5 hours of sleep per day and frequently drive during night hours. (Federal Motor Carrier Safety Administration, 1996) The National Transportation Safety Board (NTSB) determined that drowsy driving was probably the cause of more than half of crashes leading to a truck driver’s death. (NTSB, 1990a,b) For each truck driver fatality, another three to four people are killed. (NHTSA, 1994)

Shift work also makes workers more prone to workplace accidents, with studies showing a boosted rate of accidents linked to the use of sharp instruments and items by medical personnel, as well as medication and diagnostic errors and increased patient death. (Ayas, et al 2006; Lockley, et al, 2006; Barger, et al, 2006.) In industrial settings, the risk of both accidents and injuries increases by more than 30 percent on night shifts, rises over successive night shifts, and increases exponentially with successive hours on a shift. (Folkard, et al, 2003)

Shift work productivity
Presumably due to the influence of circadian rhythms and lack of sleep, studies show that productivity of workers is highest during daylight hours. Switchboard operators, meter readers, and textile workers have the highest productivity between 9 a.m. and 6 p.m., after which their productivity dips progressively to a low point occurring 3-5 a.m. Productivity levels do not return to average levels until after 6 a.m. (Folkhard, 2003) A more recent study of medical resident productivity in an emergency room found that residents working 12-hour shifts evaluated significantly less patients per hour than residents working 9-hour shifts. (Jeanmonod et al, 2008) Shift work has also been linked to reduced dexterity and/or productivity in a variety of other work settings. (Drake and Wright, 2011) Night workers are also more likely to have increased absenteeism compared with day workers, especially those experiencing sleep problems. (Drake, et al, 2004)

Shift work and physical health
Shift workers are more prone to developing certain illnesses, including cancer, cardiovascular disease, diabetes, obesity, and gastrointestinal disorders. A number of factors may contribute to these increased risks, including artificial light or lack of sleep inhibiting the normal rise or fall of various hormones during the dark hours of the night. For example, we produce more of the hormone melatonin during the night, and exposure to artificial light inhibits this production. Melatonin has been shown to have a number of positive effects on the body, including lowering blood pressure and decreasing blood clotting, which can help prevent heart attacks and strokes. (Brown, et al, 2009) Melatonin also limits the body’s production of estrogen and other hormones known to fuel breast, prostate, and endometrial cancer. (Viswanathan and Schernhammer, 2009; Costa, et al, 2010)

Shift workers are more prone to developing a number of cancers, including breast, endometrial, prostate, and colorectal cancer. (Costa et al, 2010; Stevens, 2009) Large studies have found that women shift workers have up to a 60 percent increased risk of breast cancer, (Davis, et al, 2001; Schernhammer et al, 2001) and a 35 percent greater risk of colorectal cancer. (Schernhammer et al, 2003) On the basis of evidence in both experimental animals and humans, the International Agency for Research on Cancer has classified “shift work that involves circadian disruption” as a probable human carcinogen. (Stevens et al, 2010)

Cardiovascular disease
The longer people work nights or rotating shifts, the more likely they are to develop cardiovascular disease and experience a heart attack or stroke, several studies reveal. (Haupt, et al, 2008; Knutsson, 2008, Knutsson, 1986; Brown et al, 2009) For example, one study found rotating night shift work was linked to a 4 percent increased risk of stroke for every 5 years worked. (Brown et al, 2009) Another 14-year study found that rotating shift workers were 30 percent more likely to develop elevated blood cholesterol levels than daytime workers. (Dochi, et al, 2009) In addition, nurses who did 6 years or more of rotating night shifts were 51 percent more likely to develop coronary heart disease than those nurses who worked the day shift. (Kawachi, et al, 1995)

Obesity and diabetes
Extended disruption of darkness during the late evening and early morning hours also disrupts daily rhythms governing production of the various hormones that affect appetite, diabetes and obesity. (Scheer, et al, 2009) Consequently, several studies find that people short on sleep are more likely to be obese or develop diabetes. (Knutson and Van Cauter, 2008) In addition, some studies find those that work the night shift and/or extended hours are more likely to develop metabolic syndrome, which is a precursor to diabetes. (Violanti, et al, 2009; Biggi et al, 2008) There is also some evidence that long work hours are linked to obesity (DiMilia and Mummery, 2009)

Gastrointestinal disorders
Several studies have found a greater prevalence of gastrointestinal symptoms, such as diarrhea, constipation, or abdominal pain, among shift workers, when compared to those who work days. (Nojkov et al, 2010) One study found that rotating shift nurses were 30 percent more likely to have abdominal pain than those working the day shift, and were 17 percent more likely to have irritable bowel syndrome. (Nojkov et al, 2010) Shift workers also have a fourfold increased risk of ulcers in their small intestine. (Pietroiusti, et al, 2006)

Shift-work disorder
Between one-quarter and one-third of shift workers are unable to tolerate the effects of their work schedules and have excessive sleepiness or insomnia despite regular and adequate time in bed, and a lack of other sleep disorders. This is called shift-work disorder, and researchers have shown this disorder adversely impacts job performance, driving safety, quality of life, work satisfaction, and both physical and mental health. (Drake et al, 2004; Axelsson, 2004) For example, shift workers with this disorder are more prone to ulcers, depression, absenteeism and difficulties with family and social activities compared with shift workers who lack shift-work disorder. (Drake et al, 2004)

There is some evidence that women who work at night are more likely to have trouble conceiving or to miscarry (Nurminen, 1998), but not all studies find this to be true. (Zhu et al, 2003; Tuntiseranee et al,1998) One study found that women who worked long work hours took longer to become pregnant than those who worked standard hours. (Tuntiseranee, et al, 1998) In men, there doesn’t appear to be a link between reduced fertility and working night or long shifts, although few studies have explored this possibility.

Shift work quality of life and mental health
Shift work can adversely affect workers’ quality of life and family relations. Studies show a 57 percent higher divorce rate, reduced job satisfaction and reduced family and social interactions among shift workers. (Drake, et al, 2004; Bambra, et al, 2008; White and Keith, 1990) One study showed a relationship between parental shift work and poor school performance or behavior in children 5 to 12 years old. (Hsueh and Yoshikawa, 2007) Shift workers who develop shift-work disorder (see below) are also more likely to be depressed. Drake, et al, 2004)

Shift work adversely affects worker productivity, performance, health and quality of life. Shift work also puts others at risk due to workplace or driving accidents.


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