Shift Work Disorder: Overview and Diagnosis
Thomas Roth, PhD
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Circadian Rhythm Sleep Disorders
According to the second edition of the American Academy of Sleep Medicine’s International Classification of Sleep Disorders (ICSD-2),1 the major feature of circadian rhythm sleep disorders is “a misalignment between the patient’s sleep pattern and the sleep pattern that is desired or regarded as the societal norm” (AV 1).
In addition to shift work disorder (SWD), the ICSD-2 lists 8 other types of circadian rhythm sleep disorders, including time zone change (jet lag) syndrome and delayed and advanced sleep phase syndromes. Many people have experienced jet lag syndrome, caused by a lack of synchrony between your internal clock and a new time zone in which you are trying to function. Circadian rhythm disturbances can also involve delayed or advanced sleep phases (AV 2).
Because delayed and advanced sleep phase syndromes often cause the person to be out of synchrony with the prevailing sleep/wake patterns of society, they can lead to significant morbidity. Recent studies2-5 have found that, when high school classes were started an hour or two later, the number of car accidents decreased and academic functioning improved. Problems can also arise when there is chronic dyssynchrony between the person’s internal clock and external light and dark (ie, when a person is required to stay awake and work when it is dark and sleep when it is light), which can, in some cases, lead to SWD.
It is important to distinguish between shift work and SWD. Shift work is a job description. The vast majority of people who work shifts adjust and do well. However, a subgroup of people have great difficulty adjusting their internal clocks and develop SWD due to a mismatch between the sleep/wake schedule required by their jobs and their own circadian sleep/wake cycles.
Prevalence. It is estimated that 15%–26% of the US labor force works night, evening, or rotating shifts (AV 3).6,7
Effects of shift work on the sleep/wake cycle. Shift work affects the sleep/wake cycle in a number of ways. No matter how many hours you have slept during the day, trying to work during the downside of the circadian rhythm (eg, between 12 AM and 6 AM) is very difficult unless you can shift your internal clock. Studies have found that, over a 24-hour cycle, both subjective alertness and cognitive functioning decline between 2 AM and 4 AM.8 Also, because you are not sleeping at night, the homeostatic pressure to sleep is not relieved, producing an ever-increasing pressure to sleep.9 However, only a subset of individuals who work night or rotating shifts develop SWD, because circadian rhythms are modulated not only by light and dark, but also by other factors such as “clock genes,” melatonin, and environmental cues (eg, noise).10,11
Shift Work Disorder: An Overview
Prevalence. Drake et al12 found that 28% of those who work night or rotating shifts, compared with 18% of day workers, experienced insomnia and/or excessive sleepiness, and they estimated the “true prevalence” of SWD to be approximately 10% of those who work night or rotating shifts. A study of 103 shift workers on a North Sea oil rig (working 2 weeks on 7 nights/7 days, 12-hour shifts, 4 weeks off) by Waage et al13 found a relatively high prevalence of SWD. They reported that 24 (23.3%) of the shift workers were suffering from SWD and that, during their 4-week period off work, the workers with SWD reported significantly poorer sleep quality, more subjective health complaints, and greater problems in coping than individuals who did not have SWD. Shift workers without SWD reported results similar to those of day workers on the rig with regard to sleep, sleepiness, subjective health complaints, and coping.
Diagnosis. The ICSD-2 diagnostic criteria for circadian rhythm sleep disorder, shift work type, are shown in (AV 4). The differential diagnosis of SWD includes excessive sleepiness due to obstructive sleep apnea, narcolepsy, restless legs syndrome, and chronic insufficient sleep due to daytime conflicts (eg, child care, environmental factors, “moonlighting” at a second job). Comorbid conditions (eg, increased prevalence of sleep apnea in shift workers) can complicate the diagnosis of SWD. Clinicians should also rule out comorbid disorders that can cause insomnia and excessive sleepiness (eg, primary insomnia, insomnia associated with psychiatric disorders such as major depression), as well as consider whether the person may be taking medications or abusing drugs or alcohol to help with sleep, which may be causing impairment at work.
Consequences of Shift Work Disorder
Just as animal studies have found that disruptions in circadian rhythm can affect health outcomes, studies in humans have produced similar findings.
Gastrointestinal problems (eg, ulcers, functional bowel disorders) are significantly increased in individuals who work night or rotating shifts. However, the increased prevalence of ulcers is associated not just with shift work, but also with SWD. In a study comparing 360 workers on rotating shifts, 174 on night shifts, and 2,036 on day shifts, Drake et al12 found that, among those who reported excessive sleepiness and/or insomnia, the prevalence of ulcers was higher among rotating shift workers (12.5%) and night shift workers (15.4%) than day workers (6%). This effect was not seen to any marked degree in those who worked rotating or night shifts but did not have excessive sleepiness and/or insomnia. Zhen Lu et al14 found that the prevalence of functional bowel disorders was higher in a sample of nurses who worked rotating shifts (38%) than in those who worked day shifts (20%) and that functional bowel disorder symptoms were positively correlated with level of sleep disturbance.
Cancer. Shift work (whether or not the person has SWD) has been found to be a risk factor for cancer. Increased odds ratios for breast cancer have been found in large samples of women who worked night shifts, particularly with increasing duration of nighttime employment.15-19 A study20 of 14,052 working men in Japan also found a significantly increased risk of prostate cancer in those who worked rotating shifts. The World Health Organization International Agency for Research on Cancer has concluded, “Shift work that involves circadian dysruption is probably carcinogenic to humans.”21
Depression. The prevalence of depression is significantly higher in those who work rotating and night shifts than in day workers. In addition, while insomnia or daytime sleepiness is a risk factor for depression for all individuals, it is a much greater risk factor for rotating or night shift workers.12
Cardiovascular effects. While insomnia is a risk factor for hypertension in all individuals, it is a significantly higher risk factor for shift workers with insomnia.12 In contrast, although shift work is associated with a significantly increased risk of heart disease compared with nonshift work, this increased risk is not associated with SWD.12
Excessive sleepiness and accidents. Insomnia is associated with excessive sleepiness, which can impair functioning, in rotating shift workers compared with day workers.12 Studies have found a 12% frequency of drowsy driving and an increased risk of driving accidents related to sleepiness in rotating shift workers with SWD compared with those without the disorder. Relative risk of injuries and accidents increases with each successive night shift worked.22 The effects of shift work on patient and employee safety are an important consideration in the health care field, where many workers have extended shifts.23
Productivity. Similarly, it is the combination of night or rotating shift work and daytime sleepiness or insomnia that decreases productivity, not each factor alone.12 Rotating shift workers with insomnia and/or excessive sleepiness (SWD) missed significantly more days of work (an average of 3 days per month over 3 months, a 10% decrease in productivity) than day workers with these symptoms, who missed approximately half a day of work per month over the 3-month period. This effect was not seen in shift workers who did not have insomnia or excessive sleepiness: they also missed a half day or less of work over 3 months.12 Rotating shift workers who experience both insomnia and excessive sleepiness are at the greatest risk for lost productivity. (See Keller23 for a review of potential productivity problems in health care workers on extended shifts.)
Shift work is very prevalent in our society. However, only a subset of shift workers meet criteria for SWD and need treatment. Potential targets for treatment are (1) the person’s work schedule, (2) difficulty sleeping during the day, and, most important, given the accident data discussed above, (3) difficulty functioning because of excessive sleepiness (eg, commuting home safely).
- American Academy of Sleep Medicine (AASM). International Classification of Sleep Disorders Diagnostic and Coding Manual, Second Edition (ICSD-2). Westchester, IL: American Academy of Sleep Medicine; 2005.
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- Owens JA, Belon K, Moss P. Impact of delaying school start time on adolescent sleep, mood, and behavior. Arch Pediatr Adolesc Med. 2010;164(7):608–614. doi:10.1001/archpediatrics.2010.96 PubMed
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- Bureau of Labor Statistics. Workers on Flexible and Shift Schedules in May 2004 (Data from the May 2004 Current Population Survey). Washington, DC: United States Department of Labor; 2005. www.bls.gov/news.release/pdf/flex.pdf. Accessed August 30, 2010.
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- Kryger MH, Roth T, Dement WC. Principles and Practice of Sleep Medicine. 4th ed. Philadelphia, PA: Saunders; 2005.
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- Waage S, Moen BE, Pallesen S, et al. Shift work disorder among oil rig workers in the North Sea. Sleep. 2009;32(4):558–565. PubMed
- Zhen Lu W, Ann Gwee K, Yu Ho K. Functional bowel disorders in rotating shift nurses may be related to sleep disturbances. Eur J Gastroenterol Hepatol. 2006;18(6):623–627. doi:10.1097/00042737-200606000-00008 PubMed
- Hansen J. Increased breast cancer risk among women who work predominantly at night. Epidemiology. 2001;12(1):74–77. doi:10.1097/00001648-200101000-00013 PubMed
- Davis S, Mirick DK, Stevens RG. Night shift work, light at night, and risk of breast cancer. J Natl Cancer Inst. 2001;93(20):1557–1562. doi:10.1093/jnci/93.20.1557 PubMed
- Schernhammer ES, Laden F, Speizer FE, et al. Rotating night shifts and risk of breast cancer in women participating in the Nurses’ Health Study. J Natl Cancer Inst. 2001;93(20):1563–1568. doi:10.1093/jnci/93.20.1563 PubMed
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- O’Leary ES, Schoenfeld ER, Stevens RG, et al, for the Electromagnetic Fields and Breast Cancer on Long Island Study Group. Shift work, light at night, and breast cancer on Long Island, New York. Am J Epidemiol. 2006;164(4):358–366. doi:10.1093/aje/kwj211 PubMed
- Kubo T, Ozasa K, Mikami K, et al. Prospective cohort study of the risk of prostate cancer among rotating-shift workers: findings from the Japan collaborative cohort study. Am J Epidemiol. 2006;164(6):549–555. doi:10.1093/aje/kwj232 PubMed
- Straif K, Baan R, Grosse Y, et al, on behalf of the WHO International Agency for Research on Cancer Monograph Working Group. Carcinogenicity of shift-work, painting, and fire-fighting. Lancet Oncol. 2007;8(12):1065–1066. doi:10.1016/S1470-2045(07)70373-X PubMed
- Folkard S, Tucker P. Shift work, safety and productivity. Occup Med (Lond). 2003;53(2):95–101. doi:10.1093/occmed/kqg047 PubMed
- Keller SM. Effects of extended work shifts and shift work on patient safety, productivity, and employee health. AAOHN J. 2009;57(12):497–502, quiz 503–504. doi:10.3928/08910162-20091116-01 PubMed