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Predictors of shift work disorder among nurses: a longitudinal study
Accepted Manuscript Title: Predictors of shift work disorder among nurses - a longitudinal study Author: Siri Waage, Ståle Pallesen, Bente Elisabeth Moen, Nils Magerøy, Elisabeth Flo, Lee Di Milia, Bjørn Bjorvatn PII: DOI: Reference:
S1389-9457(14)00381-5 http://dx.doi.org/doi:10.1016/j.sleep.2014.07.014 SLEEP 2551
To appear in:
Sleep Medicine
Received date: Revised date: Accepted date:
14-3-2014 7-6-2014 3-7-2014
Please cite this article as: Siri Waage, Ståle Pallesen, Bente Elisabeth Moen, Nils Magerøy, Elisabeth Flo, Lee Di Milia, Bjørn Bjorvatn, Predictors of shift work disorder among nurses - a longitudinal study, Sleep Medicine (2014), http://dx.doi.org/doi:10.1016/j.sleep.2014.07.014. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Predictors of shift work disorder among nurses - a longitudinal study Siri Waage, PhD1,2,*, Ståle Pallesen, PhD2,3, Bente Elisabeth Moen, MD, PhD1,4, Nils Magerøy, MD PhD5, Elisabeth Flo, PhD1,2, Lee Di Milia, PhD6, and Bjørn Bjorvatn, MD, PhD1,2 1
Department of Global Public Health and Primary Care, University of Bergen, Norway
2
Norwegian Competence Center for Sleep Disorders, Haukeland University Hospital, Norway
3
Department of Psychosocial Science, University of Bergen, Norway
4
Centre for International Health, University of Bergen, Norway
5
Department of occupational medicine, Haukeland University Hospital, Norway
6
Central Queensland University, School of Management and the Institute for Health and
Social Science Research, Rockhampton, QLD, Australia
*Corresponding author / Request for reprints: Siri Waage Department of Global Public Health and Primary Care Kalfarveien 31 5018 Bergen, Norway
E-mail: [email protected]
keywords: Shift work, sleep problems, sleepiness, insomnia, shift work disorder
Highlights Survey on shift work, sleep and health among 1533 nurses with a two year follow-up There was a significant reduction in shift work disorder from baseline to follow-up Several factors were identified to be predictors of shift work disorder
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Abstract Background: Shift work is associated with sleep problems and impaired health. The main aim of the present study was to explore predictors of developing shift work disorder (SWD) among Norwegian nurses using a longitudinal design.
Methods: A total of 1533 nurses participating in a survey on shift work, sleep and health responded to questionnaires at baseline and about two years later at follow-up. SWD was defined as problems of excessive sleepiness and/or complaints of insomnia related to the work schedule.
Results and Conclusions: There was a significant reduction (p<.001) in the prevalence of SWD from baseline to follow-up, from 35.7% to 28.6%. Logistic regression analyses showed significant risks of having SWD at follow-up and the following variables measured at baseline; number of nights worked the last year (OR=1.01, 95% CI=1.01-1.02), having SWD (OR=5.19, 95% CI=3.74-7.20), composite score on the Epworth Sleepiness Scale (OR=1.08, 95% CI=1.04-1.13), use of melatonin (OR=4.20, 95% CI=1.33-13.33), use of bright light therapy (OR=3.10, 95% CI 1.14-8.39), and symptoms of depression measured by the Hospital Anxiety and Depression Scale (OR=1.07, 95% CI=1.00-1.14). In addition, leaving night work between baseline and follow-up was associated with a significant reduced risk of SWD at follow-up (OR=0.12, 95% CI=0.07-0.22).
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Introduction Estimates suggest that approximately 20% of workers in Europe are employed in some form of shift work that involves night work [1]. Shift work is associated with a wide range of health problems [2, 3]. In work schedules that include night work, sleep problems are among the most frequently reported health problems [4, 5]. The most common sleep problems among night shift workers are sleep onset and maintenance difficulties, reduced sleep duration and excessive sleepiness during work [5]. For Norwegian nurses, it is common to work rotating shifts, which is a work schedule that could involve morning, evening and night shifts within the same work week. This is reported to lead to insufficient sleep duration [6], and night work is reported to be the most important cause of long-term stress and fatigue [7]. Nurses in rotating shifts have reported more mental health problems compared to nurses working nonrotating shifts [8] and associations between shift work, anxiety and depression have also been shown [9]. In a recent Norwegian study, nurses working in intensive care units were found to report poorer sleep and higher levels of sleepiness, fatigue, anxiety and depression compared to Norwegian norm groups [10]. However, results linking shift work and mental health are inconsistent; anxiety and depression were for example not associated with night work in a cross sectional study of Norwegian nurses [11].
Shift work disorder (SWD) is a circadian rhythm sleep disorder characterized by excessive sleepiness and complaints of insomnia related to the work schedule [12]. The diagnostic criteria for SWD are described in the second edition of the International Classification of Sleep Disorders (ICSD-2) and include the following four criteria: 1) Complaint of insomnia or
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excessive sleepiness temporally associated with a recurring work schedule that overlaps the usual time for sleep, 2) symptoms must be associated with the shift work schedule and present over the course of at least one month, 3) circadian and sleep-time misalignment as demonstrated by sleep log or actigraphical monitoring for 7 days or more and finally 4) sleep disturbance is not explainable by another sleep disorder, a medical or neurological disorder, mental disorder, medication use or substance use disorder [13].
Varying prevalences of SWD have been reported in cross-sectional studies, ranging from 14.5% among police officers working nights [14], 23.3% among oil rig workers [15], 24.4% among shift working nurses [16], 32.1% among Australian night workers [17], and up to 44.3% among Norwegian nurses in rotating shift work [18]. SWD is a relatively new diagnosis, and there is uncertainty and discussion about its operationalization, prevalence, consequences and treatment [19]. At present, few studies have used standardized questions to measure SWD and SWD seems to be underestimated in clinical settings. Epidemiological data on SWD are scarce [20]. One limitation of previous research on SWD is that the studies are based on cross-sectional design which prevent conclusions concerning directionality and possible cause-effect relationships related to the development and consequences of SWD.
To address this limitation, the main aim of the present study was to explore predictors of SWD among Norwegian shift working nurses using longitudinal data. We also aimed to assess the prevalence of SWD among Norwegian nurses at baseline and follow-up.
Methods Procedure and participants
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The data were collected from an on-going longitudinal cohort study “SUrvey of Shift work, Sleep and Health (SUSSH)” that was initiated in 2008/2009. A sample of 5400 nurses was randomly selected from the Norwegian Nurses Organization’s membership roll which includes most of the nurses in Norway. Survey questionnaires have been sent to this sample annually and this study presents findings from the first (2008/2009=baseline) and the third wave (2011=follow-up) of the survey.
The sample comprised five equal strata based on the numbers of years since graduation from nursing school (0-11 months, 1-3 years, 3.1-6 years, 6.1-9 years, and 9.1-12 years). A total of 2059 (response rate = 38.1%) nurses completed the questionnaire in the first wave (during the period December 2008 to March 2009). During the spring of 2011, all the nurses that completed the first wave received an invitation to participate in the third wave. A total of 1533 nurses (91% female) responded, yielding a response rate of 78.5%. All questionnaires were administered by postal mail with a pre-paid envelope for returning the completed forms. Up to two reminders were sent for each wave to those who did not respond.
Instruments Demographics The questionnaires comprised several sections. We collected data on socio-demographic variables (gender, age, married/cohabiting, children in household), work-related variables (work schedule, type and percentage of position worked in a full-time capacity, type of work place, exposure to night work (years) and number of night shifts per year); as well as life-style variables (e.g. present daily smoking and caffeine consumption/daily cups of coffee, tea or cola, use of sleep medication, use of bright light treatment).
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Shift work disorder In line with a number of previous studies three questions based on the minimal criteria from the ICSD-2 were used to assess SWD [15, 17, 18]. The questions are found to adequately assess SWD in epidemiological contexts [18]. The questions were: 1) Do you experience either difficulties sleeping or experience excessive sleepiness? (yes/no), 2) Is the sleep or sleepiness problem related to the work schedule that makes you work when you normally would sleep? (yes/no), 3) Have you had this sleep or sleepiness problem related to the work schedule for at least one month? (yes/no). Subjects were classified as suffering from SWD when they responded “yes” to all three questions.
Insomnia Insomnia symptoms were assessed with the Bergen Insomnia Scale (BIS) [21]. The questionnaire consists of 6 items, where the response alternatives reflects the number of days per week (0-7) the respondent had experienced a specific insomnia symptom, and is based on the diagnostic criteria for insomnia found in the fourth edition of the Diagnostic and Statistical Manual for Mental Disorders (DSM-IV, American Psychiatric Association, 2000). The scores on each item are added to create a total composite score (0 to 42), and higher values indicate a greater degree of insomnia symptoms [21]. The Norwegian version of the BIS has been validated and had shown good psychometric properties [21]. In the present study, the Cronbach’s alpha was .83 for the baseline data and .82 for the follow-up data.
Sleepiness Sleepiness was measured with the Norwegian version of the Epworth Sleepiness Scale (ESS) [22]. The ESS consists of 8 items that measure the subject’s general tendency to sleep or doze off in 8 different situations. Each item is scored from 0 (no probability) to 3 (high probability),
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yielding a total score between 0 and 24. The Norwegian version has shown high validity and reliability [23]. In the present study the Cronbach’s alpha coefficients were .74 for the baseline data and .75 for the follow-up data, respectively.
Anxiety and depression Symptoms of anxiety and depression were assessed with the Norwegian version of the Hospital Anxiety and Depression Scale (HADS) [24]. This scale consists of 14 items measuring symptoms of anxiety and depression experienced during the last week. Seven questions specifically address symptoms of anxiety whereas the other seven questions pertain specifically to symptoms of depression. The items are rated on a 4-point scale (0-3), yielding two scores, one for anxiety and one for depression, each ranging from 0 and 21. The HADS has shown good reliability [25]. In the present study the Cronbach’s alpha coefficients for both the anxiety and depression subscales were .81 for the baseline data. For the follow-up data the coefficients were .82 for both the scales.
Morningness A Norwegian version of the Diurnal Type Scale [26] was used to assess the morningnesseveningness dimension. The scale consists of 7 items related to this dimension, each rated on a four-point scale. Higher scores indicate higher levels of morningness (e.g. preference for rising relatively early in the day, performing activities relatively early in the day and getting to bed relatively early in the evening). The Diurnal Type Scale has been shown to have high internal reliability and validity. The Cronbach’s alpha coefficient was .64 at both baseline and follow-up in the present study.
Ethics
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The Regional Committee for Medical and Health Research Ethics of Western Norway (REKWest) approved the study.
Statistics PASW Statistics 18 for Windows was used for the statistical analyses. For comparisons of demographic variables between the nurses with and without SWD at baseline, independent ttests and Pearson chi-square tests were used. Significance level was set at .05.
The nurses were divided into four different groups based on having SWD or not at the two time points of the study. Group one consisted of workers not having SWD in either of the two waves (54.9%, n=817). The second group comprised nurses defined with SWD solely at baseline, and not at follow-up (16.5%, n=246). Group three consisted of nurses defined with SWD solely at follow-up and not at baseline (9.7%, n=144). Finally, the fourth group comprised of nurses defined with SWD at both baseline and follow-up (18.9%, n=281). Paired-sample t-tests were used to compare the change for each of the sleep and health parameters from baseline to follow-up within each of the four groups.
Logistic regression analyses were performed to assess the impact of a number of factors on the likelihood of nurses having SWD at follow-up. The latter comprised the dependent variable (0=not having SWD, 1=having SWD). We included age, gender, marital status, children in household, use of sleep medication (prescription or over the counter), use of exogenous melatonin, bright light treatment, smoking, caffeine consumption, number of nights worked the last 12 months, SWD, sleepiness score, insomnia score, anxiety score, depression score and diurnal type all measured at baseline as independent variables.
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Furthermore, we included no longer working night shifts at follow-up (yes/no) also as an independent variable. All variables were first entered separately (crude analyses) and secondly they were all entered together at the same time in an adjusted analysis. Furthermore, logistic regression analyses were performed to assess the impact of the same factors on the likelihood of disappearance of SWD among nurses having SWS at baseline.
Results Demographics At baseline (n=1533), the mean age of the nurses was 33 years, range 21-63. Among the whole sample, 76.3% worked in somatic hospital departments, 13.5% in psychiatric departments, 3.5% in nursing homes, 3.7% in home care services, and 2.1% in other work places, respectively. The mean hours worked per week were 33.9 with a distribution of 2.8% working <50% position, 28.6% working between 50-75% of full time position, 13.4% working between 76-90% of full time position, and 55.2% working more than 90% of full time position. Previous or present night work was reported by 84.3% of the nurses. Working only daytime was reported by 7.6% of the nurses, 0.1% reported only evening work, 25.0% worked a two-shift schedule involving day work and evening work, 8.2% worked nights only, 55.0% worked a three-shift schedule involving day, evening and night work, and 3.1% reported working other schedules involving night work. 14.0% of the nurses quitted night work between baseline and follow-up. A total of 74.0% of the nurses were married or cohabiting, and 48.7% reported having children at home. Mean body mass index (BMI) among the nurses was 24.4 (range 17.3-47.9). The nurses reported a mean of 3 cups of caffeinated beverages per day (range 0-30), and 10.0% were daily smokers (daily smoking, yes/no).
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Shift work disorder A total of 54.9% of the nurses did not have SWD at baseline or at follow-up, 16.5% were defined with SWD at baseline only, 9.7% at follow-up only, and 18.9% of the nurses were defined with SWD at both baseline and follow-up. Analysing data from all the nurses, there was a significant reduction (p<.001) in the prevalence of SWD between baseline (35.7%, n=538) and follow-up (28.6%, n=433). Among the nurses from wave 1 who did not respond in wave 3, the prevalence of SWD was 33.2% at baseline. The differences in demographics between nurses with and without SWD at baseline are described in table 1. Having SWD at baseline was positively associated with male gender, age, currently or previously working night shifts, numbers of nights the last year, and inversely related to the score on the morningness dimension (table 1).
Insert table 1 about here
The differences between the two measurements (baseline and follow-up) across the four groups are presented in table 2. Interestingly, the group that no longer met the criteria for SWD at follow-up worked significantly fewer nights and also reported a reduction in scores on the ESS, the BIS, and on symptoms of anxiety and depression. In that group 30.7% of the nurses had quit night work between the two assessment points, and the mean number of nights worked the last year was reduced from 27.9 to 18.5 nights. The reductions in the ESS, BIS, anxiety and depression scores were similar among the nurses who no longer worked night shifts, and those still working nights shifts at follow-up. For the group that developed SWD at follow-up, exposure to night work and scores for BIS increased.
Insert table 2 about here
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The results from the logistic regression analyses showed a risk of having SWD at follow-up and the following variables assessed at baseline; use of melatonin, use of bright light therapy, number of nights worked the last year, having SWD, sleepiness scores, and depression score (see table 3). In addition, leaving night work between baseline and follow-up significantly reduced the risk for fulfilling the criteria for SWD at follow-up. In addition, logistic regression analyses were performed among the nurses who had SWD at baseline to investigate predictors of cessation of SWD. Not surprisingly, no longer working night shift at follow-up was the strongest predictor of disappearance of SWD from baseline to follow-up. Furthermore, numbers of nights worked per year and sleepiness measured by the ESS at baseline were positively associated maintaining SWD (see table 4).
Insert table 3 and 4 about here
Discussion Having SWD at baseline, use of exogenous melatonin, use of bright light therapy, number of nights worked last year, sleepiness score, and depression score were all found to be predictors of SWD among Norwegian nurses. In addition, quitting night shifts from baseline to followup decreased the risk of SWD at follow-up. Interestingly, there was overall a significant reduction in the prevalence of shift work disorder (SWD) from 35.7% at baseline to 28.6% at follow-up.
Measured at baseline, the nurses with SWD were slightly older, comprised more males, were presently or previously working nights, and worked more nights the last year than the nurses without SWD. This is in line with studies showing that sleep problems related to shift work
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increase with age [27, 28]. However, the difference in age between the two groups was small, only about one year, and the importance of this in terms of tolerance to shift work seems negligible. Similarly, most of the other demographic differences between nurses with or without SWD at baseline are small. One exception is number of nights worked the last year, where nurses with SWD worked on average nearly 32 nights per year, while the nurses without SWD worked about 22 nights per year. Also, morning types are reported to have more difficulties adjusting their circadian rhythms to night work [29]. However, in this study the nurses with SWD scored lower on the morningness dimension compared to the nurses with SWD. In line with this, a study on oil rig workers found no significant difference in diurnal type between workers with SWD and workers without SWD [15]. Not surprisingly, subjects with SWD scored higher on measures of sleepiness, anxiety and depression than subjects without SWD.
The strongest predictor for SWD at follow-up was having SWD (OR 5.2) at baseline. This suggests that the sleep related problems that are experienced by shift workers are chronic for a significant proportion of the nurses. The number of night shifts worked per year was also a significant predictor, in line with results from a previous cross-sectional study from SUSSH including nurses from the same cohort [18]. Changing the work schedule so that it no longer included night work significantly predicted reduced risk of SWD at follow-up, supporting the fact that night work seems to be a main determinant of SWD. One of the criteria for SWD is to have complaints of insomnia or excessive sleepiness associated with a work schedule that overlaps with the usual time for sleep. Previous studies have reported that SWD is particularly prevalent among shift workers working night shift and early-morning shifts [30]. Excessive sleepiness and/or insomnia are criteria for SWD and are frequently reported among shift workers [4, 5]. Sleepiness score at baseline was one of the predictors of reporting SWD at
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follow-up. It therefore was somewhat surprising that this was not the case for insomnia. However, in our study insomnia was measured by the Bergen Insomnia Scale, which is based on the diagnostic criteria for insomnia stated in the DSM-IV (American Psychiatric Association, 2000). The criteria for SWD do not require insomnia as defined by the DSM-IV diagnostic criteria, which is likely to be more stringent than self-reported insomnia symptoms. The DSM-IV criteria requests difficulty in falling or maintaining sleep, or the feeling of not having had restitutional sleep for at least one month in addition to a daytime impairment, while the questions used to define SWD in this study did for instance not include any questions about daytime consequences of the sleep problem. However, at baseline the mean total score of the Epworth Sleepiness Scale (ESS) was about 9.5 among the nurses with SWD and even lower about (7.7) among the nurses without SWD. A value of 11 or higher on the total score of the ESS is considered as pathological sleepiness, meaning that the nurses in this study overall are feeling sleepy, albeit still not above the clinical cut-off value.
Shift workers struggling with adjustment to different work schedules are likely to search for interventions in order to better adjust to shift work and the problems such work schedules pose. Adapting circadian rhythms to the work schedule or to adjust it to a normal day schedule following e.g. a period of night shifts is assumed to be curative of shift work related problems and SWD [31]. Bright light therapy and exogenous melatonin are as such recommended in the current treatment guidelines for SWD [30]. Both use of melatonin and bright light were predictors of having SWD at follow-up. One possible explanation could be that the nurses reporting use of melatonin or bright light treatment are the nurses with the most severe sleep or circadian problems, and that the SWD remains as long as they continue working shift work. A weakness of the study is that we neither collected information on the timing and dosage of melatonin, nor information about the timing and dosage of bright light.
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Future research on the effect of treatment in large randomized clinical trials of shift workers with SWD is needed to make recommendations regarding the effects of melatonin and bright light treatment for this disorder. One of the predictors of SWD was depression at baseline. This is in accordance with results from a cross-sectional study showing that shift working nurses with SWD showed more severe depressive symptoms than those without SWD [16]. The strength of our study is the longitudinal design, being the first study to identify depression as a predictor of developing SWD. In the cross-sectional study from SUSSH by Flo and co-workers (2012) an association between SWD and anxiety was reported, but after adjusting for multiple variables, neither symptoms of anxiety nor depression were associated with SWD [18].
The prevalence of SWD reported in our study was high. Cross-sectional data from the same Norwegian nurse population including nearly 2000 nurses at baseline, have previously reported an even higher SWD prevalence ranging from 32.4% to 37.6% depending on the operationalization [18]. For comparison, a recent cross-sectional study from Japan reported a prevalence of 24.4% among 997 female hospital nurses engaged in two-shift and three-shift schedules [16]. The discrepancy between the prevalence rates across these studies could be explained by differences in working hours and working conditions between Norway and Japan. For instance the typical night shift start/end times differed between the samples (Japan; 24:00 - 09:00 compared to Norway; 21:30 - 07:00). Other possible explanations for variances in reported prevalence rates may be differences in the estimation of SWD and methodology. However, several of the reported studies have used the same questions to estimate SWD, which highlights that in addition to work schedule differences, individual differences may also play a role [17].
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In the present study, the prevalence of SWD decreased by about 7% from baseline to followup. Different possible factors could be considered to explain this reduction. Like in other studies on shift workers, it is reasonable to assume that a type of selection bias might be present, where individuals not coping with shift work tend to terminate this type of work [32]. In the present study, 30.7% of the nurses who had SWD at baseline, but not at follow-up, had quit night work between the two time points. Among the whole sample, 14% of the nurses made the same change. This suggests that vulnerable nurses self-selected out of night shift work. In addition, it is also possible that during the period from baseline to follow-up the nurses developed better coping strategies to deal with irregular work hours. It is also possible that this decline in prevalence can be explained by organizational changes in working conditions or wages, which we do not have information about. Interestingly, the reduction in SWD prevalence between baseline and follow-up was evident in all different shift work schedules.
Limitations and strengths Some limitations of the study should be mentioned. The response rate from the first wave was only 38% and some may thus question the representativeness of the population. Still, the response rate at follow-up was high (79%) ensuring that we compared a large sample across the two time points. The sample comprised only nurses, mainly females, which also pose some threat to the external validity. On the other hand the relative homogenous sample reduces the risk of work related confounders to influence the results.
Research on shift work as a risk factor for negative health has some methodological challenges regarding shift work exposure. A limitation of the study is that we do not have information about shift work experience before being included in the present cohort study.
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Future research should therefore include workers early in their shift work career, as many nurses may have been exposed to night work before and during their education. Still, a strength of the study is that the questionnaire consists of detailed information about present shift work exposure, in addition to the longitudinal design with information about changes in work schedules among the nurses.
Another limitation of the study includes its reliance on self-reported data only, and no objective assessments. However, as primarily baseline data were used as predictors for SWD at follow-up, this significantly reduces the risk of the results being distorted by the common method bias [33]. Lately, a new version of the ICSD has been published, with an important change in the criteria for SWD. In ICSD-2 the symptoms associated with the SWD should be present for at least one month, while in the ICSD-3 this is changed to three months. Future research should be conducted in accordance with the changes in the criteria.
Many of the odds ratios of the predictors on SWD in the present study were quite small. Still, it should be noted that many of the variables in the analyses are not dichotomous, and the odds ratios are consequently not expected to be high. One important factor that was significant, but had a low odd ratio, was number of night worked last year. This variable was continuous ranging from 0 to 200 nights per year, meaning that with an odd ratio of 1.01, an increase of one night shift per year would increase the risk of developing SWD with one percent. Nevertheless, when interpreting the significant results, the size of the odds ratio needs to be taken into careful consideration.
The present study also has some valuable assets worth mentioning. As far as we know, this study is the first longitudinal study assessing SWD, providing an unique opportunity to
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investigate directionality between variables and to identify possible causes of SWD. Longitudinal designs are superior to cross-sectional studies when it comes to discovering directionalities between variables. Still, the present study would be better if we had included more than two assessment times. The study furthermore included standardized and wellvalidated instruments and the relatively large sample size provides adequate statistical power to the analyses.
Conclusion Several factors measured at baseline like reporting SWD, use of exogenous melatonin, use of bright light therapy, number of nights worked last year, sleepiness score, and depression score were found to be significant predictors of SWD at follow-up. In addition, quitting night work between baseline and follow-up was significantly associated with a decreased risk of SWD at follow-up, suggesting that night work may be a major cause of SWD. There was a significant reduction in the SWD prevalence rate from 35.7% at baseline to 28.6% at follow up that might reflect selection of SWD subjects to other types of work schedules, development of better coping strategies over time or organizational changes.
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Table 1. Comparison of demographic variables between nurses with shift work disorder (SWD; n=525) and nurses without SWD (n=947) at baseline. Age mean years Sex female/male % Married/cohabiting in % Children living at home % yes Body Mass Index Daily caffeine use in cups Daily smoking % yes Work hours per week Nurse experience in years Night work (present or previous) % yes Number of nights worked last year Mean hours of sleep per night Sum Epworth Sleepiness Scale Sum Bergen Insomnia Scale Sum Anxiety (HADS-A) Sum Depression (HADS-D) Sum Diurnal Scale a
Not SWD 33.0 92.3/7.7 75.3 51.3 24.5 3.0 9.5 33.9 5.1 83.6 22.3 7.05 7.67 10.47 3.81 2.01 18.1
SWD 34.2 88.5/11.5 73.9 50.4 24.5 3.2 12.5 33.9 5.4 88.6 31.7 6.77 9.52 18.16 5.96 3.95 16.9
p-value .006 a .012 b .545 b .740 b .798 a .053 a .077 b .816 a .129 a .008 b <.001 a <.001 a <.001 a <.001 a <.001 a <.001 a <.001 a
Independent samples t-test, b Pearson chi-square test
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Table 2. Comparisons of four groups of nurses, with or without shift work disorder (SWD) at baseline and follow-up by paired-samples t-tests.
Number of nights last year Work hours per week Mean hours of sleep per night Sum Epworth Sleepiness Scale Sum Bergen Insomnia Scale Sum Anxiety (HADS-A) Sum Depression (HADS-D)
No SWD at baseline or at follow-up Baseline Follow-up p-value 21.7 21.3 .75 37.1 32.9 <.001 7.1 7.1 .65 7.5 7.2 .01 10.1 9.6 .05 3.7 3.8 .32 1.9 1.9 .58
SWD at baseline No SWD at follow-up Baseline Follow-up 27.9 18.5 33.4 31.9 6.8 6.9 9.4 7.7 17.6 13.4 5.8 5.0 3.7 3.0
p-value <.001 .01 .10 <.001 <.001 <.001 <.001
No SWD at baseline SWD at follow-up Baseline Follow-up 26.6 38.2 33.5 32.3 7.0 6.9 8.5 9.1 12.9 14.6 4.2 4.3 2.6 2.7
p-value <.001 .048 .04 .06 .005 .89 .78
SWD at both baseline and at follow-up Baseline Follow-up p-value 35.2 37.9 .17 34.2 33.6 .10 6.7 6.7 .79 9.8 9.8 .65 18.3 18.4 .80 6.1 5.9 .48 4.2 4.0 .38
HADS = Hospital Anxiety and Depression Scale
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Table 3. Logistic regression analyses predicting shift work disorder (SWD) at follow-up among Norwegian nurses. In the crude analyses each independent variable is analysed one by one against SWD, in the adjusted analysis they are all included simultaneously. Crude
Adjusted
(N=1440 to 1516)
(N=1196)
Independent variable
OR
95% CI
OR
95% CI
Age
1.02
1.01-1.04
1.02
1.00-1.04
Gender (female = 1.00)
Male
1.19
0.82-1.74
0.88
0.53-1.48
Married/Cohabiting (yes = 1.00)
No
1.18
0.92-1.52
1.24
0.86-1.79
Children in household (yes = 1.00)
No
0.99
0.79-1.24
.84
0.60-1.18
Sleep medication (no =1.00)
Yes
2.40
1.64-3.51
1.03
0.59-1.83
Melatonin (no =1.00)
Yes
3.36
1.62-6.99
4.20
1.33-13.33
Bright light (no =1.00)
Yes
2.29
1.18-4.45
3.10
1.14-8.39
Over the counter sleep medication (no =1.00)
Yes
2.64
1.64-4.25
1.51
0.78-2.93
Smoking (no = 1.00)
Yes
1.37
0.96-1.95
0.98
0.60-1.62
1.06
1.01-1.10
0.99
0.93-1.06
0.22
0.13-0.35
0.12
0.07-0.22
1.01
1.01-1.01
1.01
1.01-1.02
6.48
5.07-8.29
5.19
3.74-7.20
ESS total score at baseline
1.11
1.07-1.14
1.08
1.04-1.13
BIS total score at baseline
1.07
1.06-1.08
1.02
1.00-1.04
Sum Anxiety (HADS-A)
1.11
1.07-1.14
0.99
0.94-1.06
Sum Depression (HADS-D)
1.16
1.12-1.21
1.07
1.00-1.14
Sum Diurnal Scale
0.94
0.91-0.97
1.01
0.96-1.06
Caffeine (daily consumption) No longer working night shift at follow-up (no = 1.00)
Yes
Number of nights per year at baseline SWD at baseline (no = 1.00)
Yes
ESS = Epworth Sleepiness Scale, BIS = Bergen Insomnia Scale, HADS = Hospital Anxiety and Depression Scale, bold = p-values ≤ 0.05
23 Page 23 of 24
Table 4. Logistic regression analyses predicting disappearance of shift work disorder (SWD) at follow-up among Norwegian nurses with SWD at baseline. In the crude analyses each independent variable is analysed one by one against SWD, in the adjusted analysis they are all included simultaneously. Crude
Adjusted
(N=503 to 527)
(N=429)
Independent variable
OR
95% CI
OR
95% CI
Age
0.98
0.96-1.00
0.97
0.94-1.00
Gender (female = 1.00)
Male
0.93
0.55-1.59
1.02
0.51-2.04
Married/Cohabiting (yes = 1.00)
No
0.77
0.52-1.14
0.77
0.46-1.30
Children in household (yes = 1.00)
No
0.83
0.58-1.17
1.00
6.21-1.61
Sleep medication (no =1.00)
Yes
0.53
0.31-0.89
0.71
0.35-1.46
Melatonin (no =1.00)
Yes
0.56
0.22-1.41
0.38
0.84-1.69
Bright light (no =1.00)
Yes
0.99
0.36-2.80
0.41
0.05-3.27
Over the counter sleep medication (no =1.00)
Yes
0.65
0.35-1.21
0.54
0.23-1.25
Smoking (no = 1.00)
Yes
1.09
0.65-1.85
1.14
0.58-2.23
0.90
0.84-.0.97
0.97
0.88-1.07
8.23
4.50-15.05
8.46
4.20-17.00
Number of nights per year at baseline
0.99
0.99-1.00
0.99
0.98-1.00
ESS total score at baseline
0.98
0.93-1.02
0.94
0.89-1.00
BIS total score at baseline
0.99
0.97-1.01
1.00
0.98-1.03
Sum Anxiety (HADS-A)
0.98
0.93-1.02
1.01
0.94-1.09
Sum Depression (HADS-D)
0.96
0.90-1.01
0.93
0.85-1.02
Sum Diurnal Scale
1.02
0.97-1.07
0.99
0.93-1.06
Caffeine (daily consumption) No longer working night shift at follow-up (no = 1.00)
Yes
ESS = Epworth Sleepiness Scale. BIS = Bergen Insomnia Scale. HADS = Hospital Anxiety and Depression Scale. bold = p-values ≤ 0.05
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S1389-9457(14)00381-5 http://dx.doi.org/doi:10.1016/j.sleep.2014.07.014 SLEEP 2551
To appear in:
Sleep Medicine
Received date: Revised date: Accepted date:
14-3-2014 7-6-2014 3-7-2014
Please cite this article as: Siri Waage, Ståle Pallesen, Bente Elisabeth Moen, Nils Magerøy, Elisabeth Flo, Lee Di Milia, Bjørn Bjorvatn, Predictors of shift work disorder among nurses - a longitudinal study, Sleep Medicine (2014), http://dx.doi.org/doi:10.1016/j.sleep.2014.07.014. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Predictors of shift work disorder among nurses - a longitudinal study Siri Waage, PhD1,2,*, Ståle Pallesen, PhD2,3, Bente Elisabeth Moen, MD, PhD1,4, Nils Magerøy, MD PhD5, Elisabeth Flo, PhD1,2, Lee Di Milia, PhD6, and Bjørn Bjorvatn, MD, PhD1,2 1
Department of Global Public Health and Primary Care, University of Bergen, Norway
2
Norwegian Competence Center for Sleep Disorders, Haukeland University Hospital, Norway
3
Department of Psychosocial Science, University of Bergen, Norway
4
Centre for International Health, University of Bergen, Norway
5
Department of occupational medicine, Haukeland University Hospital, Norway
6
Central Queensland University, School of Management and the Institute for Health and
Social Science Research, Rockhampton, QLD, Australia
*Corresponding author / Request for reprints: Siri Waage Department of Global Public Health and Primary Care Kalfarveien 31 5018 Bergen, Norway
E-mail: [email protected]
keywords: Shift work, sleep problems, sleepiness, insomnia, shift work disorder
Highlights Survey on shift work, sleep and health among 1533 nurses with a two year follow-up There was a significant reduction in shift work disorder from baseline to follow-up Several factors were identified to be predictors of shift work disorder
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Abstract Background: Shift work is associated with sleep problems and impaired health. The main aim of the present study was to explore predictors of developing shift work disorder (SWD) among Norwegian nurses using a longitudinal design.
Methods: A total of 1533 nurses participating in a survey on shift work, sleep and health responded to questionnaires at baseline and about two years later at follow-up. SWD was defined as problems of excessive sleepiness and/or complaints of insomnia related to the work schedule.
Results and Conclusions: There was a significant reduction (p<.001) in the prevalence of SWD from baseline to follow-up, from 35.7% to 28.6%. Logistic regression analyses showed significant risks of having SWD at follow-up and the following variables measured at baseline; number of nights worked the last year (OR=1.01, 95% CI=1.01-1.02), having SWD (OR=5.19, 95% CI=3.74-7.20), composite score on the Epworth Sleepiness Scale (OR=1.08, 95% CI=1.04-1.13), use of melatonin (OR=4.20, 95% CI=1.33-13.33), use of bright light therapy (OR=3.10, 95% CI 1.14-8.39), and symptoms of depression measured by the Hospital Anxiety and Depression Scale (OR=1.07, 95% CI=1.00-1.14). In addition, leaving night work between baseline and follow-up was associated with a significant reduced risk of SWD at follow-up (OR=0.12, 95% CI=0.07-0.22).
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Introduction Estimates suggest that approximately 20% of workers in Europe are employed in some form of shift work that involves night work [1]. Shift work is associated with a wide range of health problems [2, 3]. In work schedules that include night work, sleep problems are among the most frequently reported health problems [4, 5]. The most common sleep problems among night shift workers are sleep onset and maintenance difficulties, reduced sleep duration and excessive sleepiness during work [5]. For Norwegian nurses, it is common to work rotating shifts, which is a work schedule that could involve morning, evening and night shifts within the same work week. This is reported to lead to insufficient sleep duration [6], and night work is reported to be the most important cause of long-term stress and fatigue [7]. Nurses in rotating shifts have reported more mental health problems compared to nurses working nonrotating shifts [8] and associations between shift work, anxiety and depression have also been shown [9]. In a recent Norwegian study, nurses working in intensive care units were found to report poorer sleep and higher levels of sleepiness, fatigue, anxiety and depression compared to Norwegian norm groups [10]. However, results linking shift work and mental health are inconsistent; anxiety and depression were for example not associated with night work in a cross sectional study of Norwegian nurses [11].
Shift work disorder (SWD) is a circadian rhythm sleep disorder characterized by excessive sleepiness and complaints of insomnia related to the work schedule [12]. The diagnostic criteria for SWD are described in the second edition of the International Classification of Sleep Disorders (ICSD-2) and include the following four criteria: 1) Complaint of insomnia or
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excessive sleepiness temporally associated with a recurring work schedule that overlaps the usual time for sleep, 2) symptoms must be associated with the shift work schedule and present over the course of at least one month, 3) circadian and sleep-time misalignment as demonstrated by sleep log or actigraphical monitoring for 7 days or more and finally 4) sleep disturbance is not explainable by another sleep disorder, a medical or neurological disorder, mental disorder, medication use or substance use disorder [13].
Varying prevalences of SWD have been reported in cross-sectional studies, ranging from 14.5% among police officers working nights [14], 23.3% among oil rig workers [15], 24.4% among shift working nurses [16], 32.1% among Australian night workers [17], and up to 44.3% among Norwegian nurses in rotating shift work [18]. SWD is a relatively new diagnosis, and there is uncertainty and discussion about its operationalization, prevalence, consequences and treatment [19]. At present, few studies have used standardized questions to measure SWD and SWD seems to be underestimated in clinical settings. Epidemiological data on SWD are scarce [20]. One limitation of previous research on SWD is that the studies are based on cross-sectional design which prevent conclusions concerning directionality and possible cause-effect relationships related to the development and consequences of SWD.
To address this limitation, the main aim of the present study was to explore predictors of SWD among Norwegian shift working nurses using longitudinal data. We also aimed to assess the prevalence of SWD among Norwegian nurses at baseline and follow-up.
Methods Procedure and participants
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The data were collected from an on-going longitudinal cohort study “SUrvey of Shift work, Sleep and Health (SUSSH)” that was initiated in 2008/2009. A sample of 5400 nurses was randomly selected from the Norwegian Nurses Organization’s membership roll which includes most of the nurses in Norway. Survey questionnaires have been sent to this sample annually and this study presents findings from the first (2008/2009=baseline) and the third wave (2011=follow-up) of the survey.
The sample comprised five equal strata based on the numbers of years since graduation from nursing school (0-11 months, 1-3 years, 3.1-6 years, 6.1-9 years, and 9.1-12 years). A total of 2059 (response rate = 38.1%) nurses completed the questionnaire in the first wave (during the period December 2008 to March 2009). During the spring of 2011, all the nurses that completed the first wave received an invitation to participate in the third wave. A total of 1533 nurses (91% female) responded, yielding a response rate of 78.5%. All questionnaires were administered by postal mail with a pre-paid envelope for returning the completed forms. Up to two reminders were sent for each wave to those who did not respond.
Instruments Demographics The questionnaires comprised several sections. We collected data on socio-demographic variables (gender, age, married/cohabiting, children in household), work-related variables (work schedule, type and percentage of position worked in a full-time capacity, type of work place, exposure to night work (years) and number of night shifts per year); as well as life-style variables (e.g. present daily smoking and caffeine consumption/daily cups of coffee, tea or cola, use of sleep medication, use of bright light treatment).
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Shift work disorder In line with a number of previous studies three questions based on the minimal criteria from the ICSD-2 were used to assess SWD [15, 17, 18]. The questions are found to adequately assess SWD in epidemiological contexts [18]. The questions were: 1) Do you experience either difficulties sleeping or experience excessive sleepiness? (yes/no), 2) Is the sleep or sleepiness problem related to the work schedule that makes you work when you normally would sleep? (yes/no), 3) Have you had this sleep or sleepiness problem related to the work schedule for at least one month? (yes/no). Subjects were classified as suffering from SWD when they responded “yes” to all three questions.
Insomnia Insomnia symptoms were assessed with the Bergen Insomnia Scale (BIS) [21]. The questionnaire consists of 6 items, where the response alternatives reflects the number of days per week (0-7) the respondent had experienced a specific insomnia symptom, and is based on the diagnostic criteria for insomnia found in the fourth edition of the Diagnostic and Statistical Manual for Mental Disorders (DSM-IV, American Psychiatric Association, 2000). The scores on each item are added to create a total composite score (0 to 42), and higher values indicate a greater degree of insomnia symptoms [21]. The Norwegian version of the BIS has been validated and had shown good psychometric properties [21]. In the present study, the Cronbach’s alpha was .83 for the baseline data and .82 for the follow-up data.
Sleepiness Sleepiness was measured with the Norwegian version of the Epworth Sleepiness Scale (ESS) [22]. The ESS consists of 8 items that measure the subject’s general tendency to sleep or doze off in 8 different situations. Each item is scored from 0 (no probability) to 3 (high probability),
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yielding a total score between 0 and 24. The Norwegian version has shown high validity and reliability [23]. In the present study the Cronbach’s alpha coefficients were .74 for the baseline data and .75 for the follow-up data, respectively.
Anxiety and depression Symptoms of anxiety and depression were assessed with the Norwegian version of the Hospital Anxiety and Depression Scale (HADS) [24]. This scale consists of 14 items measuring symptoms of anxiety and depression experienced during the last week. Seven questions specifically address symptoms of anxiety whereas the other seven questions pertain specifically to symptoms of depression. The items are rated on a 4-point scale (0-3), yielding two scores, one for anxiety and one for depression, each ranging from 0 and 21. The HADS has shown good reliability [25]. In the present study the Cronbach’s alpha coefficients for both the anxiety and depression subscales were .81 for the baseline data. For the follow-up data the coefficients were .82 for both the scales.
Morningness A Norwegian version of the Diurnal Type Scale [26] was used to assess the morningnesseveningness dimension. The scale consists of 7 items related to this dimension, each rated on a four-point scale. Higher scores indicate higher levels of morningness (e.g. preference for rising relatively early in the day, performing activities relatively early in the day and getting to bed relatively early in the evening). The Diurnal Type Scale has been shown to have high internal reliability and validity. The Cronbach’s alpha coefficient was .64 at both baseline and follow-up in the present study.
Ethics
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The Regional Committee for Medical and Health Research Ethics of Western Norway (REKWest) approved the study.
Statistics PASW Statistics 18 for Windows was used for the statistical analyses. For comparisons of demographic variables between the nurses with and without SWD at baseline, independent ttests and Pearson chi-square tests were used. Significance level was set at .05.
The nurses were divided into four different groups based on having SWD or not at the two time points of the study. Group one consisted of workers not having SWD in either of the two waves (54.9%, n=817). The second group comprised nurses defined with SWD solely at baseline, and not at follow-up (16.5%, n=246). Group three consisted of nurses defined with SWD solely at follow-up and not at baseline (9.7%, n=144). Finally, the fourth group comprised of nurses defined with SWD at both baseline and follow-up (18.9%, n=281). Paired-sample t-tests were used to compare the change for each of the sleep and health parameters from baseline to follow-up within each of the four groups.
Logistic regression analyses were performed to assess the impact of a number of factors on the likelihood of nurses having SWD at follow-up. The latter comprised the dependent variable (0=not having SWD, 1=having SWD). We included age, gender, marital status, children in household, use of sleep medication (prescription or over the counter), use of exogenous melatonin, bright light treatment, smoking, caffeine consumption, number of nights worked the last 12 months, SWD, sleepiness score, insomnia score, anxiety score, depression score and diurnal type all measured at baseline as independent variables.
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Furthermore, we included no longer working night shifts at follow-up (yes/no) also as an independent variable. All variables were first entered separately (crude analyses) and secondly they were all entered together at the same time in an adjusted analysis. Furthermore, logistic regression analyses were performed to assess the impact of the same factors on the likelihood of disappearance of SWD among nurses having SWS at baseline.
Results Demographics At baseline (n=1533), the mean age of the nurses was 33 years, range 21-63. Among the whole sample, 76.3% worked in somatic hospital departments, 13.5% in psychiatric departments, 3.5% in nursing homes, 3.7% in home care services, and 2.1% in other work places, respectively. The mean hours worked per week were 33.9 with a distribution of 2.8% working <50% position, 28.6% working between 50-75% of full time position, 13.4% working between 76-90% of full time position, and 55.2% working more than 90% of full time position. Previous or present night work was reported by 84.3% of the nurses. Working only daytime was reported by 7.6% of the nurses, 0.1% reported only evening work, 25.0% worked a two-shift schedule involving day work and evening work, 8.2% worked nights only, 55.0% worked a three-shift schedule involving day, evening and night work, and 3.1% reported working other schedules involving night work. 14.0% of the nurses quitted night work between baseline and follow-up. A total of 74.0% of the nurses were married or cohabiting, and 48.7% reported having children at home. Mean body mass index (BMI) among the nurses was 24.4 (range 17.3-47.9). The nurses reported a mean of 3 cups of caffeinated beverages per day (range 0-30), and 10.0% were daily smokers (daily smoking, yes/no).
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Shift work disorder A total of 54.9% of the nurses did not have SWD at baseline or at follow-up, 16.5% were defined with SWD at baseline only, 9.7% at follow-up only, and 18.9% of the nurses were defined with SWD at both baseline and follow-up. Analysing data from all the nurses, there was a significant reduction (p<.001) in the prevalence of SWD between baseline (35.7%, n=538) and follow-up (28.6%, n=433). Among the nurses from wave 1 who did not respond in wave 3, the prevalence of SWD was 33.2% at baseline. The differences in demographics between nurses with and without SWD at baseline are described in table 1. Having SWD at baseline was positively associated with male gender, age, currently or previously working night shifts, numbers of nights the last year, and inversely related to the score on the morningness dimension (table 1).
Insert table 1 about here
The differences between the two measurements (baseline and follow-up) across the four groups are presented in table 2. Interestingly, the group that no longer met the criteria for SWD at follow-up worked significantly fewer nights and also reported a reduction in scores on the ESS, the BIS, and on symptoms of anxiety and depression. In that group 30.7% of the nurses had quit night work between the two assessment points, and the mean number of nights worked the last year was reduced from 27.9 to 18.5 nights. The reductions in the ESS, BIS, anxiety and depression scores were similar among the nurses who no longer worked night shifts, and those still working nights shifts at follow-up. For the group that developed SWD at follow-up, exposure to night work and scores for BIS increased.
Insert table 2 about here
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The results from the logistic regression analyses showed a risk of having SWD at follow-up and the following variables assessed at baseline; use of melatonin, use of bright light therapy, number of nights worked the last year, having SWD, sleepiness scores, and depression score (see table 3). In addition, leaving night work between baseline and follow-up significantly reduced the risk for fulfilling the criteria for SWD at follow-up. In addition, logistic regression analyses were performed among the nurses who had SWD at baseline to investigate predictors of cessation of SWD. Not surprisingly, no longer working night shift at follow-up was the strongest predictor of disappearance of SWD from baseline to follow-up. Furthermore, numbers of nights worked per year and sleepiness measured by the ESS at baseline were positively associated maintaining SWD (see table 4).
Insert table 3 and 4 about here
Discussion Having SWD at baseline, use of exogenous melatonin, use of bright light therapy, number of nights worked last year, sleepiness score, and depression score were all found to be predictors of SWD among Norwegian nurses. In addition, quitting night shifts from baseline to followup decreased the risk of SWD at follow-up. Interestingly, there was overall a significant reduction in the prevalence of shift work disorder (SWD) from 35.7% at baseline to 28.6% at follow-up.
Measured at baseline, the nurses with SWD were slightly older, comprised more males, were presently or previously working nights, and worked more nights the last year than the nurses without SWD. This is in line with studies showing that sleep problems related to shift work
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increase with age [27, 28]. However, the difference in age between the two groups was small, only about one year, and the importance of this in terms of tolerance to shift work seems negligible. Similarly, most of the other demographic differences between nurses with or without SWD at baseline are small. One exception is number of nights worked the last year, where nurses with SWD worked on average nearly 32 nights per year, while the nurses without SWD worked about 22 nights per year. Also, morning types are reported to have more difficulties adjusting their circadian rhythms to night work [29]. However, in this study the nurses with SWD scored lower on the morningness dimension compared to the nurses with SWD. In line with this, a study on oil rig workers found no significant difference in diurnal type between workers with SWD and workers without SWD [15]. Not surprisingly, subjects with SWD scored higher on measures of sleepiness, anxiety and depression than subjects without SWD.
The strongest predictor for SWD at follow-up was having SWD (OR 5.2) at baseline. This suggests that the sleep related problems that are experienced by shift workers are chronic for a significant proportion of the nurses. The number of night shifts worked per year was also a significant predictor, in line with results from a previous cross-sectional study from SUSSH including nurses from the same cohort [18]. Changing the work schedule so that it no longer included night work significantly predicted reduced risk of SWD at follow-up, supporting the fact that night work seems to be a main determinant of SWD. One of the criteria for SWD is to have complaints of insomnia or excessive sleepiness associated with a work schedule that overlaps with the usual time for sleep. Previous studies have reported that SWD is particularly prevalent among shift workers working night shift and early-morning shifts [30]. Excessive sleepiness and/or insomnia are criteria for SWD and are frequently reported among shift workers [4, 5]. Sleepiness score at baseline was one of the predictors of reporting SWD at
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follow-up. It therefore was somewhat surprising that this was not the case for insomnia. However, in our study insomnia was measured by the Bergen Insomnia Scale, which is based on the diagnostic criteria for insomnia stated in the DSM-IV (American Psychiatric Association, 2000). The criteria for SWD do not require insomnia as defined by the DSM-IV diagnostic criteria, which is likely to be more stringent than self-reported insomnia symptoms. The DSM-IV criteria requests difficulty in falling or maintaining sleep, or the feeling of not having had restitutional sleep for at least one month in addition to a daytime impairment, while the questions used to define SWD in this study did for instance not include any questions about daytime consequences of the sleep problem. However, at baseline the mean total score of the Epworth Sleepiness Scale (ESS) was about 9.5 among the nurses with SWD and even lower about (7.7) among the nurses without SWD. A value of 11 or higher on the total score of the ESS is considered as pathological sleepiness, meaning that the nurses in this study overall are feeling sleepy, albeit still not above the clinical cut-off value.
Shift workers struggling with adjustment to different work schedules are likely to search for interventions in order to better adjust to shift work and the problems such work schedules pose. Adapting circadian rhythms to the work schedule or to adjust it to a normal day schedule following e.g. a period of night shifts is assumed to be curative of shift work related problems and SWD [31]. Bright light therapy and exogenous melatonin are as such recommended in the current treatment guidelines for SWD [30]. Both use of melatonin and bright light were predictors of having SWD at follow-up. One possible explanation could be that the nurses reporting use of melatonin or bright light treatment are the nurses with the most severe sleep or circadian problems, and that the SWD remains as long as they continue working shift work. A weakness of the study is that we neither collected information on the timing and dosage of melatonin, nor information about the timing and dosage of bright light.
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Future research on the effect of treatment in large randomized clinical trials of shift workers with SWD is needed to make recommendations regarding the effects of melatonin and bright light treatment for this disorder. One of the predictors of SWD was depression at baseline. This is in accordance with results from a cross-sectional study showing that shift working nurses with SWD showed more severe depressive symptoms than those without SWD [16]. The strength of our study is the longitudinal design, being the first study to identify depression as a predictor of developing SWD. In the cross-sectional study from SUSSH by Flo and co-workers (2012) an association between SWD and anxiety was reported, but after adjusting for multiple variables, neither symptoms of anxiety nor depression were associated with SWD [18].
The prevalence of SWD reported in our study was high. Cross-sectional data from the same Norwegian nurse population including nearly 2000 nurses at baseline, have previously reported an even higher SWD prevalence ranging from 32.4% to 37.6% depending on the operationalization [18]. For comparison, a recent cross-sectional study from Japan reported a prevalence of 24.4% among 997 female hospital nurses engaged in two-shift and three-shift schedules [16]. The discrepancy between the prevalence rates across these studies could be explained by differences in working hours and working conditions between Norway and Japan. For instance the typical night shift start/end times differed between the samples (Japan; 24:00 - 09:00 compared to Norway; 21:30 - 07:00). Other possible explanations for variances in reported prevalence rates may be differences in the estimation of SWD and methodology. However, several of the reported studies have used the same questions to estimate SWD, which highlights that in addition to work schedule differences, individual differences may also play a role [17].
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In the present study, the prevalence of SWD decreased by about 7% from baseline to followup. Different possible factors could be considered to explain this reduction. Like in other studies on shift workers, it is reasonable to assume that a type of selection bias might be present, where individuals not coping with shift work tend to terminate this type of work [32]. In the present study, 30.7% of the nurses who had SWD at baseline, but not at follow-up, had quit night work between the two time points. Among the whole sample, 14% of the nurses made the same change. This suggests that vulnerable nurses self-selected out of night shift work. In addition, it is also possible that during the period from baseline to follow-up the nurses developed better coping strategies to deal with irregular work hours. It is also possible that this decline in prevalence can be explained by organizational changes in working conditions or wages, which we do not have information about. Interestingly, the reduction in SWD prevalence between baseline and follow-up was evident in all different shift work schedules.
Limitations and strengths Some limitations of the study should be mentioned. The response rate from the first wave was only 38% and some may thus question the representativeness of the population. Still, the response rate at follow-up was high (79%) ensuring that we compared a large sample across the two time points. The sample comprised only nurses, mainly females, which also pose some threat to the external validity. On the other hand the relative homogenous sample reduces the risk of work related confounders to influence the results.
Research on shift work as a risk factor for negative health has some methodological challenges regarding shift work exposure. A limitation of the study is that we do not have information about shift work experience before being included in the present cohort study.
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Future research should therefore include workers early in their shift work career, as many nurses may have been exposed to night work before and during their education. Still, a strength of the study is that the questionnaire consists of detailed information about present shift work exposure, in addition to the longitudinal design with information about changes in work schedules among the nurses.
Another limitation of the study includes its reliance on self-reported data only, and no objective assessments. However, as primarily baseline data were used as predictors for SWD at follow-up, this significantly reduces the risk of the results being distorted by the common method bias [33]. Lately, a new version of the ICSD has been published, with an important change in the criteria for SWD. In ICSD-2 the symptoms associated with the SWD should be present for at least one month, while in the ICSD-3 this is changed to three months. Future research should be conducted in accordance with the changes in the criteria.
Many of the odds ratios of the predictors on SWD in the present study were quite small. Still, it should be noted that many of the variables in the analyses are not dichotomous, and the odds ratios are consequently not expected to be high. One important factor that was significant, but had a low odd ratio, was number of night worked last year. This variable was continuous ranging from 0 to 200 nights per year, meaning that with an odd ratio of 1.01, an increase of one night shift per year would increase the risk of developing SWD with one percent. Nevertheless, when interpreting the significant results, the size of the odds ratio needs to be taken into careful consideration.
The present study also has some valuable assets worth mentioning. As far as we know, this study is the first longitudinal study assessing SWD, providing an unique opportunity to
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investigate directionality between variables and to identify possible causes of SWD. Longitudinal designs are superior to cross-sectional studies when it comes to discovering directionalities between variables. Still, the present study would be better if we had included more than two assessment times. The study furthermore included standardized and wellvalidated instruments and the relatively large sample size provides adequate statistical power to the analyses.
Conclusion Several factors measured at baseline like reporting SWD, use of exogenous melatonin, use of bright light therapy, number of nights worked last year, sleepiness score, and depression score were found to be significant predictors of SWD at follow-up. In addition, quitting night work between baseline and follow-up was significantly associated with a decreased risk of SWD at follow-up, suggesting that night work may be a major cause of SWD. There was a significant reduction in the SWD prevalence rate from 35.7% at baseline to 28.6% at follow up that might reflect selection of SWD subjects to other types of work schedules, development of better coping strategies over time or organizational changes.
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Table 1. Comparison of demographic variables between nurses with shift work disorder (SWD; n=525) and nurses without SWD (n=947) at baseline. Age mean years Sex female/male % Married/cohabiting in % Children living at home % yes Body Mass Index Daily caffeine use in cups Daily smoking % yes Work hours per week Nurse experience in years Night work (present or previous) % yes Number of nights worked last year Mean hours of sleep per night Sum Epworth Sleepiness Scale Sum Bergen Insomnia Scale Sum Anxiety (HADS-A) Sum Depression (HADS-D) Sum Diurnal Scale a
Not SWD 33.0 92.3/7.7 75.3 51.3 24.5 3.0 9.5 33.9 5.1 83.6 22.3 7.05 7.67 10.47 3.81 2.01 18.1
SWD 34.2 88.5/11.5 73.9 50.4 24.5 3.2 12.5 33.9 5.4 88.6 31.7 6.77 9.52 18.16 5.96 3.95 16.9
p-value .006 a .012 b .545 b .740 b .798 a .053 a .077 b .816 a .129 a .008 b <.001 a <.001 a <.001 a <.001 a <.001 a <.001 a <.001 a
Independent samples t-test, b Pearson chi-square test
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Table 2. Comparisons of four groups of nurses, with or without shift work disorder (SWD) at baseline and follow-up by paired-samples t-tests.
Number of nights last year Work hours per week Mean hours of sleep per night Sum Epworth Sleepiness Scale Sum Bergen Insomnia Scale Sum Anxiety (HADS-A) Sum Depression (HADS-D)
No SWD at baseline or at follow-up Baseline Follow-up p-value 21.7 21.3 .75 37.1 32.9 <.001 7.1 7.1 .65 7.5 7.2 .01 10.1 9.6 .05 3.7 3.8 .32 1.9 1.9 .58
SWD at baseline No SWD at follow-up Baseline Follow-up 27.9 18.5 33.4 31.9 6.8 6.9 9.4 7.7 17.6 13.4 5.8 5.0 3.7 3.0
p-value <.001 .01 .10 <.001 <.001 <.001 <.001
No SWD at baseline SWD at follow-up Baseline Follow-up 26.6 38.2 33.5 32.3 7.0 6.9 8.5 9.1 12.9 14.6 4.2 4.3 2.6 2.7
p-value <.001 .048 .04 .06 .005 .89 .78
SWD at both baseline and at follow-up Baseline Follow-up p-value 35.2 37.9 .17 34.2 33.6 .10 6.7 6.7 .79 9.8 9.8 .65 18.3 18.4 .80 6.1 5.9 .48 4.2 4.0 .38
HADS = Hospital Anxiety and Depression Scale
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Table 3. Logistic regression analyses predicting shift work disorder (SWD) at follow-up among Norwegian nurses. In the crude analyses each independent variable is analysed one by one against SWD, in the adjusted analysis they are all included simultaneously. Crude
Adjusted
(N=1440 to 1516)
(N=1196)
Independent variable
OR
95% CI
OR
95% CI
Age
1.02
1.01-1.04
1.02
1.00-1.04
Gender (female = 1.00)
Male
1.19
0.82-1.74
0.88
0.53-1.48
Married/Cohabiting (yes = 1.00)
No
1.18
0.92-1.52
1.24
0.86-1.79
Children in household (yes = 1.00)
No
0.99
0.79-1.24
.84
0.60-1.18
Sleep medication (no =1.00)
Yes
2.40
1.64-3.51
1.03
0.59-1.83
Melatonin (no =1.00)
Yes
3.36
1.62-6.99
4.20
1.33-13.33
Bright light (no =1.00)
Yes
2.29
1.18-4.45
3.10
1.14-8.39
Over the counter sleep medication (no =1.00)
Yes
2.64
1.64-4.25
1.51
0.78-2.93
Smoking (no = 1.00)
Yes
1.37
0.96-1.95
0.98
0.60-1.62
1.06
1.01-1.10
0.99
0.93-1.06
0.22
0.13-0.35
0.12
0.07-0.22
1.01
1.01-1.01
1.01
1.01-1.02
6.48
5.07-8.29
5.19
3.74-7.20
ESS total score at baseline
1.11
1.07-1.14
1.08
1.04-1.13
BIS total score at baseline
1.07
1.06-1.08
1.02
1.00-1.04
Sum Anxiety (HADS-A)
1.11
1.07-1.14
0.99
0.94-1.06
Sum Depression (HADS-D)
1.16
1.12-1.21
1.07
1.00-1.14
Sum Diurnal Scale
0.94
0.91-0.97
1.01
0.96-1.06
Caffeine (daily consumption) No longer working night shift at follow-up (no = 1.00)
Yes
Number of nights per year at baseline SWD at baseline (no = 1.00)
Yes
ESS = Epworth Sleepiness Scale, BIS = Bergen Insomnia Scale, HADS = Hospital Anxiety and Depression Scale, bold = p-values ≤ 0.05
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Table 4. Logistic regression analyses predicting disappearance of shift work disorder (SWD) at follow-up among Norwegian nurses with SWD at baseline. In the crude analyses each independent variable is analysed one by one against SWD, in the adjusted analysis they are all included simultaneously. Crude
Adjusted
(N=503 to 527)
(N=429)
Independent variable
OR
95% CI
OR
95% CI
Age
0.98
0.96-1.00
0.97
0.94-1.00
Gender (female = 1.00)
Male
0.93
0.55-1.59
1.02
0.51-2.04
Married/Cohabiting (yes = 1.00)
No
0.77
0.52-1.14
0.77
0.46-1.30
Children in household (yes = 1.00)
No
0.83
0.58-1.17
1.00
6.21-1.61
Sleep medication (no =1.00)
Yes
0.53
0.31-0.89
0.71
0.35-1.46
Melatonin (no =1.00)
Yes
0.56
0.22-1.41
0.38
0.84-1.69
Bright light (no =1.00)
Yes
0.99
0.36-2.80
0.41
0.05-3.27
Over the counter sleep medication (no =1.00)
Yes
0.65
0.35-1.21
0.54
0.23-1.25
Smoking (no = 1.00)
Yes
1.09
0.65-1.85
1.14
0.58-2.23
0.90
0.84-.0.97
0.97
0.88-1.07
8.23
4.50-15.05
8.46
4.20-17.00
Number of nights per year at baseline
0.99
0.99-1.00
0.99
0.98-1.00
ESS total score at baseline
0.98
0.93-1.02
0.94
0.89-1.00
BIS total score at baseline
0.99
0.97-1.01
1.00
0.98-1.03
Sum Anxiety (HADS-A)
0.98
0.93-1.02
1.01
0.94-1.09
Sum Depression (HADS-D)
0.96
0.90-1.01
0.93
0.85-1.02
Sum Diurnal Scale
1.02
0.97-1.07
0.99
0.93-1.06
Caffeine (daily consumption) No longer working night shift at follow-up (no = 1.00)
Yes
ESS = Epworth Sleepiness Scale. BIS = Bergen Insomnia Scale. HADS = Hospital Anxiety and Depression Scale. bold = p-values ≤ 0.05
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