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SLEEP HAS FAR-REACHING EFFECTS ON ENDOCRI-NOLOGY, IMMUNOLOGY, AND METABOLISM.1 SEV-ERAL STUDIES HAVE SHOWN SLEEP DISTURBANCES to be associated with increased risk of health problems2-9 and, in working populations, with increased sickness absence,10-14 which in turn has been suggested as an integrated measure of physical, psychological, and social functioning.15 The etiology behind sleep disturbances can vary, and sleep disturbances can be symptoms of disease as well as risk factors for ill health. Work stress and poor working conditions have been shown to be associated with sleep disturbances16-19 and increased need for recovery.7 In a survey in the USA, the most frequent self-report-ed cause of sleeping difficulties was work-related stress.20 Sleep disturbances may indeed be a mediator between work-related factors and sickness absence.

Population-based evidence on secular trends in sleep distur-bances is scarce. A Swedish study reported a sharp increase in prevalence of sleep disturbances from 1996/1997 to 2002/2003 among both women and men,21 and corresponding findings have been reported in 2 other Scandinavian studies.22,23 A recent Finnish study showed a minor decrease in self-reported sleep duration from 1972 to 2005 and presented tentative evidence for an increase in insomnia-related problems in the working age population during the last 10 years.24 However, some com-mentators argue that there is no firm evidence of an increase in sleep debt over the years.25 Regarding sickness absence, a sub-stantial increase in sick leaves due to mental disorders has been reported in several countries in the last decades.26-30 A Swedish study reported a parallel rise in fatigue and sick-leave days be-tween 1994 and 2002, and an increased prevalence of fatigue and anxiety among younger people with a high number of sick-leave days.31 Nevertheless, little is known about the association between sleep disturbances and sickness absence, of possible changes in this over time, or about the extent to which sickness absence is attributable to work-related sleep disturbances.10 Most of the existing studies are cross-sectional,11-13,32 or focus on fatigue in specific trades.7 We are not aware of any previ-ous population-based studies that investigate the association between work-related sleep disturbances and sickness absence over time.

The aim of the present study was to examine secular trends in work-related sleep disturbances; the association between work-

Work-related Sleep diSturbance

Work-Related Sleep Disturbances and Sickness Absence in the Swedish Working Population, 1993-1999Hugo Westerlund, PhD1; Kristina Alexanderson, PhD2; Torbjörn Åkerstedt, PhD1; Linda Magnusson Hanson, PhD1; Töres Theorell, MD, PhD1; Mika Kivimäki, PhD3

1Stress Research Institute, Stockholm University, Stockholm, Sweden; 2Section of Personal Injury Prevention, Department of Clinical Neuroscience, Karolinska Institute, Stockholm, Sweden; 3Department of Epidemiology and Public Health, University College London, London, UK

Study objectives: To examine secular trends in work-related sleep disturbances and their association with sickness absence in the Swed-ish working population.Design. Nationally representative cross-sectional samples of the Swed-ish working population aged 16-64 (the biennial Swedish Work Environ-ment Survey) in 1993, 1995, and 1999 respectively. Questionnaire data on work-related sleep disturbances were linked to records of medically-certified sick-leave spells exceeding 14 days obtained from national registers.Setting: All Sweden.participants: A total of 28,424 individuals aged 16-65 with com-plete data (5162/5173 women/men in 1993; 4635/4764 in 1995; and 4422/4268 in 1999).interventions: N/A.Measurements and results: The age-adjusted proportion of women with work-related sleep disturbances at least once a week increased from 12.3% in 1993 to 21.7% in 1999 (P < 0.001). The corresponding figures for men were 12.5% to 18.6% (P < 0.001). There was a strong cross-sectional association between work-related sleep disturbances

and sickness absence in both genders and in each studied year. Using binary logistic regressions and adjusting for age, supervisory position, and geographical region, the odds ratios for sickness absence for those who reported work-related sleep disturbances every day, compared with those who answered “not at all/seldom last 3 months” varied between 3.22 (1.88-5.50) and 4.26 (2.56-7.19), with the strongest associations seen in 1999. Adjustment for health indicators, especially depressive symptoms, attenuated the relationship substantially.conclusions: Self-reported sleep disturbances attributed to work-related causes were on the rise in Sweden and were associated with medically-certified sickness absence. Most of this association seems to be accounted for by depressive symptoms.keywords: sleep disturbances, sleep, secular trend, sickness ab-sence, gender, work, depressive symptomscitation: Westerlund H; Alexanderson K; Åkerstedt T; Hanson LM; Theorell T; Kivimäki M. Work-related sleep disturbances and sick-ness absence in the swedish working population, 1993-1999. SLEEP 2008;31(8):1169-1177.

disclosure StatementThis was not an industry supported study. Dr. Åkerstedt has participated in a speaking engagement for Sanofi-Aventis. The other authors have in-dicated no financial conflicts of interest.

Submitted for publication June, 2007accepted for publication May, 2008Address correspondence to: Dr. Hugo Westerlund, Stress Research In-stitute at Stockholm University, S-106 91 Stockholm, Sweden; Tel: +46-8-5537 8926; Cell: +46-76-814 1011; Fax: +46-8-5537 8900; E-mail: hugo.westerlund@stressforskning.su.se

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related sleep disturbances and long-term sickness absence; and changes in this association over 8 years in the Swedish working population.

MetHodS

We performed cross-sectional and time trend analyses in the Swedish working population linking questionnaire data on work-related sleep disturbances to register data on medically certified sick-leave spells exceeding 14 days.

Study Sample

Participants were all respondents of the biennial Swedish Work Environment Survey (SWES), comprising representa-tive samples of the Swedish working population aged 16-64.33 These samples are derived from the respondents in the annual national Labour Force Survey (LFS). The sampling method is stratified on geographic region, sex, citizenship, and employ-ment. Our analysis included 3 steps, each of which had specific inclusion criteria:

Included in the descriptive part of the present study were participants in SWES in any of the years 1993, 1995, 1997, or 1999. There were a total of 41,912 participants with complete data on work-related sleep disturbances, ranging from 9,608 to 11,476 in each survey, corresponding to between 77.0% and 86.5% of the original samples (average 81.2%).

In the analyses of cross-sectional associations between work-related sleep disturbances and sickness absence, only subjects participating in the 1993, 1995, and 1999 SWES were included, since the rules for sickness absence benefits and reg-istration were comparable those years. Of the initial 38,897 persons in the database for those 3 years, we excluded those who did not respond to the question about work-related sleep disturbances (n = 7,190), subjects who had received unemploy-ment benefit during the year in question (n = 2,991), and sub-jects with incomplete data on supervisory position or county of employment (n = 302). A total of 28,424 subjects were thus used in the analyses of association between work-related sleep disturbances and sickness absence, corresponding to 73.1% of the total original samples for the 3 years.

In the analyses of possible explanatory factors for the associ-ation between sleep disturbances and sickness absence, another 4,851 were excluded because of missing data on one or more of the covariates. (i.e., psychosocial work environment, hospi-talizations, and self-reported health problems). Thus, a total of 23,573 subjects were included in these analyses.

Sleep disturbances

Work-related sleep disturbances were measured by one ques-tion in the self-completion questionnaire that is sent to the par-ticipants in the SWES: In the past three months how often have you had difficulties sleeping because thoughts about work have kept you awake? The response options were: “Every day”; “A couple of days a week (1 day of 2)”; “One day a week (1 day of 5)”; “A couple of days a month (1 day of 10)”; “Not at all/Seldom in the last 3 months.”

Sickness absence

All employed residents in Sweden are covered by sickness insurance entitling them to compensation for work incapacity due to disease or injury. Self-certification for sick-leave was accepted for the first 7 calendar days of a spell; thereafter a sickness certificate from a physician was required.

It has been argued that gender differences in sickness ab-sence are due to women taking sick-leave in order to care for (sick) children. In Sweden this has probably not been the case, as the parental insurance is generous. In the years studied here, it covered absence from work to care for newborn (450 days) or sick (60 days/year/child) children. Data on this type of absence were not included.

We obtained records of sickness absence from the Income and Capital Register (IoF). These records were linked to SWES and LFS data through unique personal identification numbers. In 1993, 1995, and 1999, the public social insurance office started paying sickness benefit on the 15th day of a sick-leave spell, since the the first 14 days of each spell were paid by the employer. Therefore, public registeres contain information only about sick-leave spells exceeding 14 days. For all these absenc-es a physician’s certificate was required. Since sickness absence was registered somewhat differently in 1997, that year was not included in the cross-sectional analyses. We contrasted people who had at least one sick-leave spell exceeding 14 days with those who had no such spell.

adjustment Variables

Data on age, sex, county of employment, and unemployment benefit were obtained from Statistics Sweden. Regional (coun-ty) levels of sickness absence in relation to national mean were obtained from the Swedish Social Insurance Agency. Counties were defined as being above or below the national mean. Data on whether the participants had a supervisory position or not were taken from the LFS telephone interviews. Work environ-ment was measured by proxies of psychological demands, deci-sion authority, skill discretion, and social support, as in previous studies.34 Self-reported symptoms were measured by questions (cf. Appendix) regarding depressive symptoms (2 items), physi-cal exhaustion after work, psychosomatic symptoms (2 items), and musculoskeletal symptoms (6 items). Although our measure of depressive symptoms includes tiredness and listlessness, it was only weakly correlated with work-related sleep disturbances (Spearman rho 0.34). Hospital admission data were obtained from Centre for Epidemiology, Swedish National Board of Health and Welfare, and were dichotomized into none versus any number of admissions during the survey year.

Statistical Methods

Time trends in work-related sleep disturbances were analyzed with Kruskal-Wallis H Test. In order to adjust for the potential inflation of the overall type I error rate, we used the Bonferroni method for post hoc testing of pairwise differences between years. Direct age adjustment was made with 5-year strata, with the 1993 through 1999 samples combined as norm. ANCOVA was used to test differences in psychosocial work environment

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between the years, adjusting for age. Cross-sectional associa-tions between work-related sleep disturbances and sickness ab-sence, adjusting for age, supervisor position, and region were analyzed with binary logistic regression analyses, stratified by sex, in 1993, 1995, and 1999 respectively. Further adjustments were made for psychosocial work environment and health indi-cators in models containing all 3 years, with year as an adjust-ment variable. To test whether the strength of the association between work-related sleep disturbances and sickness absence varied by year, we added the interaction term “work-related sleep disturbances × study year” in a regression model already containing the main effects. The 95% confidence intervals (CI) for odds ratios (OR) were calculated. Statistical analyses were conducted using SPSS for Windows, versions 15.0 and 16.0.

ethics

We obtained ethics approval for the study from the Regional Research Ethics Board as well as from the research ethics com-mittee at Statistics Sweden.

reSultS

Work-related sleep disturbances increased markedly in the Swedish working population during 1993–1999, especial-ly among women, (Figure 1 and 2 [trend P < 0.001 for both sexes]). This increase was statistically significant between all consecutive pairs of years for both sexes (P < 0.01 for pairwise comparisons). The age-adjusted proportion of women with work-related sleep disturbances at least once a week was 12.3% in 1993 and 21.7% in 1999; the corresponding figures for men were 12.5% and 18.6%, respectively.

Table 1 shows background characteristics for the participants with complete data for the analyses of cross-sectional associa-tions. Comparable cross-sectional data on sickness absence are available for the participants in the 1993, 1995, and 1999 SWES. The age-adjusted one-year prevalence of sick-leave spells exceeding 14 days, hospitalizations, self-reported health problems, and indicators of psychosocial work environment in

each year are shown in Table 2. There was no significant change in sickness absences over time for either sex, or in the one-year prevalence of hospital admissions. However, self-rated psy-chosocial work environment deteriorated over time, and there was an increase in self-reported health problems (all trends P < 0.001).

Tables 3 and 4 show strong and significant associations be-tween self-rated work-related sleep disturbances and the likeli-hood of having at least one recorded sick-leave spell exceeding 14 days. The association was found in both men and women, and there was a tendency towards a dose-response relation-ship, with the by far the highest risk of having at least one long sick-leave spell among those who reported having work-related sleep disturbances every day. The associations were virtually unchanged after adjustment for supervisory position and re-gional level of sickness absence. Although the prevalence of sickness absence was much higher in women than in men, the strength of the associations between work-related sleep distur-bances and sickness absence was almost the same for women and men (P for gender difference 0.70). In women but not in men, however, there was a strengthening of the association over the years (P = 0.008).

In further analyses we added proxies for stressful work envi-ronment and health in regression models with all years analyzed together (Table 5). Skill discretion in both sexes, and psychologi-cal demands and decision authority in women, were associated with risk of sickness absence, in the expected directions. Howev-er, inclusion of psychosocial work environment measures in the model only marginally affected the associations between work-related sleep disturbances and sickness absence. In contrast, in-clusion of health measures as a block substantially attenuated the relationship, leaving only work-related sleep disturbances every day significantly associated with sickness absence. Among the health measures, the strongest individual predictor of sickness absence in the mutually adjusted model was hospital admission (OR = 13.5 [11.5–15.9] in women and 11.8 [9.75–14.2] in men), which, however, did not explain any of the association between work-related sleep disturbances and sickness absence (Table 6). Instead, adjustment for depressive symptoms reduced the esti-

2.42.11.21.1

9.37.06.74.5

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93 95 97 99

A couple of days amonthOne day a week

A couple of daysa weekEvery day

Figure 1—Age-adjusted prevalence of work-related sleep distur-bances in the past 3 months among the women in stratified, ran-dom samples of the Swedish working population 1993–1999.

1.81.41.21.4

7.25.76.14.2

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A couple of days amonthOne day a week

A couple of daysa weekEvery day

Figure 2—Age-adjusted prevalence of work-related sleep distur-bances in the past 3 months among the men in stratified, random samples of the Swedish working population 1993–1999.

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for men. We also found a dose-response relationship between work-related sleep disturbances and medically-certified sick-ness absence in women and men. This association was robust to adjustments for demographic and regional characteristics, supervisory position, and measures of work environment, and was strengthened with time in women. Increasing trends in de-pressive symptoms explained most of the association between work-related sleep disturbances and sickness absence.

To the best of our knowledge, no prior population-based stud-ies of secular trends in work-related sleep disturbances are avail-

mates of this association the most (–76.6 % among the women and –65.3 % among the men), explaining virtually all of the re-duction of the association shown in Table 5.

diScuSSion

This study of the Swedish working population shows that the age-adjusted prevalence of self-reported work-related sleep disturbances, experienced at least once a week, increased from 12% in 1993 to 22% in 1999 for women, and from 12% to 19%

Table 1—Descriptive Statistics for Those Participants in the SWES Populations Who had Complete Data on All Variables Used in the Analy-ses of Cross-Sectional Associations* and Who had not Received Unemployment Benefit. Values are Numbers (Percentages of the Sample).

Women Men 1993 1995 1999 1993 1995 1999 n (%) n (%) n (%) n (%) n (%) n (%)Total number of respondents 5162 (100) 4635 (100) 4422 (100) 5173 (100) 4764 (100) 4268 (100)Age 16–20 155 ( 3.0) 123 ( 2.7) 125 ( 2.8) 97 ( 1.9) 104 ( 2.2) 100 ( 2.3) 21–25 327 ( 6.3) 263 ( 5.7) 213 ( 4.8) 346 ( 6.7) 320 ( 6.7) 265 ( 6.2) 26–30 536 (10.4) 460 ( 9.9) 388 ( 8.8) 588 (11.4) 565 (11.9) 455 (10.7) 31–35 595 (11.5) 489 (10.6) 520 (11.8) 617 (11.9) 553 (11.6) 558 (13.1) 36–40 667 (12.9) 620 (13.4) 556 (12.6) 693 (13.4) 572 (12.0) 531 (12.4) 41–45 792 (15.3) 665 (14.3) 593 (13.4) 754 (14.6) 651 (13.7) 532 (12.5) 46–50 826 (16.0) 752 (16.2) 657 (14.9) 816 (15.8) 751 (15.8) 573 (13.4) 51–55 618 (12.0) 641 (13.8) 710 (16.1) 607 (11.7) 643 (13.5) 620 (14.5) 56–60 449 ( 8.7) 425 ( 9.2) 493 (11.1) 439 ( 8.5) 411 ( 8.6) 450 (10.5) 61–65 197 ( 3.8) 197 (4.3) 167 ( 3.8) 216 ( 4.2) 194 ( 4.1) 184 ( 4.3)Employment position supervisor 1250 (24.2) 1159 (25.0) 1142 (25.8) 1943 (37.6) 1858 (39.0) 1563 (36.6) non-supervisor 3912 (75.8) 3476 (75.0) 3280 (74.2) 3230 (62.4) 2906 (61.0) 2705 (63.4)Level of sickness absence in 2002 in the county where the subjects worked below national mean 3211 (62.2) 2930 (63.2) 2874 (65.0) 3181 (61.5) 2912 (61.1) 2675 (62.7) above national mean 1951 (37.8) 1705 (36.8) 1548 (35.0) 1992 (38.5) 1852 (38.9) 1593 (37.3)Sick-leave spells >14 consecutive days No 4290 (83.1) 3843 (82.9) 3650 (82.5) 4626 (89.4) 4292 (90.1) 3829 (89.7) Yes 872 (16.9) 792 (17.1) 772 (17.5) 547 (10.6) 472 ( 9.9) 439 (10.3)

*Work-related sleep disturbances, sickness absence, age, supervisory position, and region.

Table 2—Age-Adjusted One-Year Prevalence of Sick-Leave Spells Longer than 14 Days, and Hospital Admission, Plus Mean Values for Self-Rated Psychosocial Work Environment and Health Measures, in 1993, 1995, and 1999.*

Women Men 1993 1995 1999 P for trend 1993 1995 1999 P for trendWork-related sleep disturbances at least once a week in the last 3 months** 12.6% 15.5% 22.7% < 0.001 12.3% 15.1% 18.8% < 0.001Sick-leave spells > 14 days 16.9% 16.6% 16.3% 0.914 10.1% 9.4% 9.5% 0.580Hospital admission (any cause) 8.1% 7.7% 6.9% 0.138 5.2% 4.9% 4.7% 0.695Psychological demands at work (0–10) 2.95 3.10 3.52 < 0.001 2.91 3.00 3.13 < 0.001Decision authority (0–10) 5.53 5.51 5.27 < 0.001 6.47 6.32 6.05 < 0.001Skill discretion (0–10) 5.76 5.49 5.47 < 0.001 6.07 5.86 5.74 < 0.001Social support (0–10) 7.47 7.38 7.20 < 0.001 6.53 6.62 6.74 < 0.001Depressive symptoms (0–4) 0.82 0.95 1.13 < 0.001 0.76 0.85 0.98 < 0.001Physical exhaustion (0–4) 1.33 1.47 1.57 < 0.001 1.25 1.31 1.41 < 0.001Psychosomatic symptoms (0–4) 0.83 0.92 1.07 < 0.001 0.66 0.76 0.82 < 0.001Musculoskeletal symptoms (0–4) 0.89 0.94 1.06 < 0.001 0.62 0.68 0.72 < 0.001

*For participants with complete data on all study variables including health and work environment (11,548 women and 12,025 men).**Values reported in the text are slightly different because they are based on analyses which did not require complete data.

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ses of our data affected the associations between work-related sleep disturbances and sickness absence only marginally, which indicates that work-related sleep disturbances may be indepen-dently associated with sickness absence.

Other stressors, such as job insecurity and immediate threat of job loss, have been shown to be associated with fatigue and sleep disturbances,38-40 as well as with sickness absence,41-43 and could thus potentially explain an association between work-re-lated sleep disturbances and sickness absence. However, since we excluded all subjects who had received unemployment ben-efit during the survey years, this is unlikely to explain the as-sociations found in this paper.

As our findings are based on repeated cross-sectional mea-surements, we do not know if work-related sleep disturbances caused sickness absence, or vice versa, or whether the associa-tion could be attributable to confounding. Some studies indicate that sleep disturbances lead to sickness absence,11,14 but it is also conceivable that sleep disturbances are caused by worries over work that sick-listed people have, or by other consequences of sickness absence (which could in turn impede return to work). The scientific knowledge base of any type of consequence, posi-tive or negative, of sickness absence is very limited.36 We found only one such study on consequences on sleep; a retrospec-tive study of 862 persons who were or had been on long-term sickness absence.12 That study suggested that sickness absence might have consequences for sleep: about two-thirds reported negative effects and one in ten reported positive effects.

The fact that the increasing secular trend in work-related sleep disturbances was not paralleled by a similar trend in sick-ness absence in our data suggests that sleep disturbances are not a dominant determinant of the incidence of sickness absenc-

able for comparison, but raw data on the increase in self-reported sleep disturbances in Sweden have been reported previously.21 Our findings are consistent with the international trends of in-creased mental health complaints.27,35 Increase in reported work-related sleep disturbances could reflect a number of things: in-creasing objective sleep disturbances; an increasing tendency to report sleep disturbances; or an increased tendency to attribute sleep disturbances to problems in the work situation (which in turn could reflect either deteriorating work environment, a stron-ger objective association between work and sleep, or a cultural tendency to blame health problems on work). An increased pub-lic awareness of both sleep disturbances and psychosocial work environment as reflected in the media could have contributed to both increased reporting and attribution to work.

In the years this study covers, the prevalence of long-term sickness absence increased in Sweden, especially among wom-en.36 However, this was not reflected in our data, since only currently working individuals were included in SWES (in effect excluding most cases of very long absences and all full-time disability pensioners). Plausible explanations for the increase in sickness absence in the population have been extensively discussed without reaching clear conclusions. It can be hy-pothesized that the substantial increase in work-related sleep disturbances is one explanation.14 Other surveys in Sweden have for the same years shown a trend of increasing demands among both male and female employees in female-dominated occupations.37 A corresponding increase in male-dominated or gender-integrated occupations was not found. In the present study, there were indeed significant changes in psychosocial work environment for both women and men. However, adding demands and other work environment variables to the analy-

Table 3—Cross-Sectional Association Between Self-Reported Work-Related Sleep Disturbances and Having had at Least One Sick-Leave Spell > 14 Days; Results for Women

Have you during the last 3 months n n (%) with at least one OR (95 % CI) for having at least onehad difficulties sleeping because sick-leave spell of >14 sick-leave spell of >14 consecutive daysthoughts of work have kept you awake? consecutive days Adjusted for age Adjusted for age Adjusted for age + supervisory + supervisory position position + regionSWES 1993 Not at all/Seldom last 3 months 3506 583 (16.6) 1 1 1 A couple of days a month 1014 158 (15.6) 0.89 (0.74–1.08) 0.92 (0.76–1.12) 0.93 (0.76–1.12) One day a week 352 62 (17.6) 0.99 (0.74–1.32) 1.03 (0.77–1.37) 1.04 (0.77–1.39) A couple of days a week 232 45 (19.4) 1.10 (0.78–1.54) 1.15 (0.82–1.62) 1.15 (0.82–1.62) Every day 58 24 (41.4) 3.15 (1.84–5.40) 3.20 (1.87–5.48) 3.22 (1.88–5.50)SWES 1995 Not at all/Seldom last 3 months 2894 457 (15.8) 1 1 1 A couple of days a month 976 163 (16.7) 1.04 (0.86–1.27) 1.09 (0.89–1.32) 1.09 (0.90–1.33) One day a week 393 80 (20.4) 1.31 (1.00–1.70) 1.36 (1.04–1.79) 1.38 (1.05–1.80) A couple of days a week 321 69 (21.5) 1.31 (0.98–1.75) 1.39 (1.04–1.86) 1.42 (1.06–1.89) Every day 51 23 (45.1) 3.81 (2.16–6.71) 3.88 (2.20–6.85) 3.98 (2.25–7.03)SWES 1999 Not at all/Seldom last 3 months 2376 353 (14.9) 1 1 1 A couple of days a month 1020 154 (15.1) 0.97 (0.79–1.19) 0.99 (0.80–1.22) 0.99 (0.80–1.22) One day a week 476 104 (21.8) 1.54 (1.20–1.97) 1.59 (1.24–2.04) 1.59 (1.24–2.04) A couple of days a week 431 108 (25.1) 1.75 (1.37–2.25) 1.82 (1.42–2.34) 1.83 (1.42–2.35) Every day 119 53 (44.5) 4.03 (2.74–5.92) 4.17 (2.83–6.13) 4.13 (2.81–6.08)

Odds Ratios (OR) and 95% Confidence Intervals (CI).

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disturbances and sickness absence over time in women may mean that work-related sleep disturbances and sickness absence increasingly co-occurred in this group, either because the sig-nificance of such sleep disturbances as determinants of sickness absence was increasing over time or because sickness absence or other factors increasingly caused sleep disturbances.

es longer than 14 days, that other determinants have become less important, or that there is a time lag between the trends. However, in a previous Swedish study work-related sleep dis-turbances prospectively predicted sickness absences longer than 90 days,14 an outcome which was poorly represented in our study. The strengthening of the association between sleep

Table 4—Cross-Sectional Association Between Self-Reported Work-Related Sleep Disturbances and Having had at Least One Sick-Leave Spell > 14 Days; Results for Men

Have you during the last 3 months n n (%) with at least one OR (95 % CI) for having at least onehad difficulties sleeping because sick-leave spell of >14 sick-leave spell of >14 consecutive daysthoughts of work have kept you awake? consecutive days Adjusted for age Adjusted for age Adjusted for age + supervisory + supervisory position position + regionSWES 1993 Not at all/Seldom last 3 months 3421 358 (10.5) 1 1 1 A couple of days a month 1096 90 ( 8.2) 0.74 (0.58–0.94) 0.81 (0.64–1.04) 0.81 (0.64–1.04) One day a week 370 44 (11.9) 1.08 (0.77–1.51) 1.20 (0.85–1.68) 1.20 (0.86–1.68) A couple of days a week 218 36 (16.5) 1.56 (1.07–2.28) 1.65 (1.13–2.42) 1.66 (1.13–2.42) Every day 68 19 (27.9) 3.00 (1.74–5.18) 3.26 (1.87–5.66) 3.26 (1.88–5.68)SWES 1995 Not at all/Seldom last 3 months 3043 281 ( 9.2) 1 1 1 A couple of days a month 980 85 ( 8.7) 0.87 (0.68–1.13) 0.93 (0.74–1.25) 0.97 (0.75–1.26) One day a week 384 42 (10.9) 1.10 (0.78–1.56) 1.23 (0.87–1.75) 1.23 (0.87–1.75) A couple of days a week 298 48 (16.1) 1.63 (1.16–2.28) 1.89 (1.34–2.66) 1.89 (1.34–2.67) Every day 59 16 (27.1) 2.92 (1.60–5.31) 3.29 (1.79–6.04) 3.30 (1.80–6.06)SWES 1999 Not at all/Seldom last 3 months 2529 229 ( 9.1) 1 1 1 A couple of days a month 911 80 ( 8.8) 0.91 (0.69–1.19) 0.94 (0.72–1.23) 0.94 (0.71–1.23) One day a week 424 60 (14.2) 1.53 (1.12–2.09) 1.62 (1.19–2.22) 1.62 (1.19–2.21) A couple of days a week 328 45 (13.7) 1.42 (1.01–2.02) 1.55 (1.09–2.20) 1.55 (1.09–2.20) Every day 76 25 (32.9) 4.14 (2.49–6.88) 4.27 (2.56–7.12) 4.26 (2.56–7.10)

Odds Ratios (OR) and 95% Confidence Intervals (CI).

Table 5—Effects of Adjustment for Work Environment and Health Indicators on the Associations Between Work-Related Sleep Disturbances and Sickness Absence 1993-1999

Have you in the last 3 months n Adjustmentshad difficulties sleeping because Model A: age, supervisory A + work A + hospitalthoughts of work have kept you awake? position, region, and environment admission and self- survey year reported symptoms*WOMEN Not at all/Seldom last 3 months 7112 1 1 1 A couple of days a month 2489 0.98 (0.86–1.11) 0.98 (0.86–1.12) 0.81 (0.70–0.94) One day a week 988 1.35 (1.14–1.60) 1.29 (1.08–1.55) 0.87 (0.72–1.07) A couple of days a week 791 1.44 (1.20–1.73) 1.32 (1.08–1.60) 0.80 (0.65–1.00) Every day 168 3.56 (2.59–4.90) 3.09 (2.21–4.31) 1.69 (1.16–2.46)Change in Odds Ratio** ± 0% – 18.3% – 73.0%MEN Not at all/Seldom last 3 months 7621 1 1 1 A couple of days a month 2539 0.82 (0.69–0.96) 0.88 (0.74–1.04) 0.74 (0.62–0.89) One day a week 1001 1.28 (1.03–1.58) 1.35 (1.08–1.68) 0.94 (0.74–1.19) A couple of days a week 707 1.63 (1.30–2.04) 1.75 (1.38–2.23) 0.97 (0.74–1.26) Every day 157 3.88 (2.71–5.55) 4.18 (2.86–6.12) 1.73 (1.13–2.66)Change in Odds Ratio** ± 0% + 10.4% – 74.7%

*Depressive symptoms, physical exhaustion, psychosomatic symptoms, and musculoskeletal symptoms.**Compared with Model A odds ratios for the response alternative every day.

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SLEEP, Vol. 31, No. 8, 2008 1175

absences typically caused by minor health problems such as colds.56 Records of sick leaves longer than 14 days were certi-fied by a physician and did not include absence for care of new-born or sick children, which is treated separately from sickness absence in the public records.

The exact number of people varied slightly between the years, but the sampling procedure remained the same. Since SWES is intended to be representative of the Swedish working population at the time of the survey, the demographic charac-teristics change over time as a reflection of the demographic changes in the working population. Thus, our study indicates an increased prevalence of work-related sleep disturbances in the working population and similar trends were found for depres-sive symptoms and other self-reported health problems.

A potential source of bias, however, is the fact that attri-tion increased over time in SWES (as in most other surveys in Sweden).57 Persons with health problems tend to be more likely than healthy people to become non-respondents,58,59 potentially contributing to an underestimation of the rise in work-related sleep disturbances. In the present data, men, young workers and persons with low socioeconomic status were also overrepresent-ed in the drop-out group.60 However, the multivariate analyses took into account the effect of some of these characteristics (i.e., the separate models for men and women were adjusted for mea-sures of work environment) and this did not substantially change age-standardized estimates. This is evidence against the possibil-ity that bias due to accelerated sample attrition would explain the observed increase in work-related sleep disturbances.

Finally, the two items used to measure depressive symptoms have not been validated against a diagnosis of major depres-sion. Thus, it remains unclear to what extent clinical depression, nonclinical depressive symptoms, and other characteristics of the respondents, affected the association between self-reported sleep disturbances attributed to work-related causes and medi-cally-certified sickness absence.

From other research, we know that disturbed sleep and short sleep are related to morbidity,2-9 particularly depression,32,44-47 and to mortality,48 and there is a correlation between work stress and sleep.49,50 A plausible hypothesis would thus be that sleep disturbances induce health problems that are reflected in increased sick-leave rates. The observed attenuation of the re-lationship between work-related sleep disturbances and sick-ness absence with adjustment for depressive symptoms (rather than other health measures) indicates that the association may be mediated by depressive symptoms. It is also possible that the observed sleep disturbances, despite being attributed to work-related causes, are in fact symptoms of depression, which may be the underlying cause of both sleep disturbances and sick-ness absence, or that sleep disturbances are part of a larger syndrome that includes depressive symptoms.45 From earlier research we know that sleep disturbances tend to precede the onset of depression,47,51,52 suggesting either a causal relationship from sleep disturbance to depression or that sleep disturbances are early symptoms of depression. The increasing trends found for work-related sleeping problems and depressive symptoms are largely parallel to observed increases in antidepressant treatment53 and long sick-leaves for mental disorders54 and depression,55 suggesting that the depressive symptoms mea-sured in our study may be associated also with clinical levels of depression. Further research is needed to clarify the mechanisms linking sleep disturbances, health and environmental stress.

Methodological considerations

The main strengths of the present study are that it is based on large, random samples of the national working population from several years, and that the sickness absence data are registry based and of good quality. Furthermore, we include only lon-ger sick-leave spells (>14 days) reflecting more long-standing illnesses15 and unavoidable work disability, rather than short

Table 6—Effects of Adjustment for Different Health Measures on the Associations Between Work-Related Sleep Disturbances and Sickness Absence 1993-1999

Have you in the last 3 months Adjustmentshad difficulties sleeping because A* + hospital A* + depressive A* + physical A* + psychosomatic A* + musculoskeletalthoughts of work have kept admission symptoms exhaustion symptoms symptomsyou awake? WOMEN Not at all/Seldom last 3 months 1 1 1 1 1 A couple of days a month 0.99 (0.87–1.14) 0.83 (0.73–0.95) 0.88 (0.77–1.01) 0.88 (0.78–101) 0.88 (0.77–1.00) One day a week 1.32 (1.10–1.59) 0.95 (0.80–1.14) 1.06 (0.89–1.26) 1.08 (0.91–1.29) 1.08 (0.91–1.30) A couple of days a week 1.46 (1.20–1.78) 0.84 (0.69–1.04) 1.02 (0.84–1.24) 1.05 (0.86–1.27) 1.04 (0.86–1.26) Every day 3.77 (2.69–5.28) 1.60 (1.14–2.26) 2.22 (1.60–3.10) 2.28 (1.64–3.18) 2.36 (1.69–3.31)Change in Odds Ratio** + 8.2% – 76.6% – 52.3% – 50.0% – 46.9%MEN Not at all/Seldom last 3 months 1 1 1 1 1 A couple of days a month 0.81 (0.68–0.96) 0.72 (0.61–0.85) 0.78 (0.66–0.92) 0.77 (0.65–0.91) 0.80 (0.68–0.95) One day a week 1.31 (1.05–1.63) 0.95 (0.76–1.17) 1.06 (0.86–1.32) 1.06 (0.85–1.31) 1.07 (0.86–1.33) A couple of days a week 1.63 (1.28–2.07) 1.02 (0.80–1.30) 1.22 (0.96–1.54) 1.26 (1.00–1.60) 1.19 (0.94–1.51) Every day 3.85 (2.62–5.65) 2.00 (1.36–2.95) 2.38 (1.64–3.46) 2.56 (1.76–3.73) 2.45 (1.67–3.60Change in Odds Ratio** – 1.0% – 65.3% – 52.1% – 45.8% – 49.7%

*Odd ratios for Model A are presented in Table 5.**Compared with Model A odds ratios for the response alternative every day.

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13. Ohayon MM, Lemoine P, Arnaud-Briant V, Dreyfus M. Preva-lence and consequences of sleep disorders in a shift worker popu-lation. J Psychosom Res 2002;53:577-83.

14. Akerstedt T, Kecklund G, Alfredsson L, Selen J. Predicting long-term sickness absence from sleep and fatigue. J Sleep Res 2007;16:341-5.

15. Marmot M, Feeney A, Shipley M, North F, Syme SL. Sickness absence as a measure of health status and functioning: from the UK Whitehall II study. J Epidemiol Community Health 1995;49:124-30.

16. Ota A, Masue T, Yasuda N, Tsutsumi A, Mino Y, Ohara H. Asso-ciation between psychosocial job characteristics and insomnia: an investigation using two relevant job stress models--the demand-control-support (DCS) model and the effort-reward imbalance (ERI) model. Sleep Med 2005;6:353-8.

17. Linton SJ. Does work stress predict insomnia? A prospective study. Br J Health Psychol 2004;9:127-36.

18. Dahlgren A, Kecklund G, Akerstedt T. Different levels of work-related stress and the effects on sleep, fatigue and cortisol. Scand J Work Environ Health 2005;31:277-85.

19. Greenberg J. Losing sleep over organizational injustice: attenuat-ing insomniac reactions to underpayment inequity with supervisory training in interactional justice. J Appl Psychol 2006;91:58-69.

20. Ancoli-Israel S, Roth T. Characteristics of insomnia in the United States: results of the 1991 National Sleep Foundation Survey. I. Sleep 1999;22 Suppl 2:S347-53.

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22. Lehto A-M, Sutela H. Results from work life surveys 1977-2003. Helsinki: Statistics Finland, 2004.

23. Persson G, Danielsson M, Rosen M, et al. Health in Sweden: the National Public Health Report 2005. Scand J Public Health Suppl 2006;67:3-10.

24. Kronholm E, Partonen T, Laatikainen T, et al. Trends in self-re-ported sleep duration and insomnia-related symptoms in Finland from 1972 to 2005: a comparative review and re-analysis of Finn-ish population samples. J Sleep Res 2008;17:54-62.

25. Horne J. Is there a sleep debt? Sleep 2004;27:1047-9.26. Shiels C, Gabbay MB, Ford FM. Patient factors associated with

duration of certified sickness absence and transition to long-term incapacity. Br J Gen Pract 2004;54:86-91.

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28. Stansfeld S, Feeney A, Head J, Canner R, North F, Marmot M. Sickness absence for psychiatric illness: the Whitehall II Study. Soc Sci Med 1995;40:189-97.

29. Vaez M, Rylander G, Nygren A, Asberg M, Alexanderson K. Sickness absence and disability pension in a cohort of employees initially on long-term sick leave due to psychiatric disorders in Sweden. Soc Psychiatry Psychiatr Epidemiol 2007;42:381-8.

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31. Lindholm C, Fredlund P, Backhans M. Hälsotillstånd och sjuk-skrivningsutveckling. [Health status and development of sickness absence. In Swedish.]. In: Marklund S, Bjurvald M, Hogstedt C, Palmer E, Theorell T, eds. Den höga sjukfrånvaron – problem och lösningar. [The high sickness absenteeism – problems and solu-tions. In Swedish.]. Stockholm: National Institute for Working Life, 2005:39-59.

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concluSion

Our findings from population-based surveys between 1993 and 1999 indicate that self-reported sleep disturbances attrib-uted to work-related causes were on the rise in Sweden and were associated with medically-certified sickness absence. A substantial part of the association seems to be related to depres-sive symptoms, but further research is needed to determine the exact nature of this association.

acknoWledgMentS

We wish to thank all the participants in the Swedish Work Environment Surveys, Statistics Sweden, Centre for Epidemi-ology at the Swedish National Board of Health and Welfare for their respective contributions. We also thank Dr. Gabriel Oxen-stierna for his original idea to merge the databases into the one used in this project. The database used was financed by a re-search grant from the Swedish Council for Work Life Research (Vinnova, Grant # P23216-1A). The study was funded by the Swedish Council for Work Life Research (RALF) and the Swedish Council for Working Life and Social Research (FAS) (FAS, grant #2004-2021). MK is supported by the Academy of Finland (grants number 117604,124322, and 124271).

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appendix: items used for measurement of health-related symptoms

All items are rated on a 5-point Likert scale, coded 4–0: Ev-ery day; A couple of days a week (1 day of 2); One day a week (1 day of 5); A couple of days a month (1 day of 10); Not at all/Seldom last 3 months.

depressive Symptoms (after exclusion of items referring to Sleep):

1. During the last 3 months have you been tired and list-less?

2. Does it happen that you feel ill at ease going to your job?

physical exhaustion:

1. Does it happen that you are physically exhausted when you get home from work?

psychosomatic Symptoms:

During the last 3 months have you...1. ...had heartburn, acid burping, a burn in the pit of your

stomach, or upset stomach?2. ...had headaches?

Musculoskeletal Symptoms:

After work, do you experience pain in any of the following places...

1. ...upper parts of your back or neck?2. ...lower parts of your back?3. ...shoulders or arms?4. ...wrists or hands?5. ...hips, legs, knees, or feet?

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