Int Arch Occup Environ Health DOI 10.1007/s00420-014-0945-6
ORIGINAL ARTICLE
Synergistic interaction between job control and social support at work on depression, burnout, and insomnia among Japanese civil servants Yasuaki Saijo • Shigeru Chiba • Eiji Yoshioka • Yoshihiko Nakagi Toshihiro Ito • Kazuyo Kitaoka-Higashiguchi • Takahiko Yoshida
•
Received: 5 November 2013 / Accepted: 12 May 2014 Ó Springer-Verlag Berlin Heidelberg 2014
Abstract Purpose To elucidate whether low job control and low social support at work have synergistic interaction on mental health. The synergistic interaction was also analyzed after stratification by high and low job demands. Methods Participants were 2,121 local government employees in Asahikawa city, Japan. The Brief Job Stress Questionnaire was used to assess job demands, job control, and social support. Depression was assessed using the Patient Health Questionnaire-9. The Maslach Burnout Inventory-General Survey was used to assess burnout. Insomnia was assessed using the Athens Insomnia Scale. Possible confounder-adjusted logistic regression analyses were performed to obtain odds ratios for depression, burnout, and insomnia, and synergy indices between job control and social support at work were assessed. Results The synergy indices among men and women, respectively, were 2.08 (80 % confidence interval: 1.01, 4.27) and 1.98 (0.67, 5.89) for depression, 1.79 (1.28, 2.51) and 2.62 (1.07, 6.40) for burnout, and 1.92 (1.22, 3.02) and 2.77 (0.43, 18.01) for insomnia. Men with high job demands had higher synergistic interaction on depression and burnout, compared to men with low job demands, and Y. Saijo (&) E. Yoshioka Y. Nakagi T. Ito T. Yoshida Department of Health Science, Asahikawa Medical University, Midorigaoka-higashi 2-1-1-1, Asahikawa, Hokkaido 078-8510, Japan e-mail: [email protected] S. Chiba Department of Psychiatry and Neurology, Asahikawa Medical University, Asahikawa, Japan K. Kitaoka-Higashiguchi Institute of Medical, Pharmaceutical and Health Sciences, Kanazawa University, Kanazawa, Japan
women with low job demands had higher synergistic interaction between job control and social support at work on burnout and insomnia, compared to women with high job demands. Conclusions There were more-than-additive interactions of job control and social support at work on depression, burnout, and insomnia. After stratification by job demands, the synergistic interaction may be different between men and women. To assess job stress, it is necessary to consider the interactive effect of not only job demands and job control but also job control and social support at work. Keywords Job control Social support Job demands Depression Burnout Insomnia Synergy index
Introduction The psychosocial work environment is important for mental health (Stansfeld and Candy 2006), and the demand–control–support (DCS) model (Karasek 1979) is an important job stress model that has been widely used in the field of occupational health. The model has two job stress dimensions, job demands and job control, with social support included as a third dimension. Moreover, because the DCS model focuses mainly on job demands and control, many studies have reported combined effects of job demands and control on health outcomes (Fransson et al. 2012), and few studies have reported their synergistic interactions (Santavirta et al. 2007). ‘‘Collective control’’ is the concept in which control over the work process is strongly influenced by the character of workplace social groups, based on Gardell’s suggestion of the importance of uniting the following two concepts for a strategy of work reform: (1) the combining
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of the collective influence in management decision making with the formation of an autonomous group and (2) the individual and the group achieving greater control over the work system and working methods (Gardell 1982). Increased worker control—at the individual as well as at the collective level—may be a means of reducing the risk of work environment-related stress and diseases (Aronsson 1989). This influence process, collective control, determines the possibilities for collective coping with chronic demands and the pressures of various production systems (Johnson 1989). There have been reports that a combined effect of job control and social support at work increases the risk of cardiovascular disease (Johnson 1989). Furthermore, the concept of collective control implies a synergistic effect of job control and social support at work on mental health (Choi et al. 2011). A Swedish study reported synergy indices between job control and social support on general psychological distress, as assessed by the General Health Questionnaire (GHQ) (Choi et al. 2011). The study observed a synergistic (i.e., more-than-additive) effect of low job control and low social support at work on general psychological distress. The results showed that the synergistic effect was stronger when job demands were low compared to when job demands were high, and particularly in men, an antagonistic effect was observed when job demands were high. However, to our knowledge, only one study has reported a synergistic effect of job control and social support at work on mental health or the confidence interval of this measure (Choi et al. 2011). Burnout is a psychological outcome of chronic stress, which is characterized by emotional exhaustion, cynicism, and negative self-evaluation (Maslach et al. 2001). Burnout is a predictor of depression and life dissatisfaction among dentists (Hakanen and Schaufeli 2012). Though burnout was initially explored among medical and human services professionals, it has been recently described in various other occupational areas. A population-based study reported a significant association of burnout with sick leaves (Ahola et al. 2008) and severe injury (Ahola et al. 2014). Studies have reported that depression and insomnia are negatively related to occupational performance (Bender and Farvolden 2008; Kucharczyk et al. 2012), and effects of job strain, a combined index of high job demand and low job control, on depression and insomnia have also been reported (Bonde 2008; Ota et al. 2009). However, the interactive effects of low job control and low social support at work on depression and insomnia have not been fully investigated. The present study aimed to elucidate whether low job control and low social support at work have synergistic interactions on mental health indicators such as burnout,
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depression, and insomnia, with analysis of their confidence intervals. The synergistic interactions were analyzed after stratification by high and low job demands.
Methods Participants The original participants were 2,811 local government employees in Asahikawa city, Hokkaido, Japan: the entire workforce except for 75 new recruits hired after April 2010 and employees taking medical leave between July and August 2010. Self-administered questionnaires were distributed to the employees on June 28, 2010. Responses were obtained from 2,514 employees (response rate: 89.4 %) between July and August 2010. After excluding 393 questionnaires because of missing values, 2,121 questionnaires (75.4 % of the original participants) were analyzed. This study was approved by the Institutional Ethical Board for Epidemiological Studies of Asahikawa Medical University. Demographics and work-related factors The questionnaire included sex, age (ranges 19–30, 31–40, 41–50, 51–60 years old), marital status, occupation, job ranking, shift work, and use of hypnotics. The recall period for the variables was the preceding 1 month. Classification of socioeconomic status was performed similar to a previous Japanese civil servants study (Nishi et al. 2004; Saijo et al. 2008). Employment grade was categorized into ‘‘higher-level non-manual,’’ ‘‘lower-level non-manual,’’ and ‘‘manual.’’ ‘‘Non-manual’’ workers included clerical workers, professional workers (e.g., physicians, nurses, public health nurses, pharmacists, radiographers, dieticians, researchers, and technicians), and firefighters. ‘‘Higher-level non-manual’’ workers included non-manual workers whose ranks were department director or section chief for men, and department director, section chief, or chief clerk for women. ‘‘Lower-level non-manual’’ workers included non-manual workers whose ranks were chief clerk or who were in a non-managerial position for both men and women. Regardless of rank, ‘‘manual’’ workers included sanitation workers, cooks, and janitors. Firefighters are not classified as ‘‘manual’’ workers in the Japanese civil servants system, and their work includes various jobs such as ‘‘firefighting,’’ ‘‘ambulance work,’’ ‘‘rescue work,’’ ‘‘command and control center work,’’ and ‘‘fire investigation.’’ Therefore, firefighters were categorized as ‘‘non-manual’’ workers.
Int Arch Occup Environ Health
Job stress and social support The Brief Job Stress Questionnaire (BJSQ) (Shimomitsu et al. 2000), which is based on the DCS model (Karasek 1979), was used to assess two job stress dimensions (job demands and job control) and social support from supervisors, co-workers, and family/friends. The BJSQ has been widely used in Japan for practical occupational health evaluation and occupational health research (Kawada and Otsuka 2011; Suwazono et al. 2008; Umehara et al. 2007). The questionnaire for job demands included three items: (1) You have to do an enormous amount of work, (2) You cannot complete all your work in the allotted time, and (3) You have to work very hard. In addition, there were three items about job control: (1) You can work at your own pace, (2) You can decide the order in which you do your work and the way you do it, and (3) You can provide your opinions on the work strategy of your workplace. The responses were scored on a 4-point Likert-type scale (1, agree; 2, somewhat agree; 3, somewhat disagree; and 4, disagree) (Suwazono et al. 2008). To assess job demands and job control, the total score for each dimension was used; higher scores denoted higher job demands or control. Social support from supervisors, co-workers, and family/friends was also assessed using three items: (1) You can often communicate with supervisors/co-workers, (2) You can strongly rely on supervisors/co-workers when you have problems, and (3) Your supervisors/co-workers are prepared to spend their time on your personal problems. The responses to these items were also scored on a 4-point Likert-type scale (1, agree; 2, somewhat agree; 3, somewhat disagree; and 4, disagree). The total score was calculated for each of the three questions; higher scores denoted higher social support (Kawada and Otsuka 2011). Job demands and job control scores were each dichotomized at the median value. The social support at work score was the sum of social support from supervisors and co-workers, and it was also dichotomized at the median value. The social support from family/friends score, which was used as a confounder, was a continuous variable. Depression Depression was assessed using the Japanese version of Patient Health Questionnaire-9 (PHQ-9) (Muramatsu et al. 2007), the depression subscale of the self-report Patient Health Questionnaire (Spitzer et al. 1999). The PHQ-9 consists of nine items, and the total score varies between 0 and 27 (Kroenke et al. 2001). Standard PHQ-9 cutoff scores were used to classify minimal (0–4), mild (5–9), and moderate to severe (C10) symptoms of depression (Roy et al. 2012). In this study, ‘‘positive for depression’’ was
defined as PHQ-9 scores C10. Cronbach’s alpha coefficient for the PHQ-9 was 0.88. Burnout The Japanese version of the Maslach Burnout InventoryGeneral Survey (MBI-GS) was used to assess burnout (Kitaoka-Higashiguchi et al. 2004a, b; Maslach et al. 1996, 2001). The MBI-GS has 16 items with three subscales: exhaustion (5 items), cynicism (5 items), and professional efficacy (6 items). Each item was scored using a 7-point Likert-type scale ranging from ‘‘never’’ (0 points) to ‘‘every day’’ (6 points), based on the frequency of occurrence. The total scores for each subscale were divided by the number of items on the subscale; higher exhaustion and cynicism scores and a lower professional efficacy score denote higher degrees of burnout. Each subscale was tertilized, and the upper tertiles of exhaustion and cynicism and the lower tertile of professional efficacy were defined as subscale positives. Then, on the basis of the common burnout process in which exhaustion comes first, followed by cynicism and diminished professional efficacy, the participants who were at least exhaustion positive and were either cynicism positive or professional efficacy positive were defined as burnout positive (Brenninkmeijer and VanYperen 2003; KitaokaHigashiguchi et al. 2009). Insomnia Insomnia was assessed using the Athens Insomnia Scale (AIS). The AIS is a validated instrument based on the International Classification of Disease, 10th Revision (ICD-10) criteria (Soldatos et al. 2000, 2003). The scale is a self-administered inventory consisting of eight items. The first five items assess difficulty with sleep induction, awakening during the night, final awakening earlier than desired, total sleep duration, and overall quality of sleep. The remaining three items pertain to the next-day consequences of insomnia: sense of well-being during the day, functioning during the day, and sleepiness during the day. Each item of the AIS was rated from 0 (no problem at all) to 3 (very serious problem). The total score ranged from 0 to 24, and having six or more points was defined as the presence of insomnia. The participants were requested to choose rating items only if they had experienced sleep difficulty at least 3 times a week during the last month. Statistical analysis We analyzed data for men and women separately, because a previous study speculated that the synergistic effect might vary by sex (Choi et al. 2011). Crude odds ratios
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(ORs) of age, marital status, employment grade, shift work, hypnotic use, social support from family/friends, job demands, job control, and social support at work concerning depression, burnout, and insomnia were obtained by univariate logistic regression analysis, and adjusted ORs of all variables were obtained in full multivariable models. To investigate synergistic effects of job control and social support at work on depression, burnout, and insomnia, we created four categorical variables: high control and high social support at work (reference); high control and low social support at work; low control and high social support at work; and low control and low social support at work. The ORs of the latter three variables were obtained after adjustment for age, marital status, employment grade, shift work, hypnotic use, social support from family/friends, and job demands. Because hypnotic use was introduced into the model in previous studies of job strain and insomnia (Ota et al. 2005; Sinokki et al. 2010) and was significantly related to three outcomes of both sex and job control for men, it was introduced in the adjusted model as a possible confounder. Then, a synergy index (Rothman 1986) was obtained using following formula: S¼
ORðABÞ 1 ½OR ðAbÞ 1 þ ½ORðaBÞ 1
S: synergy index; OR: odds ratio; Ab: exposed to one factor; aB: exposed to the other factor; AB: exposed to both factors. The synergy index (S) was greater than one (S [ 1) if a synergistic interaction of both exposures was present and less than one (S \ 1) if an antagonistic interaction was present. The confidence interval (CI) was estimated (Hosmer and Lemeshow 1992) after calculating the standard error of the synergy index, using an asymptotic covariance matrix analyzed using SPSS syntax (Andersson et al. 2005). Because of low statistical power in the interaction test (Greenland 1993; Marshall 2007), we calculated 95 and 80 % CIs of the synergy index, as previously reported (Choi et al. 2011). We also estimated the synergy index after stratification by levels of job demands. P values \0.05 of ORs were considered statistically significant. All calculations were conducted using IBM SPSS Statistics 21.0 for Windows (SPSS Inc., Chicago, IL, USA).
Results For the subscales of the BJSQ, Cronbach’s alpha coefficients for job demands, job control, social support from supervisors, social support from co-workers, and social support from family/friends were 0.78, 0.71, 0.83, 0.81, and 0.89, respectively. For the subscales of the MBI-GS,
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Table 1 Characteristics of male and female participants Men (N = 1,459)
Women (N = 662)
Age (years) B30
191 (13.1)
192 (29.0)
31–40
525 (36.0)
279 (42.1)
41–50
351 (24.1)
110 (16.6)
51–60
392 (26.9)
81 (12.2)
180 (12.3) 1,184 (81.2)
105 (15.9) 500 (75.5)
Employment grade Higher-level non-manual Lower-level non-manual
95 (6.5)
57 (8.6)
Married
Manual
1,094 (75.0)
311 (47.0)
Shift work
317 (21.7)
249 (37.6)
No
1,357 (93.0)
579 (87.5)
Yes
58 (4.0)
48 (7.3)
Former
44 (3.0)
35 (5.3)
8.0 ± 2.0
8.5 ± 3.2
Hypnotic
Job stress (score) Demand
8.0 ± 1.9
7.7 ± 1.8
Social support at work (score)
Control
15.7 ± 3.7
15.7 ± 3.9
Social support from family/friends (score)
9.6 ± 2.1
10.1 ± 2.0
Burnout (score) Exhaustion
2.3 ± 1.5
3.4 ± 1.6
Cynicism
1.8 ± 1.5
2.1 ± 1.5
Reduced professional efficacy Burnout positive
2.2 ± 1.2
2.1 ± 1.2
375 (25.7)
157 (23.7) 4.8 ± 3.5
AIS (score)
4.1 ± 3.3
Insomnia positive (AIS C 6)
399 (27.3)
228 (34.4)
PHQ-9 (score)
4.3 ± 4.6
6.0 ± 4.9
PHQ-9 C10
187 (11.4)
133 (20.1)
Variables are presented as number (percentage) or mean ± SD
Cronbach’s alpha coefficients for exhaustion, cynicism, and professional efficacy were 0.92, 0.88, and 0.86, respectively. The Cronbach’s alpha coefficient for the AIS was 0.83. Table 1 shows the demographic and work-related factors; scores on the BJSQ, MBI-GS, AIS, and PHQ-9 scales; and the number of participants who were burnout positive, insomnia positive, and depression positive (PHQ-9 C 10). The prevalence of insomnia and depression was higher in women than in men. One hundred and thirty-three men had both depression and burnout (79.6 % of men with depression: 35.5 % of men with burnout); 142 men had both depression and insomnia (85.0 % of men with depression: 35.6 % of men with insomnia); and 228 men had both burnout and insomnia (60.8 % of men with burnout: 57.1 % of men
0.003 0.227 0.007 1.22, 2.57 0.86, 1.85 1.15, 2.46 1.77 1.27 1.68 1.30, 2.56 0.001 1.03, 2.06 0.032 1.45, 2.85 \0.001 1.82 1.46 2.03 Adjusted for age, marital status, employment grade, shift work, hypnotic use, and social support from family/friends a
3.19 3.24 1.51 1.11, 2.45 0.014 1.44, 3.63 \0.001 1.38, 3.20 0.001 1.65 2.29 2.10
1.64 2.24 1.38
1.05, 2.58 1.33, 3.75 0.85, 2.23
0.031 0.002 0.188
3.57 3.95 1.75
2.46, 5.19 \0.001 2.47, 6.31 \0.001 1.20, 2.56 0.004
2.10, 4.83 \0.001 1.94, 5.42 \0.001 0.96, 2.35 0.072
1.51, 2.54 \0.001 1.39, 2.42 \0.001 1.53, 2.79 \0.001 1.96 1.83 2.07 1.52, 2.42 \0.001 1.86, 3.05 \0.001 2.30, 3.89 \0.001 1.92 2.38 2.99 1.91, 3.23 \0.001 2.08, 3.73 \0.001 1.39, 2.58 \0.001 2.48 2.79 1.89 2.09, 3.39 \0.001 2.82, 4.83 \0.001 1.96, 3.32 \0.001 2.67 3.69 2.55 1.34, 2.74 \0.001 1.52, 3.47 \0.001 1.16, 2.86 \0.001 1.92 2.30 1.82 1.41, 2.74 \0.001 1.77, 3.82 \0.001 1.68, 3.74 \0.001 1.97 2.60 2.51
Men Job stress High demand Low control Low social support at work Women Job stress High demand Low control Low social support at work
95 % CI OR 95 % CI OR 95 % CI OR 95 % CI OR OR 95 % CI
P OR
95 % CI
P
Crude Adjusted Crude
a
Depression
Table 2 Crude and adjusted odds ratios for depression, burnout, and insomnia
Burnout
P
Adjusted
a
P
Crude
Insomnia
P
Adjusteda
P
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with insomnia). Among women, 78 had both depression and burnout (58.6 % of women with depression: 49.7 % of women with burnout); 112 had both depression and insomnia (84.2 % of women with depression: 34.4 % of women with insomnia); and 95 had both burnout and insomnia (60.5 % of women with burnout: 53.7 % of women with insomnia). Table 2 shows crude and adjusted ORs for depression, burnout, and insomnia. After adjustment, job demands (OR 1.92; 95 % CI 1.34, 2.74), job control (OR 2.30; 95 % CI 1.52, 3.47), and social support at work (OR 1.82; 95 % CI 1.16, 2.86) were significantly related to depression among men, and job demands (OR 1.64; 95 % CI 1.05, 2.58) and job control (OR 2.24; 95 % CI 1.33, 3.75) were significantly related to depression among women. After adjustment, job demands (OR 2.48; 95 % CI 1.91, 3.23), job control (OR 2.79; 95 % CI 2.08, 3.73), and social support at work (OR 1.89; 95 % CI 1.39, 2.58) were significantly related to burnout among men, and job demands (OR 3.19; 95 % CI 2.10, 4.83) and job control (OR 3.24; 95 % CI 1.94, 5.42) were significantly related to burnout among women. After adjustment, job demands (OR 1.96; 95 % CI 1.51, 2.54), job control (OR 1.83; 95 % CI 1.39, 2.42), and social support at work (OR 2.07; 95 % CI 1.53, 2.79) were significantly related to insomnia among men, and job demands (OR 1.77; 95 % CI 1.22, 2.57) and social support at work (OR 1.68; 95 % CI 1.15, 2.46) were significantly related to insomnia among women. Next, the synergistic effects of job control and social support at work on depression, burnout, and insomnia were estimated. Table 3 shows the ORs and synergy indices for depression, burnout, and insomnia. The synergy indices for burnout, for both men and women, were 2.08 (80 % CI 1.01, 4.27) and 1.98 (0.67, 5.89), respectively. In the analysis stratified by level of job demands (Table 4), the synergy indices for men with low job demands, men with high job demands, women with low job demands, and women with high job demands were 1.13 (0.53, 3.39), 2.85 (0.97, 8.43), 1.19 (0.57, 2.48), and -0.51 (not calculable), respectively. The synergy indices for burnout, for both men and women, were 1.79 (1.28, 2.51 *) (where * means that the 95 % CI did not include 1.0) and 2.62 (1.07, 6.40 *), respectively (Table 3). In the analysis stratified by level of job demands (Table 5), the synergy indices among men with low job demands, men with high job demands, women with low job demands, and women with high job demands were 1.29 (0.84, 1.97), 2.48 (1.41, 4.34 *), 4.61 (0.67, 31.72), and 1.11 (0.47, 2.58). The synergy indices for insomnia, for both men and women, were 1.92 (1.22, 0.43) and 2.77 (0.43, 18.01), respectively (Table 3). In the analysis stratified by level of job demands (Table 6), the synergy indices for men with
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Int Arch Occup Environ Health Table 3 Synergistic interactive effects of job control and social support at work on depression Sex
Job control
Social support at work High % (n)
Odds ratios (95 % CI)
Synergy index (95 % CI; 80 % CI)
Low % (n)
Depression Men
High
3.9 (331)
7.6 (317)
1.00
1.47 (0.71, 3.07)
2.08 (0.69, 6.26; 1.01, 4.27)
Women
Low High
7.8 (257) 9.1 (132)
19.9 (554) 14.7 (102)
1.84 (0.87, 3.87) 1.00
3.72 (1.98, 6.98) 1.07 (0.44, 2.59)
1.98 (0.38, 10.50; 0.67, 5.89)
Low
16.7 (150)
29.1 (278)
1.82 (0.82, 4.02)
2.76 (1.35, 5.66)
High
9.7 (331)
16.7 (317)
1.00
1.72 (1.04, 2.84)
Low
23.7 (257)
41.3 (554)
2.54 (1.57, 4.11)
5.05 (3.29, 7.77)
Burnout Men Women
High
9.8 (132)
10.8 (102)
1.00
0.89 (0.36, 2.20)
Low
25.3 (150)
34.2 (278)
2.23 (1.07, 4.67)
3.95 (1.99, 7.81)
High
12.4 (331)
24.3 (317)
1.00
1.78 (1.14, 2.79)
Low
19.5 (257)
41.7 (554)
1.55 (0.96, 2.48)
3.54 (2.38, 5.28)
High
25.0 (132)
34.3 (102)
1.00
1.33 (0.72, 2.45)
Low
26.0 (150)
43.5 (278)
1.02 (0.57, 1.82)
1.97 (1.18, 3.28)
1.79 (1.07, 3.01; 1.28, 2.51) 2.62 (0.67, 10.28; 1.07, 6.40)
Insomnia Men Women a
1.92 (0.96, 3.84; 1.22, 3.02) 2.77 (0.16, 48.66; 0.43, 18.01)
Adjusted for age, marital status, employment grade, shift work, hypnotic use, social support from family/friends, and job demand
low job demands, men with high job demands, women with low job demands, and women with high job demands were 1.81 (1.06, 3.07), 1.87 (0.87, 4.03), 2.40 (0.32, 17.84), and 1.71 (0.19, 15.66), respectively.
Discussion In this study, there were more-than-additive effects of job control and social support at work on depression, burnout, and insomnia, and the 80 % CI of these synergistic indices among men did not include 1.0. Moreover, men with high job demands had higher synergistic interactions for depression and burnout compared with men with low job demands, and women with low job demands had higher synergistic interaction between job control and social support at work for burnout and insomnia compared with women with high job demands. A previous Swedish study reported the synergy indices between job control and social support for general psychological distress (men: 1.68; women: 1.83), and the 80 % CI among women did not cross 1.0 (Choi et al. 2011). The synergy indices in our study, in which we used other mental health outcomes (depression, burnout, and insomnia), were almost the same or slightly larger (range 1.79–2.77), though the 80 % CI of depression and insomnia among women included 1.0. Thereby, before stratification by level of job demands, these synergistic effects were compatible with those reported in the previous study.
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After stratification by level of job demands in the Swedish study, the synergy index was larger for men with low job demands (9.24), antagonistic for men with high demands (0.52), and slightly higher for women with low job demands (2.16) compared with women with high job demands (1.51) (Choi et al. 2011). In our study, the synergy indices for women in the analysis of burnout and insomnia stratified by job demands were consistent with the previous Swedish study. However, the synergy indices for depression and burnout for men with low job demands were higher than those for men with high job demands. A Norwegian study, though not actually reporting a synergistic index, reported synergistic effects of job control and social support at work on anxiety and depression, particularly among men and women with high job demands (Sanne et al. 2005). Hence, the present synergy indices for depression and burnout in the analysis of data for men stratified by job demands are consistent with the previous Norwegian study. After stratification by level of job demands, there was no consistent tendency to the synergistic effect. The authors of the Swedish study therefore speculated that the synergistic effect might vary by the level of job demands, sex, and study context (Choi et al. 2011). Job strain, which is an interaction effect between job demands and control, has frequently been studied (Fransson et al. 2012), and it has been used for practical purposes (Choi et al. 2008) in the DCM model. However, other interactions, particularly job control and social support at
Confidence interval: not calculable c
Reference: high job control, high social support at work, and low job demands. Adjusted for age, marital status, employment grade, shift work, hypnotic use, and social support from family/ friends
b
Reference: high job control and high social support at work in low and high job demands. Adjusted for age, marital status, employment grade, shift work, hypnotic use, and social support from family/friends
a
1.18 (0.25, 5.09)
5.57 (2.25,13.83) 3.64 (1.34, 9.95) 1.44 (0.42, 4.99) 0.70 (0.19, 2.63) 22.2 (63) Low
37.2 (94)
16.7 (24) High
High
11.5 (26)
1.00
0.44 (0.08, 2.34)
-0.51c
3.10 (0.80, 11.93)
3.23 (1.37, 7.63)
1.62 (0.58, 4.52) 1.00
2.11 (0.78, 5.76)
1.19 (0.38, 3.67; 0.57, 2.48) 1.70 (0.59, 4.91)
3.58 (1.46, 8.82) 2.48 (0.89, 6.92)
1.00 15.8 (76)
25.0 (184)
7.2 (108)
12.6 (87)
High
Low
Low Women
2.25 (0.80, 6.38) 6.55 (2.86, 14.98) 1.31 (0.42, 4.06) 2.14 (0.77, 5.90) 2.85 (0.54, 14.99; 0.97, 8.43) 1.64 (0.52, 5.14) 4.86 (1.91, 12.39) 1.00 1.71 (0.57, 5.15) 11.4 (88) 26.8 (272) 4.8 (124) 7.8 (257) High Low
1.13 (0.36, 3.56; 0.53, 2.39) 1.53 (0.58, 4.04)
3.32 (1.38, 7.96) 2.52 (0.90, 7.04)
1.00 6.1 (229)
13.1 (282)
3.4 (207)
8.5 (118)
High Low
Low
Low High
Social support at work
Men
High
1.28 (0.49, 3.33) 1.00
2.25 (0.81, 6.25)
Odds ratios (95 % CI)b Synergy index (95 % CI; 80 % CI) Odds ratios (95 % CI)a Depression % (n) Job control Job demands Sex
Table 4 Interactive effects of job control and social support at work on depression stratified by the level of job demands
2.75 (1.17, 6.46)
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work, should be taken into account to estimate job stress in detail, because both the present and previous studies have indicated that job control and social support at work have more-than-additive effects on mental health. Both burnout and depression were used as mental health outcomes in this study. A review described burnout as more job-related and situation-specific than general depression, though depression-prone people are more vulnerable to burnout (Maslach et al. 2001). Burnout estimated using the MBI is discriminative because exhaustion and cynicism/reduced professional efficacy are required in the work context (Schaufeli et al. 2009). The analysis of depression in women stratified by level of job demands did not detect the synergistic effect. This may be because the sample size of women was smaller than that of men, and the rate of depression reported was smaller than that for burnout and insomnia. Moreover, although social support from family/friends had a significant protective effect on depression in the overall analysis of women’s data (Table 2), it also had a significant effect on burnout and insomnia. As previously mentioned, depression is less job-related and situation-specific than burnout (Maslach et al. 2001); this may be one reason for the unclear interactive effect on women’s depression after stratification by level of job demands. Insomnia is one of the most common psychiatric complaints in the general population (Kim et al. 2012), and it is a risk factor for depression (Benca and Peterson 2008). A meta-analysis has shown that insomnia predicted depression with an odds ratio of 2.1 (Baglioni et al. 2011). Thus, evaluation of insomnia is useful for mental health care. In our study, the synergy index for insomnia among men with higher job demands was almost the same as that for men with lower demands. The reason for this small difference in synergy indices, compared to depression and burnout, is unknown. This may be due to a less weaker relationship of insomnia to job status. We calculated the 80 % CI of the synergy index because of the low statistical power of the interaction test (Greenland 1993; Marshall 2007). In the previous Swedish study, the 80 % CI of the synergy indices did not include 1.0 in women, in analyses conducted both before and after stratification by level of job demands; however, the 80 % CI did include 1.0 in men (Choi et al. 2011). In our study, the 80 % CI of the synergy indices did not include 1.0 in men for depression, in either men or women for burnout, in men for insomnia, and in men with high job demands for burnout. The smaller sample size among women may explain why the 80 % CI of the synergy index included 1.0 in our study. Despite the fact that the statistical significance of the synergy indices was not necessarily clear in the present study and the previous Swedish study, more-thanadditive effects between job control and social support at
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Reference: high job control, high social support at work, and low job demands. Adjusted for age, marital status, employment grade, shift work, hypnotic use, and social support from family/ friends
b
Reference: high job control and high social support at work in low and high job demands. Adjusted for age, marital status, employment grade, shift work, hypnotic use, and social support from family/friends
a
10.51 (4.36, 25.37)
6.55 (2.09, 20.53) 3.69 (1.05, 12.90)
7.20 (2.86,18.15)
1.11 (0.30, 4.05; 0.47, 2.58) 1.80 (0.47, 7.00)
3.29 (1.01, 10.67) 2.27 (0.68, 7.53)
1.00 34.6 (26)
47.9 (94)
20.8 (24)
39.7 (63)
High
Low
High
0.28 (0.06, 1.42) 1.00
4.51 (1.96, 10.37) 4.26 (1.79, 10.14) 2.46 (0.92, 6.58)
1.00 2.6 (76) 7.4 (108)
14.9 (87)
High Women
Low
Low
27.2 (184)
0.25 (0.05, 1.31)
4.61 (0.24, 88.41; 0.67, 31.72)
2.40 (0.92, 6.26)
13.21 (7.19, 24.24)
3.05 (1.42, 6.53) 2.21 (1.05, 4.66)
1.90 (0.97, 3.72) 1.00
5.39 (2.77, 10.47)
2.48 (1.05, 5.86; 1.41, 4.34) 1.25 (0.57, 2.72)
5.52 (3.02, 10.09) 2.58 (1.34, 4.95)
1.00 22.7 (88)
54.8 (272)
14.5 (124)
28.8 (257)
High
Low
High
5.02 (2.67, 9.45) 2.87 (1.38, 5.98)
1.00 14.4 (229) 6.8 (207)
17.8 (118)
High Low Men
Low
Low High
Social support at work
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28.4 (282)
2.26 (1.14, 4.49)
1.29 (0.67, 2.48; 0.84, 1.97)
2.74 (1.32, 5.70)
Odds ratios (95 % CI)b Synergy index (95 % CI; 80 % CI) Odds ratios (95 % CI)a Burnout % (n) Job control Job demands Sex
Table 5 Interactive effects of job control and social support at work on burnout stratified by the level of job demands
4.17 (2.25, 7.74)
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work should be considered because the point estimates of all of the synergy indices before stratification by job demands were greater than 1.68 in both studies. Three outcome variables were used in our study, and over 80 % of men and women with depression also had insomnia. The PHQ-9 includes a sleep disturbance item, and insomnia is a risk factor for depression, as previously described (Benca and Peterson 2008). Those were the reasons for the overlap between depression and insomnia. However, burnout and depression overlapped less. As previously mentioned, burnout is more job-related and situation-specific than general depression (Maslach et al. 2001), and the odds ratios of high job demand, low job control, and low social support for burnout were higher than those for depression in our study. Thus, it was meaningful that three different established outcomes had a different prevalence, though there were some overlaps. This study has several limitations. First, because it was cross-sectional, a cause–effect relationship cannot be inferred. If more stressed persons had been prone to take sick leaves or to quit, the influence of stress on mental health may have been attenuated. However, self-reports of both job characteristics and health simultaneously may inflate estimates of associations owing to personality and occasion factors. Second, although the response rate was high, 15 % of the questionnaires had to be excluded because of missing values. If participants who did not complete the questionnaire had a greater job burden, the CIs may have been widened. Third, the number of female participants was smaller than that of males, which may have caused the lack of significance for the synergy indices among women. Among the twofold interactions among women, the 80 % CI for burnout did not include 1.0. Nevertheless, we believe that it was necessary to present the threefold interaction among women in order to compare with the results among males in our study and with the results of previous studies. In Japan, the number of full-time female employees is smaller than that of full-time male employees. Therefore, studies of an entire local government workforce in Japan almost always have fewer female employees. Fourth, we used the 80 % confidence interval of the synergy index, because of the low statistical power of the interaction test as previously mentioned. However, this may overestimate the synergistic interaction. Finally, because the participants were local government employees, generalization from this study may be limited to employees having a steady job. However, the participants were various types of workers, such as clerical workers, professional workers (e.g., physicians, nurses, public health nurses, pharmacists, radiographers, dieticians, researchers, and technicians), firefighters, cooks, and janitors. In conclusion, our results suggest that there were morethan-additive interactions between job control and social
Reference: high job control, high social support at work, and low job demands. Adjusted for age, marital status, employment grade, shift work, hypnotic use, and social support from family friends
b
a Reference: high job control and high social support at work in low and high job demands. Adjusted for age, marital status, employment grade, shift work, hypnotic use, and social support from family friends
6.55 (2.09, 20.53)
10.51 (4.36, 25.37)
3.69 (1.05, 12.90)
7.20 (2.86,18.15)
1.65 (0.46, 5.96) 1.00
0.98 (0.31, 3.09) 56.4 (94)
29.2 (24)
30.2 (63)
High High
Low
46.2 (26)
2.07 (0.69, 6.21)
1.71 (0.06, 50.75; 0.19, 15.66)
0.28 (0.06, 1.42)
4.51 (1.96, 10.37)
1.00
2.40 (0.92, 6.26)
2.40 (0.11, 51.76; 032, 17.84) 1.91 (1.07, 3.42) 1.28 (0.63, 2.60)
1.00 30.3 (76) 24.1 (108)
23.0 (87)
High Women
Low
Low
37.0 (184)
1.28 (0.63, 2.60)
3.32 (1.69, 6.54) 7.24 (4.18, 12.56) 2.18 (1.11, 4.30) 3.33 (1.78, 6.26) 1.87 (0.58, 6.05; 0.87, 4.03) 1.00 1.62 (0.85, 3.08) 33.0 (88) 48.9 (272) 17.7 (124) 23.7 (257) High Low High
2.27 (1.24, 4.14)
1.48 (0.73, 3.02) 3.05 (1.71, 5.44)
1.97 (1.09, 3.55)
3.79 (2.18, 6.59)
1.00
1.45 (0.70, 3.01) 1.51 (0.73, 3.13)
1.00 21.0 (229)
34.8 (282)
9.2 (207)
14.1 (118)
High Low Men
Low
Low High
Social support at work
4.32 (2.45, 7.61)
1.81 (0.80, 4.07; 1.06, 3.07)
Synergy index (95 % CI; 80 % CI) Odds ratios (95 % CI)a Insomnia, % (n) Job control Job demands Sex
Table 6 Interactive effects of job control and social support at work on insomnia stratified by the level of job demands
Odds ratios (95 % CI)b
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support at work on depression, burnout, and insomnia among male and female local government employees. Moreover, compared to men with low job demands, men with high job demands may have higher synergistic interactions between job control and social support at work on depression and burnout, in particular; however, compared to women with high job demands, women with low job demands may have higher synergistic interactions between job control and social support at work on burnout and insomnia, in particular. To assess job stress, we need to consider interactive effects not only between job demands and job control but also between job control and social support at work. Acknowledgments This work was supported by Grants-in-Aid for Scientific Research (No. 21590549) from the Japan Society for the Promotion of Science. Conflict of interest of interest.
The authors declare that they have no conflict
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ORIGINAL ARTICLE
Synergistic interaction between job control and social support at work on depression, burnout, and insomnia among Japanese civil servants Yasuaki Saijo • Shigeru Chiba • Eiji Yoshioka • Yoshihiko Nakagi Toshihiro Ito • Kazuyo Kitaoka-Higashiguchi • Takahiko Yoshida
•
Received: 5 November 2013 / Accepted: 12 May 2014 Ó Springer-Verlag Berlin Heidelberg 2014
Abstract Purpose To elucidate whether low job control and low social support at work have synergistic interaction on mental health. The synergistic interaction was also analyzed after stratification by high and low job demands. Methods Participants were 2,121 local government employees in Asahikawa city, Japan. The Brief Job Stress Questionnaire was used to assess job demands, job control, and social support. Depression was assessed using the Patient Health Questionnaire-9. The Maslach Burnout Inventory-General Survey was used to assess burnout. Insomnia was assessed using the Athens Insomnia Scale. Possible confounder-adjusted logistic regression analyses were performed to obtain odds ratios for depression, burnout, and insomnia, and synergy indices between job control and social support at work were assessed. Results The synergy indices among men and women, respectively, were 2.08 (80 % confidence interval: 1.01, 4.27) and 1.98 (0.67, 5.89) for depression, 1.79 (1.28, 2.51) and 2.62 (1.07, 6.40) for burnout, and 1.92 (1.22, 3.02) and 2.77 (0.43, 18.01) for insomnia. Men with high job demands had higher synergistic interaction on depression and burnout, compared to men with low job demands, and Y. Saijo (&) E. Yoshioka Y. Nakagi T. Ito T. Yoshida Department of Health Science, Asahikawa Medical University, Midorigaoka-higashi 2-1-1-1, Asahikawa, Hokkaido 078-8510, Japan e-mail: [email protected] S. Chiba Department of Psychiatry and Neurology, Asahikawa Medical University, Asahikawa, Japan K. Kitaoka-Higashiguchi Institute of Medical, Pharmaceutical and Health Sciences, Kanazawa University, Kanazawa, Japan
women with low job demands had higher synergistic interaction between job control and social support at work on burnout and insomnia, compared to women with high job demands. Conclusions There were more-than-additive interactions of job control and social support at work on depression, burnout, and insomnia. After stratification by job demands, the synergistic interaction may be different between men and women. To assess job stress, it is necessary to consider the interactive effect of not only job demands and job control but also job control and social support at work. Keywords Job control Social support Job demands Depression Burnout Insomnia Synergy index
Introduction The psychosocial work environment is important for mental health (Stansfeld and Candy 2006), and the demand–control–support (DCS) model (Karasek 1979) is an important job stress model that has been widely used in the field of occupational health. The model has two job stress dimensions, job demands and job control, with social support included as a third dimension. Moreover, because the DCS model focuses mainly on job demands and control, many studies have reported combined effects of job demands and control on health outcomes (Fransson et al. 2012), and few studies have reported their synergistic interactions (Santavirta et al. 2007). ‘‘Collective control’’ is the concept in which control over the work process is strongly influenced by the character of workplace social groups, based on Gardell’s suggestion of the importance of uniting the following two concepts for a strategy of work reform: (1) the combining
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of the collective influence in management decision making with the formation of an autonomous group and (2) the individual and the group achieving greater control over the work system and working methods (Gardell 1982). Increased worker control—at the individual as well as at the collective level—may be a means of reducing the risk of work environment-related stress and diseases (Aronsson 1989). This influence process, collective control, determines the possibilities for collective coping with chronic demands and the pressures of various production systems (Johnson 1989). There have been reports that a combined effect of job control and social support at work increases the risk of cardiovascular disease (Johnson 1989). Furthermore, the concept of collective control implies a synergistic effect of job control and social support at work on mental health (Choi et al. 2011). A Swedish study reported synergy indices between job control and social support on general psychological distress, as assessed by the General Health Questionnaire (GHQ) (Choi et al. 2011). The study observed a synergistic (i.e., more-than-additive) effect of low job control and low social support at work on general psychological distress. The results showed that the synergistic effect was stronger when job demands were low compared to when job demands were high, and particularly in men, an antagonistic effect was observed when job demands were high. However, to our knowledge, only one study has reported a synergistic effect of job control and social support at work on mental health or the confidence interval of this measure (Choi et al. 2011). Burnout is a psychological outcome of chronic stress, which is characterized by emotional exhaustion, cynicism, and negative self-evaluation (Maslach et al. 2001). Burnout is a predictor of depression and life dissatisfaction among dentists (Hakanen and Schaufeli 2012). Though burnout was initially explored among medical and human services professionals, it has been recently described in various other occupational areas. A population-based study reported a significant association of burnout with sick leaves (Ahola et al. 2008) and severe injury (Ahola et al. 2014). Studies have reported that depression and insomnia are negatively related to occupational performance (Bender and Farvolden 2008; Kucharczyk et al. 2012), and effects of job strain, a combined index of high job demand and low job control, on depression and insomnia have also been reported (Bonde 2008; Ota et al. 2009). However, the interactive effects of low job control and low social support at work on depression and insomnia have not been fully investigated. The present study aimed to elucidate whether low job control and low social support at work have synergistic interactions on mental health indicators such as burnout,
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depression, and insomnia, with analysis of their confidence intervals. The synergistic interactions were analyzed after stratification by high and low job demands.
Methods Participants The original participants were 2,811 local government employees in Asahikawa city, Hokkaido, Japan: the entire workforce except for 75 new recruits hired after April 2010 and employees taking medical leave between July and August 2010. Self-administered questionnaires were distributed to the employees on June 28, 2010. Responses were obtained from 2,514 employees (response rate: 89.4 %) between July and August 2010. After excluding 393 questionnaires because of missing values, 2,121 questionnaires (75.4 % of the original participants) were analyzed. This study was approved by the Institutional Ethical Board for Epidemiological Studies of Asahikawa Medical University. Demographics and work-related factors The questionnaire included sex, age (ranges 19–30, 31–40, 41–50, 51–60 years old), marital status, occupation, job ranking, shift work, and use of hypnotics. The recall period for the variables was the preceding 1 month. Classification of socioeconomic status was performed similar to a previous Japanese civil servants study (Nishi et al. 2004; Saijo et al. 2008). Employment grade was categorized into ‘‘higher-level non-manual,’’ ‘‘lower-level non-manual,’’ and ‘‘manual.’’ ‘‘Non-manual’’ workers included clerical workers, professional workers (e.g., physicians, nurses, public health nurses, pharmacists, radiographers, dieticians, researchers, and technicians), and firefighters. ‘‘Higher-level non-manual’’ workers included non-manual workers whose ranks were department director or section chief for men, and department director, section chief, or chief clerk for women. ‘‘Lower-level non-manual’’ workers included non-manual workers whose ranks were chief clerk or who were in a non-managerial position for both men and women. Regardless of rank, ‘‘manual’’ workers included sanitation workers, cooks, and janitors. Firefighters are not classified as ‘‘manual’’ workers in the Japanese civil servants system, and their work includes various jobs such as ‘‘firefighting,’’ ‘‘ambulance work,’’ ‘‘rescue work,’’ ‘‘command and control center work,’’ and ‘‘fire investigation.’’ Therefore, firefighters were categorized as ‘‘non-manual’’ workers.
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Job stress and social support The Brief Job Stress Questionnaire (BJSQ) (Shimomitsu et al. 2000), which is based on the DCS model (Karasek 1979), was used to assess two job stress dimensions (job demands and job control) and social support from supervisors, co-workers, and family/friends. The BJSQ has been widely used in Japan for practical occupational health evaluation and occupational health research (Kawada and Otsuka 2011; Suwazono et al. 2008; Umehara et al. 2007). The questionnaire for job demands included three items: (1) You have to do an enormous amount of work, (2) You cannot complete all your work in the allotted time, and (3) You have to work very hard. In addition, there were three items about job control: (1) You can work at your own pace, (2) You can decide the order in which you do your work and the way you do it, and (3) You can provide your opinions on the work strategy of your workplace. The responses were scored on a 4-point Likert-type scale (1, agree; 2, somewhat agree; 3, somewhat disagree; and 4, disagree) (Suwazono et al. 2008). To assess job demands and job control, the total score for each dimension was used; higher scores denoted higher job demands or control. Social support from supervisors, co-workers, and family/friends was also assessed using three items: (1) You can often communicate with supervisors/co-workers, (2) You can strongly rely on supervisors/co-workers when you have problems, and (3) Your supervisors/co-workers are prepared to spend their time on your personal problems. The responses to these items were also scored on a 4-point Likert-type scale (1, agree; 2, somewhat agree; 3, somewhat disagree; and 4, disagree). The total score was calculated for each of the three questions; higher scores denoted higher social support (Kawada and Otsuka 2011). Job demands and job control scores were each dichotomized at the median value. The social support at work score was the sum of social support from supervisors and co-workers, and it was also dichotomized at the median value. The social support from family/friends score, which was used as a confounder, was a continuous variable. Depression Depression was assessed using the Japanese version of Patient Health Questionnaire-9 (PHQ-9) (Muramatsu et al. 2007), the depression subscale of the self-report Patient Health Questionnaire (Spitzer et al. 1999). The PHQ-9 consists of nine items, and the total score varies between 0 and 27 (Kroenke et al. 2001). Standard PHQ-9 cutoff scores were used to classify minimal (0–4), mild (5–9), and moderate to severe (C10) symptoms of depression (Roy et al. 2012). In this study, ‘‘positive for depression’’ was
defined as PHQ-9 scores C10. Cronbach’s alpha coefficient for the PHQ-9 was 0.88. Burnout The Japanese version of the Maslach Burnout InventoryGeneral Survey (MBI-GS) was used to assess burnout (Kitaoka-Higashiguchi et al. 2004a, b; Maslach et al. 1996, 2001). The MBI-GS has 16 items with three subscales: exhaustion (5 items), cynicism (5 items), and professional efficacy (6 items). Each item was scored using a 7-point Likert-type scale ranging from ‘‘never’’ (0 points) to ‘‘every day’’ (6 points), based on the frequency of occurrence. The total scores for each subscale were divided by the number of items on the subscale; higher exhaustion and cynicism scores and a lower professional efficacy score denote higher degrees of burnout. Each subscale was tertilized, and the upper tertiles of exhaustion and cynicism and the lower tertile of professional efficacy were defined as subscale positives. Then, on the basis of the common burnout process in which exhaustion comes first, followed by cynicism and diminished professional efficacy, the participants who were at least exhaustion positive and were either cynicism positive or professional efficacy positive were defined as burnout positive (Brenninkmeijer and VanYperen 2003; KitaokaHigashiguchi et al. 2009). Insomnia Insomnia was assessed using the Athens Insomnia Scale (AIS). The AIS is a validated instrument based on the International Classification of Disease, 10th Revision (ICD-10) criteria (Soldatos et al. 2000, 2003). The scale is a self-administered inventory consisting of eight items. The first five items assess difficulty with sleep induction, awakening during the night, final awakening earlier than desired, total sleep duration, and overall quality of sleep. The remaining three items pertain to the next-day consequences of insomnia: sense of well-being during the day, functioning during the day, and sleepiness during the day. Each item of the AIS was rated from 0 (no problem at all) to 3 (very serious problem). The total score ranged from 0 to 24, and having six or more points was defined as the presence of insomnia. The participants were requested to choose rating items only if they had experienced sleep difficulty at least 3 times a week during the last month. Statistical analysis We analyzed data for men and women separately, because a previous study speculated that the synergistic effect might vary by sex (Choi et al. 2011). Crude odds ratios
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(ORs) of age, marital status, employment grade, shift work, hypnotic use, social support from family/friends, job demands, job control, and social support at work concerning depression, burnout, and insomnia were obtained by univariate logistic regression analysis, and adjusted ORs of all variables were obtained in full multivariable models. To investigate synergistic effects of job control and social support at work on depression, burnout, and insomnia, we created four categorical variables: high control and high social support at work (reference); high control and low social support at work; low control and high social support at work; and low control and low social support at work. The ORs of the latter three variables were obtained after adjustment for age, marital status, employment grade, shift work, hypnotic use, social support from family/friends, and job demands. Because hypnotic use was introduced into the model in previous studies of job strain and insomnia (Ota et al. 2005; Sinokki et al. 2010) and was significantly related to three outcomes of both sex and job control for men, it was introduced in the adjusted model as a possible confounder. Then, a synergy index (Rothman 1986) was obtained using following formula: S¼
ORðABÞ 1 ½OR ðAbÞ 1 þ ½ORðaBÞ 1
S: synergy index; OR: odds ratio; Ab: exposed to one factor; aB: exposed to the other factor; AB: exposed to both factors. The synergy index (S) was greater than one (S [ 1) if a synergistic interaction of both exposures was present and less than one (S \ 1) if an antagonistic interaction was present. The confidence interval (CI) was estimated (Hosmer and Lemeshow 1992) after calculating the standard error of the synergy index, using an asymptotic covariance matrix analyzed using SPSS syntax (Andersson et al. 2005). Because of low statistical power in the interaction test (Greenland 1993; Marshall 2007), we calculated 95 and 80 % CIs of the synergy index, as previously reported (Choi et al. 2011). We also estimated the synergy index after stratification by levels of job demands. P values \0.05 of ORs were considered statistically significant. All calculations were conducted using IBM SPSS Statistics 21.0 for Windows (SPSS Inc., Chicago, IL, USA).
Results For the subscales of the BJSQ, Cronbach’s alpha coefficients for job demands, job control, social support from supervisors, social support from co-workers, and social support from family/friends were 0.78, 0.71, 0.83, 0.81, and 0.89, respectively. For the subscales of the MBI-GS,
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Table 1 Characteristics of male and female participants Men (N = 1,459)
Women (N = 662)
Age (years) B30
191 (13.1)
192 (29.0)
31–40
525 (36.0)
279 (42.1)
41–50
351 (24.1)
110 (16.6)
51–60
392 (26.9)
81 (12.2)
180 (12.3) 1,184 (81.2)
105 (15.9) 500 (75.5)
Employment grade Higher-level non-manual Lower-level non-manual
95 (6.5)
57 (8.6)
Married
Manual
1,094 (75.0)
311 (47.0)
Shift work
317 (21.7)
249 (37.6)
No
1,357 (93.0)
579 (87.5)
Yes
58 (4.0)
48 (7.3)
Former
44 (3.0)
35 (5.3)
8.0 ± 2.0
8.5 ± 3.2
Hypnotic
Job stress (score) Demand
8.0 ± 1.9
7.7 ± 1.8
Social support at work (score)
Control
15.7 ± 3.7
15.7 ± 3.9
Social support from family/friends (score)
9.6 ± 2.1
10.1 ± 2.0
Burnout (score) Exhaustion
2.3 ± 1.5
3.4 ± 1.6
Cynicism
1.8 ± 1.5
2.1 ± 1.5
Reduced professional efficacy Burnout positive
2.2 ± 1.2
2.1 ± 1.2
375 (25.7)
157 (23.7) 4.8 ± 3.5
AIS (score)
4.1 ± 3.3
Insomnia positive (AIS C 6)
399 (27.3)
228 (34.4)
PHQ-9 (score)
4.3 ± 4.6
6.0 ± 4.9
PHQ-9 C10
187 (11.4)
133 (20.1)
Variables are presented as number (percentage) or mean ± SD
Cronbach’s alpha coefficients for exhaustion, cynicism, and professional efficacy were 0.92, 0.88, and 0.86, respectively. The Cronbach’s alpha coefficient for the AIS was 0.83. Table 1 shows the demographic and work-related factors; scores on the BJSQ, MBI-GS, AIS, and PHQ-9 scales; and the number of participants who were burnout positive, insomnia positive, and depression positive (PHQ-9 C 10). The prevalence of insomnia and depression was higher in women than in men. One hundred and thirty-three men had both depression and burnout (79.6 % of men with depression: 35.5 % of men with burnout); 142 men had both depression and insomnia (85.0 % of men with depression: 35.6 % of men with insomnia); and 228 men had both burnout and insomnia (60.8 % of men with burnout: 57.1 % of men
0.003 0.227 0.007 1.22, 2.57 0.86, 1.85 1.15, 2.46 1.77 1.27 1.68 1.30, 2.56 0.001 1.03, 2.06 0.032 1.45, 2.85 \0.001 1.82 1.46 2.03 Adjusted for age, marital status, employment grade, shift work, hypnotic use, and social support from family/friends a
3.19 3.24 1.51 1.11, 2.45 0.014 1.44, 3.63 \0.001 1.38, 3.20 0.001 1.65 2.29 2.10
1.64 2.24 1.38
1.05, 2.58 1.33, 3.75 0.85, 2.23
0.031 0.002 0.188
3.57 3.95 1.75
2.46, 5.19 \0.001 2.47, 6.31 \0.001 1.20, 2.56 0.004
2.10, 4.83 \0.001 1.94, 5.42 \0.001 0.96, 2.35 0.072
1.51, 2.54 \0.001 1.39, 2.42 \0.001 1.53, 2.79 \0.001 1.96 1.83 2.07 1.52, 2.42 \0.001 1.86, 3.05 \0.001 2.30, 3.89 \0.001 1.92 2.38 2.99 1.91, 3.23 \0.001 2.08, 3.73 \0.001 1.39, 2.58 \0.001 2.48 2.79 1.89 2.09, 3.39 \0.001 2.82, 4.83 \0.001 1.96, 3.32 \0.001 2.67 3.69 2.55 1.34, 2.74 \0.001 1.52, 3.47 \0.001 1.16, 2.86 \0.001 1.92 2.30 1.82 1.41, 2.74 \0.001 1.77, 3.82 \0.001 1.68, 3.74 \0.001 1.97 2.60 2.51
Men Job stress High demand Low control Low social support at work Women Job stress High demand Low control Low social support at work
95 % CI OR 95 % CI OR 95 % CI OR 95 % CI OR OR 95 % CI
P OR
95 % CI
P
Crude Adjusted Crude
a
Depression
Table 2 Crude and adjusted odds ratios for depression, burnout, and insomnia
Burnout
P
Adjusted
a
P
Crude
Insomnia
P
Adjusteda
P
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with insomnia). Among women, 78 had both depression and burnout (58.6 % of women with depression: 49.7 % of women with burnout); 112 had both depression and insomnia (84.2 % of women with depression: 34.4 % of women with insomnia); and 95 had both burnout and insomnia (60.5 % of women with burnout: 53.7 % of women with insomnia). Table 2 shows crude and adjusted ORs for depression, burnout, and insomnia. After adjustment, job demands (OR 1.92; 95 % CI 1.34, 2.74), job control (OR 2.30; 95 % CI 1.52, 3.47), and social support at work (OR 1.82; 95 % CI 1.16, 2.86) were significantly related to depression among men, and job demands (OR 1.64; 95 % CI 1.05, 2.58) and job control (OR 2.24; 95 % CI 1.33, 3.75) were significantly related to depression among women. After adjustment, job demands (OR 2.48; 95 % CI 1.91, 3.23), job control (OR 2.79; 95 % CI 2.08, 3.73), and social support at work (OR 1.89; 95 % CI 1.39, 2.58) were significantly related to burnout among men, and job demands (OR 3.19; 95 % CI 2.10, 4.83) and job control (OR 3.24; 95 % CI 1.94, 5.42) were significantly related to burnout among women. After adjustment, job demands (OR 1.96; 95 % CI 1.51, 2.54), job control (OR 1.83; 95 % CI 1.39, 2.42), and social support at work (OR 2.07; 95 % CI 1.53, 2.79) were significantly related to insomnia among men, and job demands (OR 1.77; 95 % CI 1.22, 2.57) and social support at work (OR 1.68; 95 % CI 1.15, 2.46) were significantly related to insomnia among women. Next, the synergistic effects of job control and social support at work on depression, burnout, and insomnia were estimated. Table 3 shows the ORs and synergy indices for depression, burnout, and insomnia. The synergy indices for burnout, for both men and women, were 2.08 (80 % CI 1.01, 4.27) and 1.98 (0.67, 5.89), respectively. In the analysis stratified by level of job demands (Table 4), the synergy indices for men with low job demands, men with high job demands, women with low job demands, and women with high job demands were 1.13 (0.53, 3.39), 2.85 (0.97, 8.43), 1.19 (0.57, 2.48), and -0.51 (not calculable), respectively. The synergy indices for burnout, for both men and women, were 1.79 (1.28, 2.51 *) (where * means that the 95 % CI did not include 1.0) and 2.62 (1.07, 6.40 *), respectively (Table 3). In the analysis stratified by level of job demands (Table 5), the synergy indices among men with low job demands, men with high job demands, women with low job demands, and women with high job demands were 1.29 (0.84, 1.97), 2.48 (1.41, 4.34 *), 4.61 (0.67, 31.72), and 1.11 (0.47, 2.58). The synergy indices for insomnia, for both men and women, were 1.92 (1.22, 0.43) and 2.77 (0.43, 18.01), respectively (Table 3). In the analysis stratified by level of job demands (Table 6), the synergy indices for men with
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Int Arch Occup Environ Health Table 3 Synergistic interactive effects of job control and social support at work on depression Sex
Job control
Social support at work High % (n)
Odds ratios (95 % CI)
Synergy index (95 % CI; 80 % CI)
Low % (n)
Depression Men
High
3.9 (331)
7.6 (317)
1.00
1.47 (0.71, 3.07)
2.08 (0.69, 6.26; 1.01, 4.27)
Women
Low High
7.8 (257) 9.1 (132)
19.9 (554) 14.7 (102)
1.84 (0.87, 3.87) 1.00
3.72 (1.98, 6.98) 1.07 (0.44, 2.59)
1.98 (0.38, 10.50; 0.67, 5.89)
Low
16.7 (150)
29.1 (278)
1.82 (0.82, 4.02)
2.76 (1.35, 5.66)
High
9.7 (331)
16.7 (317)
1.00
1.72 (1.04, 2.84)
Low
23.7 (257)
41.3 (554)
2.54 (1.57, 4.11)
5.05 (3.29, 7.77)
Burnout Men Women
High
9.8 (132)
10.8 (102)
1.00
0.89 (0.36, 2.20)
Low
25.3 (150)
34.2 (278)
2.23 (1.07, 4.67)
3.95 (1.99, 7.81)
High
12.4 (331)
24.3 (317)
1.00
1.78 (1.14, 2.79)
Low
19.5 (257)
41.7 (554)
1.55 (0.96, 2.48)
3.54 (2.38, 5.28)
High
25.0 (132)
34.3 (102)
1.00
1.33 (0.72, 2.45)
Low
26.0 (150)
43.5 (278)
1.02 (0.57, 1.82)
1.97 (1.18, 3.28)
1.79 (1.07, 3.01; 1.28, 2.51) 2.62 (0.67, 10.28; 1.07, 6.40)
Insomnia Men Women a
1.92 (0.96, 3.84; 1.22, 3.02) 2.77 (0.16, 48.66; 0.43, 18.01)
Adjusted for age, marital status, employment grade, shift work, hypnotic use, social support from family/friends, and job demand
low job demands, men with high job demands, women with low job demands, and women with high job demands were 1.81 (1.06, 3.07), 1.87 (0.87, 4.03), 2.40 (0.32, 17.84), and 1.71 (0.19, 15.66), respectively.
Discussion In this study, there were more-than-additive effects of job control and social support at work on depression, burnout, and insomnia, and the 80 % CI of these synergistic indices among men did not include 1.0. Moreover, men with high job demands had higher synergistic interactions for depression and burnout compared with men with low job demands, and women with low job demands had higher synergistic interaction between job control and social support at work for burnout and insomnia compared with women with high job demands. A previous Swedish study reported the synergy indices between job control and social support for general psychological distress (men: 1.68; women: 1.83), and the 80 % CI among women did not cross 1.0 (Choi et al. 2011). The synergy indices in our study, in which we used other mental health outcomes (depression, burnout, and insomnia), were almost the same or slightly larger (range 1.79–2.77), though the 80 % CI of depression and insomnia among women included 1.0. Thereby, before stratification by level of job demands, these synergistic effects were compatible with those reported in the previous study.
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After stratification by level of job demands in the Swedish study, the synergy index was larger for men with low job demands (9.24), antagonistic for men with high demands (0.52), and slightly higher for women with low job demands (2.16) compared with women with high job demands (1.51) (Choi et al. 2011). In our study, the synergy indices for women in the analysis of burnout and insomnia stratified by job demands were consistent with the previous Swedish study. However, the synergy indices for depression and burnout for men with low job demands were higher than those for men with high job demands. A Norwegian study, though not actually reporting a synergistic index, reported synergistic effects of job control and social support at work on anxiety and depression, particularly among men and women with high job demands (Sanne et al. 2005). Hence, the present synergy indices for depression and burnout in the analysis of data for men stratified by job demands are consistent with the previous Norwegian study. After stratification by level of job demands, there was no consistent tendency to the synergistic effect. The authors of the Swedish study therefore speculated that the synergistic effect might vary by the level of job demands, sex, and study context (Choi et al. 2011). Job strain, which is an interaction effect between job demands and control, has frequently been studied (Fransson et al. 2012), and it has been used for practical purposes (Choi et al. 2008) in the DCM model. However, other interactions, particularly job control and social support at
Confidence interval: not calculable c
Reference: high job control, high social support at work, and low job demands. Adjusted for age, marital status, employment grade, shift work, hypnotic use, and social support from family/ friends
b
Reference: high job control and high social support at work in low and high job demands. Adjusted for age, marital status, employment grade, shift work, hypnotic use, and social support from family/friends
a
1.18 (0.25, 5.09)
5.57 (2.25,13.83) 3.64 (1.34, 9.95) 1.44 (0.42, 4.99) 0.70 (0.19, 2.63) 22.2 (63) Low
37.2 (94)
16.7 (24) High
High
11.5 (26)
1.00
0.44 (0.08, 2.34)
-0.51c
3.10 (0.80, 11.93)
3.23 (1.37, 7.63)
1.62 (0.58, 4.52) 1.00
2.11 (0.78, 5.76)
1.19 (0.38, 3.67; 0.57, 2.48) 1.70 (0.59, 4.91)
3.58 (1.46, 8.82) 2.48 (0.89, 6.92)
1.00 15.8 (76)
25.0 (184)
7.2 (108)
12.6 (87)
High
Low
Low Women
2.25 (0.80, 6.38) 6.55 (2.86, 14.98) 1.31 (0.42, 4.06) 2.14 (0.77, 5.90) 2.85 (0.54, 14.99; 0.97, 8.43) 1.64 (0.52, 5.14) 4.86 (1.91, 12.39) 1.00 1.71 (0.57, 5.15) 11.4 (88) 26.8 (272) 4.8 (124) 7.8 (257) High Low
1.13 (0.36, 3.56; 0.53, 2.39) 1.53 (0.58, 4.04)
3.32 (1.38, 7.96) 2.52 (0.90, 7.04)
1.00 6.1 (229)
13.1 (282)
3.4 (207)
8.5 (118)
High Low
Low
Low High
Social support at work
Men
High
1.28 (0.49, 3.33) 1.00
2.25 (0.81, 6.25)
Odds ratios (95 % CI)b Synergy index (95 % CI; 80 % CI) Odds ratios (95 % CI)a Depression % (n) Job control Job demands Sex
Table 4 Interactive effects of job control and social support at work on depression stratified by the level of job demands
2.75 (1.17, 6.46)
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work, should be taken into account to estimate job stress in detail, because both the present and previous studies have indicated that job control and social support at work have more-than-additive effects on mental health. Both burnout and depression were used as mental health outcomes in this study. A review described burnout as more job-related and situation-specific than general depression, though depression-prone people are more vulnerable to burnout (Maslach et al. 2001). Burnout estimated using the MBI is discriminative because exhaustion and cynicism/reduced professional efficacy are required in the work context (Schaufeli et al. 2009). The analysis of depression in women stratified by level of job demands did not detect the synergistic effect. This may be because the sample size of women was smaller than that of men, and the rate of depression reported was smaller than that for burnout and insomnia. Moreover, although social support from family/friends had a significant protective effect on depression in the overall analysis of women’s data (Table 2), it also had a significant effect on burnout and insomnia. As previously mentioned, depression is less job-related and situation-specific than burnout (Maslach et al. 2001); this may be one reason for the unclear interactive effect on women’s depression after stratification by level of job demands. Insomnia is one of the most common psychiatric complaints in the general population (Kim et al. 2012), and it is a risk factor for depression (Benca and Peterson 2008). A meta-analysis has shown that insomnia predicted depression with an odds ratio of 2.1 (Baglioni et al. 2011). Thus, evaluation of insomnia is useful for mental health care. In our study, the synergy index for insomnia among men with higher job demands was almost the same as that for men with lower demands. The reason for this small difference in synergy indices, compared to depression and burnout, is unknown. This may be due to a less weaker relationship of insomnia to job status. We calculated the 80 % CI of the synergy index because of the low statistical power of the interaction test (Greenland 1993; Marshall 2007). In the previous Swedish study, the 80 % CI of the synergy indices did not include 1.0 in women, in analyses conducted both before and after stratification by level of job demands; however, the 80 % CI did include 1.0 in men (Choi et al. 2011). In our study, the 80 % CI of the synergy indices did not include 1.0 in men for depression, in either men or women for burnout, in men for insomnia, and in men with high job demands for burnout. The smaller sample size among women may explain why the 80 % CI of the synergy index included 1.0 in our study. Despite the fact that the statistical significance of the synergy indices was not necessarily clear in the present study and the previous Swedish study, more-thanadditive effects between job control and social support at
123
Reference: high job control, high social support at work, and low job demands. Adjusted for age, marital status, employment grade, shift work, hypnotic use, and social support from family/ friends
b
Reference: high job control and high social support at work in low and high job demands. Adjusted for age, marital status, employment grade, shift work, hypnotic use, and social support from family/friends
a
10.51 (4.36, 25.37)
6.55 (2.09, 20.53) 3.69 (1.05, 12.90)
7.20 (2.86,18.15)
1.11 (0.30, 4.05; 0.47, 2.58) 1.80 (0.47, 7.00)
3.29 (1.01, 10.67) 2.27 (0.68, 7.53)
1.00 34.6 (26)
47.9 (94)
20.8 (24)
39.7 (63)
High
Low
High
0.28 (0.06, 1.42) 1.00
4.51 (1.96, 10.37) 4.26 (1.79, 10.14) 2.46 (0.92, 6.58)
1.00 2.6 (76) 7.4 (108)
14.9 (87)
High Women
Low
Low
27.2 (184)
0.25 (0.05, 1.31)
4.61 (0.24, 88.41; 0.67, 31.72)
2.40 (0.92, 6.26)
13.21 (7.19, 24.24)
3.05 (1.42, 6.53) 2.21 (1.05, 4.66)
1.90 (0.97, 3.72) 1.00
5.39 (2.77, 10.47)
2.48 (1.05, 5.86; 1.41, 4.34) 1.25 (0.57, 2.72)
5.52 (3.02, 10.09) 2.58 (1.34, 4.95)
1.00 22.7 (88)
54.8 (272)
14.5 (124)
28.8 (257)
High
Low
High
5.02 (2.67, 9.45) 2.87 (1.38, 5.98)
1.00 14.4 (229) 6.8 (207)
17.8 (118)
High Low Men
Low
Low High
Social support at work
123
28.4 (282)
2.26 (1.14, 4.49)
1.29 (0.67, 2.48; 0.84, 1.97)
2.74 (1.32, 5.70)
Odds ratios (95 % CI)b Synergy index (95 % CI; 80 % CI) Odds ratios (95 % CI)a Burnout % (n) Job control Job demands Sex
Table 5 Interactive effects of job control and social support at work on burnout stratified by the level of job demands
4.17 (2.25, 7.74)
Int Arch Occup Environ Health
work should be considered because the point estimates of all of the synergy indices before stratification by job demands were greater than 1.68 in both studies. Three outcome variables were used in our study, and over 80 % of men and women with depression also had insomnia. The PHQ-9 includes a sleep disturbance item, and insomnia is a risk factor for depression, as previously described (Benca and Peterson 2008). Those were the reasons for the overlap between depression and insomnia. However, burnout and depression overlapped less. As previously mentioned, burnout is more job-related and situation-specific than general depression (Maslach et al. 2001), and the odds ratios of high job demand, low job control, and low social support for burnout were higher than those for depression in our study. Thus, it was meaningful that three different established outcomes had a different prevalence, though there were some overlaps. This study has several limitations. First, because it was cross-sectional, a cause–effect relationship cannot be inferred. If more stressed persons had been prone to take sick leaves or to quit, the influence of stress on mental health may have been attenuated. However, self-reports of both job characteristics and health simultaneously may inflate estimates of associations owing to personality and occasion factors. Second, although the response rate was high, 15 % of the questionnaires had to be excluded because of missing values. If participants who did not complete the questionnaire had a greater job burden, the CIs may have been widened. Third, the number of female participants was smaller than that of males, which may have caused the lack of significance for the synergy indices among women. Among the twofold interactions among women, the 80 % CI for burnout did not include 1.0. Nevertheless, we believe that it was necessary to present the threefold interaction among women in order to compare with the results among males in our study and with the results of previous studies. In Japan, the number of full-time female employees is smaller than that of full-time male employees. Therefore, studies of an entire local government workforce in Japan almost always have fewer female employees. Fourth, we used the 80 % confidence interval of the synergy index, because of the low statistical power of the interaction test as previously mentioned. However, this may overestimate the synergistic interaction. Finally, because the participants were local government employees, generalization from this study may be limited to employees having a steady job. However, the participants were various types of workers, such as clerical workers, professional workers (e.g., physicians, nurses, public health nurses, pharmacists, radiographers, dieticians, researchers, and technicians), firefighters, cooks, and janitors. In conclusion, our results suggest that there were morethan-additive interactions between job control and social
Reference: high job control, high social support at work, and low job demands. Adjusted for age, marital status, employment grade, shift work, hypnotic use, and social support from family friends
b
a Reference: high job control and high social support at work in low and high job demands. Adjusted for age, marital status, employment grade, shift work, hypnotic use, and social support from family friends
6.55 (2.09, 20.53)
10.51 (4.36, 25.37)
3.69 (1.05, 12.90)
7.20 (2.86,18.15)
1.65 (0.46, 5.96) 1.00
0.98 (0.31, 3.09) 56.4 (94)
29.2 (24)
30.2 (63)
High High
Low
46.2 (26)
2.07 (0.69, 6.21)
1.71 (0.06, 50.75; 0.19, 15.66)
0.28 (0.06, 1.42)
4.51 (1.96, 10.37)
1.00
2.40 (0.92, 6.26)
2.40 (0.11, 51.76; 032, 17.84) 1.91 (1.07, 3.42) 1.28 (0.63, 2.60)
1.00 30.3 (76) 24.1 (108)
23.0 (87)
High Women
Low
Low
37.0 (184)
1.28 (0.63, 2.60)
3.32 (1.69, 6.54) 7.24 (4.18, 12.56) 2.18 (1.11, 4.30) 3.33 (1.78, 6.26) 1.87 (0.58, 6.05; 0.87, 4.03) 1.00 1.62 (0.85, 3.08) 33.0 (88) 48.9 (272) 17.7 (124) 23.7 (257) High Low High
2.27 (1.24, 4.14)
1.48 (0.73, 3.02) 3.05 (1.71, 5.44)
1.97 (1.09, 3.55)
3.79 (2.18, 6.59)
1.00
1.45 (0.70, 3.01) 1.51 (0.73, 3.13)
1.00 21.0 (229)
34.8 (282)
9.2 (207)
14.1 (118)
High Low Men
Low
Low High
Social support at work
4.32 (2.45, 7.61)
1.81 (0.80, 4.07; 1.06, 3.07)
Synergy index (95 % CI; 80 % CI) Odds ratios (95 % CI)a Insomnia, % (n) Job control Job demands Sex
Table 6 Interactive effects of job control and social support at work on insomnia stratified by the level of job demands
Odds ratios (95 % CI)b
Int Arch Occup Environ Health
support at work on depression, burnout, and insomnia among male and female local government employees. Moreover, compared to men with low job demands, men with high job demands may have higher synergistic interactions between job control and social support at work on depression and burnout, in particular; however, compared to women with high job demands, women with low job demands may have higher synergistic interactions between job control and social support at work on burnout and insomnia, in particular. To assess job stress, we need to consider interactive effects not only between job demands and job control but also between job control and social support at work. Acknowledgments This work was supported by Grants-in-Aid for Scientific Research (No. 21590549) from the Japan Society for the Promotion of Science. Conflict of interest of interest.
The authors declare that they have no conflict
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