JOB STRESS, SOCIAL SUPPORT AT WORK, AND INSOMNIA IN JAPANESE SHIFT WORKERS

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1 J. Human Ergol., 30: , 2001 JOB STRESS, SOCIAL SUPPORT AT WORK, AND INSOMNIA IN JAPANESE SHIFT WORKERS Akinori NAKATA, Takashi HARATANI, Masaya TAKAHASHI, Norito KAWAKAMI, Heihachiro ARITO, Yosei FUJIOKA, Hiroyuki SHIMIZU, Fumio KOBAYASHI and Shunichi ARAKI National Institute of Industrial Health, Kawasaki, Japan,Department of Public Health, Okayama University School of Medicine, Okayama, Japan,Department of Public Health and Occupational Medicine, Graduate School of Medicine, University of Tokyo, Japan,Department of Public Health, Gifu University School of Medicine, Gifu, Japan, Department of Health and Psychosocial Medicine, Aichi Medical University, Aichi, Japan A cross-sectional study was conducted to clarify the contribution of psychological job stress to insomnia in shift workers. A self administered questionnaire concerning job stress, sleep, depressive symptoms and lifestyle factors was submitted to a sample of 530 rotating shift workers of age years (mean age 27) in an electric equipment manufacturing company. Perceived job stress, i.e., job demands, job control and social support at work, was assessed using the Japanese version of the Job Content Questionnaire. Insomnia was regarded as prevalent if the workers had at least one of the following symptoms in the last year; less than 30 minutes to fall asleep, difficulty in maintaining sleep, or early morning awakening almost everyday. Overall prevalence was 37.8%. Logistic regression analyses while adjusting relevant factors showed that lower social support at work was significantly associated with a greater risk of insomnia than the higher social support (adjusted OR 2.5). Higher job strain with lower social support at work increased the risk, compared to lower strain with higher support at work (crude OR 1.8; adjusted OR 1.5). Our findings suggest the low social support at work independently associated with insomnia in shift workers Introduction Insomnia, defined as difficulty in initiating and maintaining sleep, is one of the most commonly reported health problems in shift workers (KNAUTH et al., 1980). Although its prevalence varies considerably (10-90% ), it is clear that insomnia has important implications for shift workers since evidence suggests associations with poor health, reduced productivity, increased accidents at work and absenteeism (STOLLER,1994). Epidemiological studies designed to determine the risk factors for insomnia suggested that aging, female genders, shift work, psychological stress, perceived poor health status, low life quality, and/or environmental disturbances are at higher risks (HARMA et al., 1998; NAKATA et al., 2000). Among them, shift work and job stress are suspected to be two major occupational risk factors (KAWACHI et al., 1995; COSTA, 1997; MURATA et al., 1999). In contrast, only a few studies on the effects of job stress on sleep have been conducted, mostly in industrialized countries. In these studies, high job demands (CAHILL and LANDSBERGIS, 1996; RIBET and DERRIENNIC, 1999), low job control (KALIMO et al., 2000), high job strain as defined as the combination of greater job demands and lower job control according to the job-demands control model (THEORELL et al., 1988; CAHILL and LANDSBERGIS, 1996), low social support at workplace (LANDSBERGIS, 1988) and less satisfaction with work (KUPPERMANN et al., 1995) are reported to relate to an increase in sleep problems. A poor atmosphere at work (JACQUINET-

2 204 A. NAKATA et al. SALORD et al., 1993) as well as overall mental workload (URPONEN et al., 1988) are also proposed as the factors that disturb sleep. KAGEYAMA et al. (1998) found that poor sleepers had significantly higher scores in job difficulty, lower scores in achievement in job and support by colleagues. TACHIBANA et al. (1996) showed over-involvement in job to be a high risk factor for insomnia in male industrial workers. In our previous report, we found an inverse relationship between social support and sleep complaints in female daytime employees (NAKATA et al., 2001). These findings suggest that certain types and amounts of job stress affect sleep or increase sleep problems. Since shift workers are reported to receive lower social support from family and friends (HOSSAIN and SHAPIRO, 1999) and higher amount of job stress than do permanent day workers (TENKANEN et al., 1997), the effects of job stress factors on their sleep should be examined. In the present study, a cross-sectional study was conducted in an electric equipment manufacturing company. Perceived job stress was assessed by the Japanese version of the Job Content Questionnaire (JCQ) developed by KARASEK (1979) (KAWAKAMI et al., 1996). Subjective sleep habits (length of time to fall asleep, frequency of awakening during sleep, and early morning awakening) were assessed by a previously used measure (NAKATA et al., 2000a, 2001). Additionally, in an attempt to assess the main effect and buffering effect of social support at work on insomnia individually, we tested two research hypotheses on the relationship between social support at work and insomnia as in Fig. 1. Fig. 1. Two hypothesized models for the effects of social support on insomnia. Subjects and methods A self administered questionnaire concerning demographic information, current job type, work schedule, job stress, sleep habits, lifestyle factors, and depressive symptoms was submitted to a total of 2,625 full-time employees in an electric equipment manufacturing company in April A total of 2,420 (92.2%) workers responded to the survey. We excluded day workers (n=1367); female workers were also excluded because of the skewed distributions in age and job type (n=466). Some workers who had a missing response were eliminated from the analyses. Finally, a total of 530 workers were subjected to the analyses. Job type was categorized into four groups, i.e., mechanics/repair workers, machine operators, manual laborers, and others (Table 1). The present study was carried out with the informed consent of all the workers.

3 205 Table 1. Selected characteristics, JCQ, and CES-D scale scores among shift workers of an electric equipment manufacturing company (n=530). &Job strain score was defined as the ratio of the job demands score to job control score. Assessment of job stress and social support at work: The Japanese version of the Job Content Questionnaire (JCQ) was used to assess job stress according to the job demand-control model and the demand-control-support model (KARASEK, 1979), as previously described (NAKATA et al., 2000b, 2002). According to our previous study, this version has acceptable levels of reliability and validity with Cronbach's alpha reliability coefficients from 0.61 to 0.89 for males and from 0.65 to 0.87 for females. (KAWAKAMI et al., 1996). Assessment of insomnia A self-administered sleep questionnaire was developed for this study and three questions about subjective sleep habits during the last one-year period were included as follows: 1. "How long does it usually take you to fall asleep in bed?"(0-10 minutes / minutes / minutes / 1-2 hours /2+ hours) 2. "How often do you have difficulty staying asleep? "(never or almost never) / few times a year / more than once a month / more than once a week / almost every day) 3. "How often do you wake up too early and can't fall asleep again?" (never or almost never) / few times a year / more than once a month / more than once a week / almost every day) The presence of difficulty initiating sleep (DIS) was defined when one took more than 30 min to fall asleep; difficulty maintaining sleep (DMS) and early morning awakening (EMA) were defined when one answered "almost always" to the questions 2 and 3, respectively. The presence of insomnia was defined when at least one of DIS, DMS, or EMA was reported. Potential confounding factors: Other covariates included age, marital status (married/not married), smoking (never or past/current smoker), drinking habit (> 4 days/week) (yes/no) caffeine intake (> 3 cups/day) (yes/no), habitual exercise (> 2 days/week) (yes/no), and depressive symptoms. The

4 206 A. NAKATA et al. depressive symptoms were measured by the Center for Epidemiologic Studies Depression Scale (CES-D). Statistical analyses: To examine the associations of job control, job demands, and social support at work with insomnia, we performed a series of univariate and multivariate logistic regression analyses individually. Age, marital status, smoking, drinking habit, caffeine intake, habitual exercise and depressive symptoms were adjusted in the multivariate analyses. To assess buffering effects of social support at work for the relation between job strain and insomnia, analysis of covariance controlling for the potential confounding factors was used. The significance level of all statistical analyses was set at a probability of less than 0.05 (two-tailed test). All data in the present study were analysed by the Statistical Package for the Social Sciences version 10.0 (SPSS Inc., Chicago, USA). Results Overall prevalence of insomnia was 37.8% with a 95% confidence interval (CI) of %. Lower social support at work (Odds ratio (OR) 2.3) was significantly and directly associated with an increased risk of insomnia; this association was confirmed by multivariate analyses adjusted for the relevant factors (Table 2). In contrast, job control and job demands did not show significant differences. Table 2 Crude and adjusted OR and 95% CI for insomnia at each level of job stress among shift workers (n= ). a All the workers were divided into three groups according to their job stress scores: Lower (or higher) scores for doe control ana social support at work (or job demands) were one standard deviation below (or above) the mean; higher (or lower) scores were above (or below) the mean, and medium between the higher and the lower. b Adjusted for all the other covariates in multivariate logistic regression analyses. * p<0.05, ** p<0.01, n.s, non significant. Workers with a combination of high job strain and low social support at work showed a significantly higher risk of insomnia compared to a combination of low strain with high support (OR 1.8). This association was not confirmed by the multivariate analyses, adjusted for age, marital status, smoking, drinking habit, caffeine intake, habitual exercise and depressive symptoms (Table 3). No significant interaction was found between job strain and social support.

5 Table 3. Crude and adjusted OR and 95% CI for insomnia at each level of a combination of job strain and social support at work amone shift workers (n-5o ajob strain and social support scores were dichotomised into higher or lower groups by respective mean scores, and created the four job stress level groups (thereby constructing a higher and lower group for each scale). b See the footnote as in Table 2. * p<0.05, n.s, non significant. Discussion The principal findings in this study are that the overall prevalence of insomnia in this population was 37.8% and that insomnia appeared to be directly associated with social support at work, rather than with job control or job demands. The results supported the hypothesis in the present study that social support at work had a main effect on insomnia but did not support a buffering effect, since no significant interaction was found between job strain and social support (Fig. 1). Social support at work in shift workers is thus independently associated with insomnia, even after controlling for important confounding factors. The prevalence of insomnia was the highest among the workers with lower social support at work (Table 2). The finding agrees with our previous study for the daytime female employees, which found inverse relationships between social support from supervisors and frequency of early morning awakening, and support from colleagues and frequency of awakening during sleep (NAKATA et al., 2001). The results of the current study are also in agreement with two previous studies. LANDSBERGIS (1988) reported an inverse relationship between social support and sleep problems in female health care workers, which also used JCQ as job stress measure. Another study by KAGEYAMA et al. (1998) revealed a significant decrease in scores of support by colleagues in poor sleepers compared to good sleepers in white-collar male workers. These reports support the findings in the present study that social support at work has a main effect on insomnia. By contrast, although the prevalence of insomnia was higher in the high strain/low support group (OR 1.8) compared to high strain/high support group (OR 1.0), a buffering effect was not supported since there was no significant interaction between job strain and social support. Social support at work itself may play an important role in the occurrence and/or persistence of insomnia in Japanese shift workers. Further well-designed studies are required to confirm this relationship. No clear associations were found between job control or job demands and prevalence of insomnia in this study. In contrast, three studies reported significant associations with job control or job demands with sleep. LANDSBERGIS (1988) found a significant positive correlation between job demands and sleep disturbances. THEORELL et al. (1988) reported that sleep disturbances increased from the lowest to the highest degree of job strain in a sample of 71 men and women. In a recent study, an increase in job demands or a decrease in job control have been observed to increase the prevalence of insomnia in male shift workers (KALIMO et al., 2000); the prevalence of insomnia was higher in the highest quartile of job strain. The discrepancy between the current and these previous studies might be attributable to the fact that our samples were from a single occupation in an electric equipment manufacturing company compared to a variety of occupational categories in other three studies; the job demands-control model is not clear in studies

6 208 A. NAKATA et al. with a single occupation group since limited distribution of job demands and job control could be observed (SPECTOR et al., 1987), as suggested by the standard deviations in the Table 1. Thus a wide variety of occupational categories of shift workers should be examined in future studies. There is evidence that shift work contributes to excess cardiovascular morbidity and mortality (e.g. AKERSTEDT et al., 1984; KNUTSSON et al., 1986). On the other hand, it has been reported that insomnia is observed prior to the occurrence of myocardial infarction (CARNEY et al., 1990). Thus it could be hypothesized that insomnia induced by low social support at work in addition to the effect of shift work may give rise to excess in the development of cardiovascular diseases. Additional studies are needed to confirm this hypothesis. Finally, several limitations of our study should be noted. The study design was crosssectional, and therefore only associations but not causal relationships could be identified. Also, the sample size was small, and only male workers from a single company were included. Comprehensive research is required to provide more practical knowledge about the role of social support at work on insomnia in our working life. Acknowledgement This work was supported by the Special Coordination Funds of the Ministry of Education, Culture, Sports, Science and Technology of the Japanese Government. References AKERSTEDT, T., KNUTTSON, A., ALFREDSSON, L. and THEORELL, T. (1984) Shift work and cardiovascular disease. Scand. J. Work Environ. Health, 10: CAHILL, J. and LANDSBERGIS, P.A. (1996) Job strain among post office mailhandlers. Int. J. Health. Services, 26; CARNEY, R.M., FREEDLAND, K.E. and JAFFE, A.S. (1990) Insomniand depression prior to myocardial infarction. Psychosom. Med., 52: COSTA, G. (1997) The problem: shiftwork. Chronobiol, Int.,14: HARMA, M., TENKANEN, L., SJOBLOM, T., ALIKOSKI, T. and HEINSALMI, P. (1998) Combined effects of shift work and life-style on the prevalence of insomnia, sleep deprivation and daytime sleepiness. Scand. J. Work Environ. Health, 24: HOSSAJN, J.L. and SHAPIRO, CM. (1999) Considerations and possible consequences of shift work. J. Psycosom. Res., 47: JACQUINET-SALORD, MC., LANG, T., FOURIAUD, C., NICOULET, I. and BINGHAM, A. (1993) Sleeping tablet consumption, self reported quality of sleep, and working conditions. J. Epedemiol. Community Health, 47: KAGEYAMA, T., NISHIKIDO, N., KOBAYASHI, T. et al. (1998) Self-reported sleep quality, job stress, and daytime autonomic activities assessed in terms of short-term heart rate variability among male white-collar workers. Ind. Health, 36: KALIMO, R., TENKANEN, L., HARMA, M., POPPIUS, E. and HEINSALMI, P. (2000) Job stress and sleep disorders: findings from the Helsinki Heart Study. Stress Med., 16: KARASEK, R. A. (1979) Job demand, job decision latitude, and mental strain: Implications for job redesign. Adm. Sci. Q., 24: KAWACHI, I., COLDITZ, G. A., STAMPFER, M. J. et al. (1995) Prospective study of shift workers and risk of coronary heart disease in women. Circulation, 92: KAWAKAMI, N. and FUJIGAKI, Y. (1996) Reliability and validity of the Japanese version of Job Content Questionnaire: Replication and extension in computer company employees. Ind. Health, 34: KNAUTH, P., LANDAU, K., DROGE, C., SCHWITTECK, M., WIDYNSKI, M. and RUTENFRANZ, J. (1980) Duration of sleep depending on the type of shift work. Int. Arch. Occup. Environ. Health, 46: KNUTSSON, A., AKERSTEDT, T., JONSSON, B.G. et al. (1986) Increased risk of ischaemic heart disease in shift workers. Lancet, 2: KUPPERMANN, M., LUBECK, D.P., MAZONSON, P.D. et al. (1995) Sleeproblems and their correlates in a working population. J. Gen. Intern. Med., 10: LANDSBERGIS, P.A. (1988) Occupational stress among health care workers: A test of the job demands-control model. J. Org. Behav., 9: MURATA, K., YANO, E. and SHINOZAKI, T. (1999) Cardiovascular dysfunction due to shift work. J. Occup. Environ. Med., 41:

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