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Occupational Medicine Advance Access originally published online on November 10, 2008
Occupational Medicine 2009 59(1):14-19; doi:10.1093/occmed/kqn141
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© The Author 2008. Published by Oxford University Press on behalf of the Society of Occupational Medicine. All rights reserved. For Permissions, please email: journals.permissions@oxfordjournals.org

Overtime work and stress response in a group of Japanese workers

Yuji Sato1,2, Hitoshi Miyake2 and Gilles Thériault1

1 Department of Occupational Health, Faculty of Medicine, McGill University, Montreal, Quebec, Canada
2 Central Health Support and Promotion Division, Fujitsu Limited, Kawasaki, Kanagawa, Japan

Correspondence to: Gilles Thériault, Department of Occupational health, Faculty of Medicine, McGill University, 1020 Pine Avenue West, Montreal, Quebec H3A1A2, Canada. Tel: +1 514 398 5110; fax: +1 514 398 7435; e-mail: gilles.theriault{at}mcgill.ca


   Abstract
Top
 Abstract
Introduction
 Methods
 Results
 Discussion
 Conflicts of interest
 References
 
Background Working long overtime hours is considered a cause of mental health problems among workers but such a relationship has yet to be empirically confirmed.

Aim To clarify the influence of overtime work on response to stress and to assess the role of other stress-related factors on this relationship.

Methods The study was conducted among 24 685 employees of a company in Japan. Stress response, job stressors and social supports were assessed by the Brief Job Stress Questionnaire. Participants were divided into five categories of overtime (0–19, 20–39, 40–59, ≥60 h of overtime per month and exempted employees).

Results The nonadjusted odds ratios for stress response for 40–59 and ≥60 overtime hours per month in reference to 0–19 overtime hours were 1.11 [95% confidence interval (CI) 1.03–1.19] and 1.62 (95% CI 1.50–1.76), respectively. After adjustment for self-assessed amount of work, mental workload and sleeping time, the association between overtime work and stress response disappeared.

Conclusions This large cross-sectional study shows that overtime work appears to influence stress response indirectly through other stress factors such as self-assessed amount of work, mental workload and sleeping time.

Keywords      Long working hours; mental health; occupational health; psychological stress


   Introduction
Top
 Abstract
 Introduction
Methods
 Results
 Discussion
 Conflicts of interest
 References
 
The contribution of working long overtime hours to mental health problems at work is currently the subject of much research. The relationship between overtime work and stress is particularly important in countries like Japan where working is deeply engrained in the national culture. In the last decade, the annual rate of suicide has grown from 18.4 to 25.5 per 100 000 people and the annual number of workers who committed suicide has increased from 5800 to 8800 [1]. Work-related claims for mental disorders have sharply increased from 155 to 819 per year in the last 7 years and the annual number of work-related suicides accepted by the Japanese Labour Standards Inspection Office has increased from 11 to 66 during the same period [2,3].

Overtime work has been proposed as a factor that contributes to this phenomenon because overtime is seen as causing increasing stress among workers. As a consequence, the Ministry of Health, Labour and Welfare of Japan has published a set of guidelines entitled ‘Comprehensive Program for Prevention of Health Impairment Due to Overwork’ [4]. According to these guidelines, Japanese employers are required to limit to 45 h the number of overtime hours an employee can work in 1 month. These limits are based on epidemiological studies that have shown an increased risk of cerebrovascular and cardiovascular disorders among employees who work >45 overtime hours per month. The existing literature [5–10] suggests that overtime work is associated with depression, fatigue and confusion which may lead to mental illnesses and suicide. However, the evidence is not clear so that some authors [11–13] have proposed that the association between overtime and mental health problems was more spurious than real, arguing that it was not overtime per se that causes health problems but other confounding factors such as demographic variables and psychosocial work characteristics. These possible confounders may have been wrongly attributed to overtime work. Other authors [12,13] have not observed any association between overtime work and mental health disorders. The association between overtime work and mental health is a complex issue that has yet to be empirically measured.

We took advantage of a survey conducted in a large company to address the influence of overtime work on stress response and to assess the role of other stress-related factors on this relationship.


   Methods
Top
 Abstract
 Introduction
 Methods
Results
 Discussion
 Conflicts of interest
 References
 
The study was conducted in a computer, software and network company that employs ~40 000 employees. In July 2004, the central Health Care Department invited all employees to voluntarily answer the web-based Brief Job Stress Questionnaire (BJSQ), a questionnaire developed and validated by the Ministry of Health, Labour and Welfare of Japan [14]. This invitation was posted on the internal company's website and invited participants to visit the company's Health Care Department website to answer the questionnaire. The responses were then sent to the Health Care Department. Only authorized staff of the department had access to the data. The entire process was web-based with manual data entry. Since this study was compiled from a survey conducted as a mental health promotion activity in the company and was administered by the medical personal staff, no institutional review board approval was solicited. However, before conducting the survey, the design and the conduct of the survey were the object of an agreement between the employers and the employees union. The study protected the participants’ privacy and participation to the survey was entirely voluntary.

The BJSQ consists of 57 items that cover stress response, job stressors and social supports. Gender, age of the participant, number of hours slept and the number of overtime hours worked in the most recent month were also collected. Sleeping time and overtime hours were self-assessed by each participant. Overtime hours per month was defined by calculating total hours worked minus the standard eight working hours per day in weekdays plus number of hours worked on holidays during the month. The participants reported their overtime work into one of 10-h categories of overtime work.

Upon completion of the questionnaire, the participants obtained their own personal profile of stress response, job stressors and social supports. In addition, the system gave them some recommendations on how to manage their stress. The BJSQ assesses the following stress responses: lack of vigor, anger, fatigue, depressive symptom, anxiety and physical distress. Each stress response was rated from 1 = low distress to 5 = high distress. The stress response for each factor was added to yield a total stress response score that ranged from 6 to 30.

In order to contrast different levels of stress in the analysis, the response scores were dichotomized at the median and at the 90th percentile value.

Job stressors in the BJSQ include the following variables: self-assessed amount of work, mental workload, physical workload, job control, skill utilization, interpersonal conflict, poor physical environment, job suitability and work satisfaction. Social supports include supervisor support, coworker support and family/friend support. The following three statements in the BJSQ were used to generate the score ‘amount of work’: (i) ‘I always have a great deal of work to do.’ (ii) ‘It is impossible to complete my work in the time available.’ (iii) ‘I have to put all my effort into my work.’ The following three statements were used to generate the score ‘mental workload’: (i) ‘I need to pay careful attention.’ (ii) ‘My job requires great knowledge and skill.’ (iii) ‘While I'm on the job, I have to keep my mind on my work all the time.’ Respondents rated each job stressor and each social support from 1 = low degree to 5 = high degree [14].

In the analysis, the participants were divided into five categories of overtime work (0–19, 20–39, 40–59, ≥60 and exempts) according to the number of overtime hours worked in the most recent month. Exempts were employees for whom overtime work was not monitored.

To select potential confounding factors of the association between overtime work and stress response, Spearman's correlation coefficients were obtained for each of these two factors with several job stressors and social support variables as well as with age and sex. Self-assessed amount of work, mental workload and sleeping time showed a correlation coefficient of |r| > 0.2 with both overtime work and stress response. Thereby, these three factors were chosen as possible confounders (Table 1).


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  		Table 1. Correlation coefficients (r) of overtime work and stress response with stressors, social supports, age and sex

 
Odds ratio (OR) and the 95% confidence intervals (CIs) were estimated by multiple logistic regression analysis using the SPSS version 15.0 statistical package (SPSS Inc., Chicago, IL, USA).

Multicollinearity among the dependent variables was verified by calculating the variance inflation factor of each independent variable.


   Results
Top
 Abstract
 Introduction
 Methods
 Results
Discussion
 Conflicts of interest
 References
 
In total, 24 685 employees (20 091 males and 4594 females) completed the BJSQ giving a response rate of ~60%.

Table 2 shows the distribution of respondents by age and overtime work. Two-thirds of the participants were <40 years of age. The median number of overtime was 30 h per month. In total, 18.5% of the participants reported <10 h of overtime per month, 6% of the participants worked ≥80 h and only ~2% of the participants worked ≥100 h of overtime per month. The exempt employees, mostly executive and professional for whom overtime work was not monitored, accounted for 12.5% of the respondents.


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  		Table 2. Distribution of respondents by age and number of overtime hours worked per month

 
Figure 1 shows the unadjusted risk ratio for stress response according to the number of hours of overtime per month. In Figure 1, when the stress score was dichotomized at a value of 17, the median score, the risk ratios started to increase in the overtime category 40–49 h and increased steadily thereafter to reach a risk ratio of 1.45 in the group that did ≥100 overtime hours. Then the stress score was dichotomized at a value of 23, which corresponded to the 90th percentile. For this more restrictive definition of stress response, the risk started to increase in the 50–59 overtime hours category to reach a maximum of 2.18 for ≥100 overtime hours.


Figure 1
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  		Figure 1. Risk ratio for stress response according to hours of overtime worked.

 
The risk of the exempted workers was <1.0 for both definitions of the stress response.

Table 3 shows the OR for stress response according to overtime work. Before adjustment, overtime work of >40 h per month was likely to cause a stress response. For above-median stress response, the 40- to 59-h overtime work category shows a significantly increased OR of 1.11 (95% CI 1.03–1.19) and overtime work of >60 h per month shows a significantly increased OR of 1.62 (95% CI 1.50–1.76). For the above 90th percentile stress response, overtime work of >60 h per month shows a significantly increased OR of 1.58 (95% CI 1.42–1.76). After adjustment for the three potentially confounding variables, amount of work, mental workload and sleeping time, the trend toward an increased OR with increased overtime work disappears completely (Table 3). The adjustment factors accounted entirely for the trend observed in the nonadjusted analysis. For the exempted employees, both the adjusted and the nonadjusted ORs for stress response show significantly lower risks than even the employees in the 0- to 19-h overtime work per month category.


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  		Table 3. OR of stress response according to overtime work

 

   Discussion
Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
Conflicts of interest
 References
 
In the present study, the unadjusted data show an increased risk of stress with increased overtime work. However, after adjustment for amount of work, mental workload and sleeping time, the association between overtime work and stress response completely disappeared, indicating that the association between stress and overtime work appeared to be attributable to amount of work, mental workload and lack of sleep.

The strength of this study was the large sample size of ~25 000 workers compared to the previous studies with smaller sample sizes [5–8,10–13,15,16]. This has given our study more power for statistical discriminate analysis. Also, this study considered job stressors and social supports as possible confounders between overtime work and stress response, whereas most previous studies have considered other less directly related variables such as gender, ethnicity, age and education.

While it has distinct advantages, our study has some methodological limitations. First, the participation rate was ~60% indicating some possibility for selection bias. Answering the BJSQ was voluntary, so respondents were likely to have a more positive attitude toward their own mental health than nonrespondents (healthy volunteer effect) [17]. Even though we cannot claim that the data are representative of the entire working population, the large sample size suggests that it is unlikely that selection would have introduced a significant bias in the results. Although the target population of the study is a large Japanese workers company (40 000 workers) and consists of variety of job types, we cannot clearly say that the findings apply to all workers in Japan. Second, the present study has a cross-sectional design, so any cause-and-effect relationship among variables cannot be claimed. Within the limitation of the cross-sectional study, care must be taken in interpreting the effect of overtime work on stress response. Third, overtime work does not correlate with stress response after adjustment for self-assessed amount of work, mental workload and sleeping time. The amount of work, mental workload and sleeping time were confounders of the association between overtime work and stress response. Some respondents may have confused the notions ‘amount of work’ and ‘overtime work’. By adjusting in the analysis for amount of work, this may have led to over adjustment and consequently may have hidden a residual relationship between overtime work and stress response. Our study did not collect information on marital status, education, work schedule, rank within the organization and lifestyle (smoking, alcohol consumption and exercise) and as a consequence could not adjust for these variables.

A significant association between overtime work and mental health has been reported by some researchers [5–8,10,15] but not by others [11–13,16].

Shields [6] found that long working hours had a significant effect on the morbidity of major depressive episodes in women (OR = 2.2, 95% CI 1.1–4.4) but not in men (OR = 0.6, 95% CI 0.3–1.3). However, the definition that Shields used for overtime work expressed as working ≥35 h per week is imprecise and makes it difficult to interpret his results. Proctor et al. [8] reported the effect of overtime hours on depressive symptom using the Profile of Mood States questionnaire, adjusted for demographic and work-related variables. In their report, increased overtime was significantly associated with increased feeling of depression. Park et al. [15] compared the stress response between three categories of weekly working hours (<60, 60–70, >70 h) adjusted only for age. They reported that working >60 h per week was associated with stress response. Grosch et al. [7] also reported a positive association between overtime work and job stress. They considered gender, ethnicity, age and education as confounders.

Aside from Stavem et al. [11] who observed a nonsignificant association between working hours per week and mental health among doctors after adjustment for demographic and health-related variables, most authors who controlled for job stressors reported an absence of association between overtime work and mental health after controlling for these variables.

Van der Hulst [18] pointed out in her review on long working hours and health that the most likely reason why the evidence of a relationship between long working hours and adverse health effect is inconclusive is that many studies have not controlled for covariates. In her review, she argued that other psychological work characteristics, such as job control and social support, may covary with work hours.

For Spurgeon et al. [19], long working hours are likely to coincide with high job demands. It is difficult to determine whether long hours are themselves a direct source of stress or if they simply contribute to increase the impact of other stressors. In particular, it is difficult to separate the effects of long hours from those of occupational stress in general.

Hobson and Beach [16] reported that it was not the number of work hours but the perceived workload that was associated with psychological health. They pointed out that perceived workload was more important in determining psychological health than actual workload. In the present study, amount of work and mental workload that had a stronger association with stress response were self-assessed which seems to indicate that our observation meets with Hobson's view.

In our study, employees who were exempted from monitoring overtime showed less stress response with overtime work than the nonexempts. The exempts showed higher job control, skill utilization, job suitability and job satisfaction in comparison with the nonexempts (data not shown). This is in agreement with the Japanese Government's report that exempts are likely to have higher job control and skill utilization [20]. However, the exempted employees are managers or regular employees preselected for their capacity to work at their own pace. The lower stress response among these employees may be the result of preselection rather than an absence of response to work stressors.

To focus on the effect of more severe overtime work, ≥50 h per month, on stress response, we reanalyzed the results using the overtime categories defined in the Ministry of Health, Labour and Welfare of Japan Guideline [21]. Before adjustment, the 50- to 79-h overtime work category showed significantly increased ORs of 1.30 (95% CI 1.22–1.39) and 1.30 (95% CI 1.19–1.43) for both median and 90th percentile stress responses, respectively. As overtime work increased, ORs increased and reached a maximum OR in the ≥100 overtime work category for both the median (OR = 2.84, 95% CI 2.26–3.56) and the 90th percentile responses (OR = 2.72, 95% CI 2.19–3.38). After adjustment for amount of work, mental workload and sleeping time, the relationship between overtime and stress response disappeared except in the ≥100 overtime hours work category for median stress response (OR = 1.37, 95% CI 1.08–1.73) (Table 4). These results suggest that if excessive overtime work carries an elevated risk for stress response in the ≥100-h overtime per month, this increased risk is weak.


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  		Table 4. OR for stress response according to overtime work categorized as per the MHLWJ guidelines

 
Since the Japanese government amended the Industrial Safety and Health Law in 2006 [22], employers are required to make arrangements for workers to receive health guidance by a medical doctor if they work ≥100 overtime hours per month. Physicians are required to review the work conditions of the workers to assess their physical and mental health and to give appropriate health advice to the workers. The present study supports the necessity of such specific care for workers who work excessive overtime, ≥100 h per month.

Although excessive overtime, ≥100 h per month, may be harmful for the mental health of workers, this study suggests that hours of overtime per se are not as important as the perception of the amount of work, mental workload and amount of sleep. This indicates that regulation of the number of overtime hours is not enough to eliminate the health effects of overtime.

It is empirically known that some workers are free from mental stress even though they do a lot of overtime work. One thing that seems to characterize these people is that they accomplish self-imposed work as opposed to work imposed upon them. Perception of workload is a complex feeling. Further studies on worker's subjective feeling about the voluntary or compulsory characteristics of their work will be needed to clarify the effects of overtime work on mental health. Future additional studies on workers who work excessively and compulsively will also shed new light on the relationship between overtime work and mental health.

In conclusion, overtime work is associated with stress response through other stress factors such as self-assessed amount of work, mental workload and sleeping time, though it remains possible that overtime work in excess of 100 h per month carries some risk for stress response regardless of these three factors.


Key points
  • Overtime work is associated with stress response through self-assessed amount of work, mental workload and sleeping time.
  • Regulation of the number of overtime hours will not eliminate the health effects of overtime unless one also addresses other work components such as amount of work, mental workload and lack of sleeping time.

 


   Conflicts of interest
Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
 Conflicts of interest
References
 
None declared.


   References
Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
 Conflicts of interest
 References
 

  1. Tokyo Metropolitan Police Department. Summary Document of Suicide in 2006. http://www.npa.go.jp/toukei/chiiki8/20070607.pdf (25 March 2008, date last accessed) [in Japanese].

  2. Ministry of Health, Labour and Welfare of Japan. Workers’ Compensation for Brain and Heart Diseases and Mental Health Disorders etc. in 2002. http://www.mhlw.go.jp/houdou/2003/06/h0610-4.html (25 March 2008, date last accessed) [in Japanese].

  3. Ministry of Health, Labour and Welfare of Japan. Workers’ Compensation for Brain and Heart Diseases and Mental Health Disorders etc. in 2006. http://www.mhlw.go.jp/houdou/2007/05/h0516-2.html (25 March 2008, date last accessed) [in Japanese].

  4. Ministry of Health, Labour and Welfare of Japan. Comprehensive Program for Prevention of Health Impairment Due to Overwork (2002) http://www.jniosh.go.jp/icpro/jicosh_old/english/guideline/OverworkMeasures.html (20 October 2008, date last accessed).

  5. Suwazono Y, Nagashima S, Okubo Y, et al. Estimation of the number of working hours critical for the development of mental and physical fatigue symptoms in Japanese male workers—application of benchmark dose method. Am J Ind Med (2007) 50:173–182.[CrossRef][Web of Science][Medline]

  6. Shields M. Long working hours and health. Health Rep (1999) 11:33–48.[Medline]

  7. Grosch JW, Caruso CC, Rosa RR, Sauter SL. Long hours of work in the U.S.: associations with demographic and organizational characteristics, psychosocial working conditions, and health. Am J Ind Med (2006) 49:943–952.[CrossRef][Web of Science][Medline]

  8. Proctor SP, White RF, Robins TG, Echeverria D, Rocskay AZ. Effect of overtime work on cognitive function in automotive workers. Scand J Work Environ Health (1996) 22:124–132.[Web of Science][Medline]

  9. Suwazono Y, Okubo Y, Kobayashi E, Kido T, Nogawa K. A follow-up study on the association of working conditions and lifestyles with the development of (perceived) mental symptoms in workers of a telecommunication enterprise. Occup Med (Lond) (2003) 53:436–442.[CrossRef][Medline]

  10. Nagashima S, Suwazono Y, Okubo Y, et al. Working hours and mental and physical fatigue in Japanese workers. Occup Med (Lond) (2007) 57:449–452.[CrossRef][Medline]

  11. Stavem K, Hofoss D, Aasland OG. Work characteristics and morbidity as predictors of self-perceived health status in Norwegian physicians. Scand J Public Health (2003) 31:375–381.[Abstract/Free Full Text]

  12. Nishikitani M, Nakao M, Karita K, Nomura K, Yano E. Influence of overtime work, sleep duration, and perceived job characteristics on the physical and mental status of software engineers. Ind Health (2005) 43:623–629.[CrossRef][Web of Science][Medline]

  13. Beckers DG, van der Linden D, Smulders PG, Kompier MA, van Veldhoven MJ, van Yperen NW. Working overtime hours: relations with fatigue, work motivation, and the quality of work. J Occup Environ Med (2004) 46:1282–1289.[Web of Science][Medline]

  14. Kato M. Study on prevention of work-related diseases. In: A Research Report Concerning Stress and Its Effects on Health in Workplace—Kato M, ed. (2000) Tokyo: Ministry of Labour of Japan. 1–411. [in Japanese].

  15. Park J, Kim Y, Chung HK, Hisanaga N. Long working hours and subjective fatigue symptoms. Ind Health (2001) 39:250–254.[Web of Science][Medline]

  16. Hobson J, Beach JR. An investigation of the relationship between psychological health and workload among managers. Occup Med (Lond) (2000) 50:518–522.[Medline]

  17. Froom P, Melamed S, Kristal-Boneh E, Benbassat J, Ribak J. Healthy volunteer effect in industrial workers. J Clin Epidemiol (1999) 52:731–735.[CrossRef][Web of Science][Medline]

  18. Van der Hulst M. Long workhours and health. Scand J Work Environ Health (2003) 29:171–188.[Web of Science][Medline]

  19. Spurgeon A, Harrington JM, Cooper CL. Health and safety problems associated with long working hours: a review of the current position. Occup Environ Med (1997) 54:367–375.[Abstract/Free Full Text]

  20. Labour Policy Council of the Ministry of Health, Labour and Welfare of Japan. Report on Working Hours Legislation. http://www.mhlw.go.jp/shingi/2006/03/s0329-9f.html (25 March 2008, date last accessed) [in Japanese].

  21. Ministry of Health, Labour and Welfare of Japan. Amended Comprehensive Program for Prevention of Health Impairment Due to Overwork (2006) http://www.mhlw.go.jp/topics/bukyoku/roudou/an-eihou/dl/ka060317008a.pdf (25 March 2008, date last accessed) [in Japanese].

  22. Ministry of Health, Labour and Welfare of Japan. Amended Industrial Safety and Health Law (2006) http://www.mhlw.go.jp/topics/bukyoku/roudou/an-eihou/dl/05-108c.pdf (25 March 2008, date last accessed) [in Japanese].


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