A total of 467 office workers employed in a service industry organization in August 2012 participated in this study. They worked during the day almost always seated at a personal computer, performing similar tasks such as making documents, responding to telephone calls, filing materials, and attending meetings. At the beginning of the study, leaflets explaining the study procedure were distributed to all workers. Written informed consent was obtained from all participants. All data were provided and analyzed anonymously. Of the 467 workers, 330 (70.7%) completed the questionnaire. After excluding 31 workers for whom complete work hour information was unavailable, 18 higher level managers who did not record their work time, 8 workers with severe illnesses, and 6 workers who submitted incomplete questionnaires, 267 workers were enrolled (57.2%). The study design and procedure were reviewed and approved by the Committee for Medical Ethics of Shinshu University (approval number 1790).
A self-administered questionnaire comprising items regarding socio-demographic factors, workplace-related factors, and Big Five personality factors was distributed to every participant. Participants completed and immediately returned the questionnaire in a sealed envelope. The questions were based on those in previous published reports [20, 24, 34, 35]. Socio-demographic factors included sex (male, female), age (years), height (cm), weight (kg), average sleep per night (hours), drinking alcoholic beverages (every day, sometimes, occasionally, never), smoking (current smoker, ex-smoker, never smoker), physical exercise (every day, sometimes, occasionally, never), hobbies (yes, no), meal times (regular, irregular), current illness (yes, no), marital status (married, unmarried (including divorced or widowed)), educational level (postgraduate, graduate, graduate of junior college or technical training, no academic qualifications), stress at home (yes, no), and human relations at home (good, poor). Workplace-related factors included position within the company (manager or higher, assistant manager, chief, clerk, part-time worker) and commuting time (minutes). The factors also included job demand, decision latitude, and social support [36, 37] according to the Japanese version of Brief Job Stress Questionnaire . In addition, work-time related factors were assessed using the following negative and positive attitude questions. Negative factors: can you take paid holidays?; do you have an opportunity to stop work to go home?; do you have difficulty going home due to concern about other workers?; do you have a fear of the boss?: Positive factors: do you have work motivation?; do you have a work purpose?; do you have success motivation?; do you feel gratitude for your employment? These questions were also based on those in previous reports [20, 24, 34]. These factors were assessed on a 4-point Likert scale (ranging from a score of 4 for high and 1 for low). The internal consistencies (Cronbach’s alpha) of the Negative and Positive attitude questions were 0.55 and 0.57, respectively.
We assessed personality characteristics using the Big Five personality test , based on the five-factor model which is generally accepted worldwide. The model describes an individual’s personality along five dimensions, Extraversion, Agreeableness, Conscientiousness, Emotional stability, and Openness. In this study, the Japanese version of the Big Five personality test  including 70 questions was applied. Scores for each characteristic range from 0 to 12 with 12 indicating a high degree of the characteristic. Scores were determined using Windows software accompanying the manual  and applied for analysis in this study. In this study, the internal consistencies (Cronbach’s alpha) of Extraversion, Agreeableness, Conscientiousness, Emotional stability and Openness were found to be 0.90, 0.76, 0.84, 0.91 and 0.81, respectively.
Number of working hours
In this sample, full-time workers were contracted to work from 08:30 to 17:15 daily with a 1 h lunch break amounting to a standard 7 h 45 min/day, 38 h 45 min/week and part-time workers were contracted to work a standard 30 h/week. The principal of the organization encouraged all workers to work a limited number of overtime hours. Additional work hours beyond the standard were counted as overtime work hours. All workers recorded their working time into their personal computers daily to be checked by their boss, thus allowing accurate determination of the number of hours worked by each subject. Such data recorded for the previous 12 months were obtained and used in this study.
As adverse health effects can be caused by not only permanent long work hours but also accumulation of overtime hours within a short period , two outcome measures were employed to evaluate number of working hours. First, average number of overtime work hours, calculated as the average of 12 months of overtime for each worker and expressed as “hours per month”, and second, excessive overtime work. In Japan, workers can work overtime by either working beyond the set number of hours per day or by working on their scheduled day off, to a maximum of 45 h/month, as prescribed by labor-management agreements; if these agreements include special clauses, workers can work more than this upper limit . Consequently, all workers are aware of their upper limit for overtime work hours, therefore, it is important to know whether workers breach the limit or not when evaluating long work hours. In this study, excessive overtime work was defined as that exceeding 45 h/month at least once during the previous 12 months.
Participants were divided into 4 groups by age (20 – 29, 30 – 39, 40 – 49, ≥ 50 years), 3 by BMI (< 18.5, 18.5 – 25, ≥ 25), and 4 by work status (Assistant manager or higher, Chief, Clerk, Part-time worker). Other factors were dichotomized (yes/no, Likert scale 1 – 2/3 – 4 or suitable division according to preliminary calculations). In this study, each personality factor was dichotomized by median score into high-score group or low-score group to determine whether high personality score group did more overtime work than low personality score group or otherwise, respectively. Average numbers of overtime work hours were similarly treated with participants classified as long overtime work group or short overtime work group according to median overtime work hours. Subjects were classified as excessive overtime work occurrence group if they worked overtime beyond 45 h/month at least once during 12 months or excessive overtime work non-occurrence group if they did not. For statistical analysis, univariate and multivariate logistic regression models were used. First, a univariate regression model of socio-demographic and workplace-related factors for the two outcome measures were used as preliminary analysis. Subsequently, significant factors which were observed in the first step were used as adjusting factors for a multivariate regression model, and associations between the Big Five personality factors and the two outcome measures were evaluated. All dimensions of personality factors were analyzed independently to avoid overadjustment . Associations were expressed as odds ratios (OR) and 95% confidence interval (95%CI). PASW 18.0 software (SPSS Inc., Chicago, IL) was used for all analyses, and P < 0.05 was taken to indicate statistical significance.