2025 Volume 47 Issue 2 Pages 87-93
Work with night shifts has been reported to have a variety of negative health effects. Self-rated health is an important indicator of health, yet studies regarding the relationship of work with night shifts on self-rated health have reported inconsistent findings. The present study aimed to examine that relationship. We conducted a prospective cohort study using a self-administered online questionnaire on workers aged 20 years or older in Japan. The relationship between work with night shifts and self-rated health was examined by logistic regression analysis. Compared with participants who work without night shifts, the odds ratio of poor self-rated health was significantly higher among participants who work with night shifts (odds ratio: 1.19), after adjusting for age, sex, annual household income, industry category and self-rated health at baseline. The present study suggests that self-rated health, a commonly used simple and general indicator of health effects, can be used for workers with night shifts.
Work with night shifts is globally prevalent, with reports indicating that 19% of workers in 28 EU countries engage in night shift work [1]. Similarly, work with night shifts is widespread in Japan, and the proportion of workers with night shifts has shown a marked increase over time, from 13.3% in 1997 to 21.8% in 2012 [2].
Work with night shifts has been shown to have a variety of negative health effects. Early effects include sleep disorders and gastrointestinal disorders; medium-term effects include obesity, diabetes, and hypertension; and long-term effects include cardiovascular diseases such as coronary artery disease, and an increased risk of malignant tumors, such as prostate cancer and breast cancer [3–9]. Additionally, work with night shifts has been independently linked to increased all-cause mortality [10].
Self-rated health, a personal assessment of one’s health status, has been reported to be an independent contributor to all-cause mortality [11]. Although few studies have examined the relationship between self-rated health and specific diseases, it has been reported that self-rated health is related to the morbidity of chronic diseases (excluding malignant tumors) [12] and the incidence of coronary heart disease [13]. Thus, self-rated health is an important indicator of general health.
Previous studies regarding the impact of work with night shifts on self-rated health have reported inconsistent findings. Although some studies have indicated that workers with night shifts report lower self-rated health [14, 15], others have found no relationship [16–19] or even higher self-rated health among groups of workers with night shifts [20], reflecting inconsistencies that are potentially attributable to sample limitations. For example, a longitudinal study of Norwegian oil drillers showed no relationship between work with night shifts and self-rated health; however, all of the participants in the study were oil drillers, potentially limiting the generalizability of the results [17]. Conversely, research encompassing a broader range of industries, such as the Atlantic Partnership for Tomorrow’s Health (PATH) cohort study in Canada [15] and a study of industrial company employees in Germany [19], can provide more comprehensive insights, although these cross-sectional studies were unable to establish causality. Future studies of the relationship between work with night shifts and self-rated health should use longitudinal study designs to cover a wide variety of industries.
The present study therefore aimed to examine the relationship between work with night shifts and self-rated health in a prospective cohort study of workers in a wide variety of industries.
Study design and participants
This is a prospective cohort study using a self-administered online questionnaire [21]. The baseline survey was conducted in March, 2022 among workers in Japan aged 20 years or older. We used a balanced sampling method to ensure that the sample of workers matched the general population in terms of sex, age, and region of residence. In the baseline survey, 27,693 workers were included. The follow-up survey was conducted in March, 2023, 1 year after the baseline survey. In total, 16,629 participants responded to the follow-up survey (60.0% follow-up rate).
This study was approved by the Ethics Committee of University of Occupational and Environmental Health, Japan (No. R3-076). Informed consent was obtained from all participants via the online form provided by the survey website.
Assessment of self-rated health
We asked the participants, “How do you rate your current health status?” to assess their self-rated health. Answers were selected from among the following six options: very good, good, slightly good, slightly poor, poor, or very poor. This method of questioning is used in the Longitudinal Survey of Middle-aged and Elderly Persons in Japan [22]. We used this method of questioning in both the baseline and follow-up surveys. We then classified those who responded, “slightly poor,” “poor,” or “very poor” as having poor self-rated health; those who responded, “very good,” “good,” and “slightly good” were classified as having good self-rated health. Self-rated health in the follow-up survey was used as the outcome, and self-rated health at baseline was adjusted as a covariate.
Assessment of work with night shifts
We asked the participants, “Do you have work with night shifts?” to assess whether they worked with night shifts. Responses were selected from among the following two options: “Yes” or “No.” Work with night shifts was defined as work performed between 10:00 p.m. and 5:00 a.m. at least four times per month on average. This was based on the definition of night work in Japanese law [23].
Covariates
The covariates comprised age, sex, annual household income, industry category, and self-rated health at the baseline. Age was expressed as a continuous variable. Sex was expressed as a categorical variable. We categorized annual household income into six groups: < 4.00 million Japanese yen (JPY), 4.00–5.99 million JPY, 6.00–7.99 million JPY, 8.00–9.99 million JPY, 10.00–11.99 million JPY, and ≥ 12.00 million JPY. Participants chose one of 20 options for the industry category, according to Japan’s standard industry classification [24]: agriculture and forestry; fisheries; mining and quarrying of stone; construction; manufacturing; electricity, gas, heat supply, and water; information and communications; transport and postal services; wholesale and retail trade; finance and insurance; real estate and goods, retail and leasing; scientific research, professional and technical services; accommodation, eating and drinking services; living-related and personal services and amusement services; education and learning support; medical, health care and welfare; compound services; services, not elsewhere classified (n.e.c.); government, except elsewhere classified; and industries unable to be classified. Self-rated health at the baseline was conducted in accordance with the method previously described.
Statistical analysis
Logistic regression analysis was used to examine associations between work with night shifts and self-rated health. We calculated the odds ratios and 95% confidence intervals (CIs) for self-rated health adjusting for covariates. We calculated the odds ratio adjusted for age, sex, annual household income, industry category and self-rated health at baseline. We considered P-values < 0.05 to indicate statistical significance. All analyses were conducted using Stata statistical software (Release 18 StataCorp LLC, College Station, TX, USA).
Table 1 shows the characteristics of participants divided into groups on the basis of whether they worked night shifts. Of the total of 16,629 participants, 2,505 were in the “work with night shifts” group, and 14,124 were in the “work without night shifts” group. The “work with night shifts” group was younger and contained a higher proportion of men compared with the “work without night shifts” group. The “work with night shifts” group contained a higher proportion of participants with poor self-rated health compared with the “work without night shifts” group.
|
Work with night shifts group |
Work without night shifts group |
||
|---|---|---|---|
| Total | 2,505 | 14,124 | |
| Age (mean [SD]) | 44.8 (12.5) | 48.2 (13.2) | |
| Sex | |||
| Male | 1,895 (75.6%) | 7,639 (54.1%) | |
| Female | 610 (24.4%) | 6,485 (45.9%) | |
| Annual household income (million Japanese Yen) | |||
| 0-3.99 | 595 (23.8%) | 3,621 (25.6%) | |
| 4.00-5.99 | 671 (26.8%) | 3,416 (24.2%) | |
| 6.00-7.99 | 504 (20.1%) | 2,784 (19.7%) | |
| 8.00-9.99 | 330 (13.2%) | 1,987 (14.1%) | |
| 10.00-11.99 | 158 (6.3%) | 924 (6.5%) | |
| 12.00- | 247 (9.9%) | 1,392 (9.9%) | |
| Self-rated health at baseline | |||
| Very good | 236 (9.4%) | 1,261 (8.9%) | |
| Good | 5,744 (29.7%) | 4,655 (33.0%) | |
| Slightly good | 978 (39.0%) | 5,586 (39.5%) | |
| Slightly poor | 416 (16.6%) | 2,091 (14.8%) | |
| Poor | 98 (3.9%) | 402 (2.8%) | |
| Very poor | 33 (1.3%) | 129 (0.9%) | |
SD: standard deviation, JPY: Japanese yen.
Table 2 shows the association between work with night shifts and self-rated health. Compared with the “work without night shifts” group, the odds ratio of the “work with night shifts” group was 1.19 (95% CI: 1.05–1.35, P = 0.007).
| OR | 95% CI | P-value | ||
|---|---|---|---|---|
|
Work without night shifts group |
reference | |||
|
Work with night shifts group |
1.19 | 1.05 | 1.35 | 0.007 |
OR: odds ratio, CI: confidence interval.
Adjusted for age, sex, household income, industry category and self-rated health at baseline.
Previous studies regarding the impact of work with night shifts on self-rated health have reported inconsistent findings [14–20], but the present study revealed a relationship between work with night shifts and self-rated health. Although some studies have indicated that workers with night shifts report lower self-rated health, others have found no relationship or even higher self-rated health among groups of workers with night shifts, reflecting inconsistencies that are potentially attributable to sample limitations. The present study targeted a wide variety of workers, increasing the generalizability of the results. Moreover, previous studies covering a wide variety of workers were cross-sectional studies [15, 19], whereas this study is a prospective cohort study, which yielded a new finding.
The mechanism underlying the relationship between work with night shifts and self-rated health has not been examined in detail in previous studies. Various factors can be considered as contributing to this relationship. It is possible that this relationship is affected by sleep problems and psychological distress. It has been reported that work with night shifts causes sleep problems [25] and psychological distress [26], which are health effects that have been shown to decrease workers’ self-rated health. A Japanese study conducted in 2014 found that workers experiencing less than 6 hours of sleep, difficulty falling asleep, difficulty staying asleep, early morning awakenings, difficulty waking, fatigue upon waking, and excessive sleepiness at work reported significantly worse self-rated health [27]. An earlier Japanese survey conducted in 2006 highlighted the potential influence of psychological factors such as perceived stress levels on self-rated health [28].
Additional factors other than sleep problems and psychological distress may also be influential. Work with night shifts has been shown to disrupt circadian rhythms and cause a variety of health problems, including increased risk of diseases such as cardiovascular disease, cancer and metabolic syndrome [29], which may lead to decreased self-rated health. Disruptions in circadian rhythms have also been reported to lead to the development of subjective symptoms such as fatigue [30] and pain [31]. The decline of self-rated health and the onset of subjective symptoms are known to decrease self-rated health [32]. Work with night shifts also can cause irregular lifestyles, leading to poor eating habits [33] and reduced opportunities for exercise [34]. Because healthy eating habits and regular exercise are known to enhance self-rated health [35], their reduction may decrease self-rated health. Furthermore, an individual’s belief in leading a regular life itself has been reported to be a factor that increases self-rated health [36], raising the possibility that work with night shifts may decrease self-rated health by making it difficult to maintain a regular lifestyle.
The current study suggests that self-rated health, the commonly used simple and general indicator of health effects, can be used for workers with night shifts. Self-rated health has the advantage of being relatively easy to measure. Many companies in Japan check daily health conditions before the start of work to prevent heat stroke in summer. Similarly, self-rated health can be used as an indicator by taking advantage of the simplicity of the measurement and using it as a health check before the night shift. It is also possible to record changes in health conditions through the results of daily measurements, capturing overall health status that cannot be expressed by objective indicators that are usually measured in health examinations. It would be meaningful to measure self-rated health daily and to use it for the early assessment of the health effects on workers with night shifts.
This study has several limitations. First, the survey was conducted using a web-based questionnaire distributed to a panel by an internet research firm. This could potentially introduce a bias toward workers who have access to the internet. However, to reduce the possibility of bias, sampling was balanced to include a representative population of workers in Japan in terms of age, sex, and region of residence, and to exclude fraudulent responses [15]. Second, because the survey only considered the presence or absence of night shift work as an explanatory variable, it was not possible to analyze the effect of frequency of night shift work. Third, the definition of night shift work encompasses a variety of different types of work, adding another layer of complexity to the analysis. These factors should be considered when interpreting the results of this study. Fourth, assessments of work with night shifts were made only during the baseline survey. The working conditions of night shifts may have changed between the baseline and follow-up surveys. Fifth, regarding the assessment of self-rated health, we asked ‘How do you rate your current health status?’. The responses may have depended on the condition at the time of answering, and one’s condition after night shift will be different from their condition on a holiday, so the timing of the survey may have affected the answers.
The results of our study indicate the existence of a relationship between work with night shifts and self-rated health. Moreover, our findings suggest that self-rated health, the commonly used simple and general indicator of health effects, can be used for workers with night shifts.
The current members of the W2S-Ohpm Study, in alphabetical order, are as follows: Akiko Matsuyama, Asumi Yama, Ayaka Yamamoto, Ayana Ogasawara, Hideki Fujiwara, Junta Naka, Juri Matsuoka, Kakeru Tsutsumi, Kazufumi Matsuyama, Kenta Moriya, Kiminori Odagami, Koji Mori, Kosuke Sakai, Madoka Miyashita, Mako Masuda, Masahiro Tanaka, Masako Nagata, Miho Omori, Mika Kawasumi, Misako Uetsuki, Mizuho Inagaki, Naoto Ito, Osamu Une, Rina Minohara, Shigeki Morioka, Shunusuke Inoue, Suo Taira, Takahiro Jinnouchi, Takahiro Mori, Tomohisa Nagata (present chairperson of the study group), Tomoko Sawajima, Yuki Hino, and Yuto Fukui. All members are affiliated with the University of Occupational and Environmental Health, Japan. We thank Benjamin Knight, MSc., from Edanz (https://jp.edanz.com/ac) for editing a draft of this manuscript. The content of this paper was presented at the 34th International Commission on Occupational Health. The abstract is published in Occupational Medicine (Yuki Hino, Tomohisa Nagata, Kiminori Odagami, Nuri Purwito Adi, Koji Mori (2024): O-168 RELATIONSHIP BETWEEN WORK WITH NIGHT SHIFTS AND SELF-RATED HEALTH. Volume 74, Issue Supplement_1).
TN reports personal fees from BackTech Inc., EWEL Inc. and Sompo Health Support Inc., outside the submitted work. KM reports research grants from DAIDO LIFE INSURANCE COMPANY and Komatsu Ltd., scholarship grants from AORC, BackTech Inc., DAIDO LIFE INSURANCE COMPANY, EWEL Inc., iSEQ.Inc., JMA Research Institute Inc., MEDIVA Inc., SMS Co., Ltd., Sompo Health Support Inc., and T-PEC CORPORATION, and personal fees from BackTech Inc. and Sompo Health Support Inc., outside the submitted work. The other authors declare no conflicts of interest associated with this manuscript.
