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Objectives: This study aimed to investigate whether occupational noise exposure is a risk factor for insomnia among male night-shift production workers. Methods: This study followed 623 male night-shift production workers at a tire manufacturing factory without insomnia for 4 years. Insomnia was evaluated based on the insomnia severity index at baseline and at 4-year follow-up. A score of ≥15 was defined as insomnia. The higher occupational noise exposure group was defined as those individuals exposed to 8-hour time-weighted-average noise above 80 dB (A). Results: Participants’ mean age was 46.3 ± 5.6 years. Of the 623 participants, 362 (58.1%) were in the higher occupational noise exposure group. At 4-year follow-up, insomnia occurred in 3.2% (n = 20) of the participants. In a multiple logistic regression analysis, the odds ratio of insomnia was 3.36 (95% confidence interval 1.083–10.405, P = 0.036) in the higher occupational noise exposure group when compared with the lower noise exposure group after adjusting for confounders. Conclusion: Our findings suggested that occupational noise exposure affected insomnia in male night-shift production workers. To prevent insomnia, efforts are required to reduce workplace noise exposure levels. Alternatively, moving to a less noisy work environment should be considered for workers with severe insomnia.
Keywords: insomnia, occupational noise, sleep problems, shift worker
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Insomnia is one of the most common sleep disorders worldwide and can cause a variety of health problems. According to a review article on insomnia, which was based on >50 worldwide studies, the prevalence of insomnia symptoms is approximately 33% in the general population.[1] Additionally, the prevalence of insomnia diagnoses, according to the Diagnostic and Statistical Manual of Mental Disorders, fourth edition (DSM-IV) classification, is 6%.[1] Studies of Korea have shown that 22.8% of 5,000 adults aged 20 to 69 years experienced insomnia symptoms in 2005; 5% of 3,719 adults aged ≥15 years had a diagnosis of DSM-IV insomnia in 2002.[2],[3] Adequate sleep is an important factor in maintaining an optimal quality of life. Poor quality of sleep or sleep disturbances can negatively affect quality of life and health.[4] Previous studies have suggested that insomnia is associated with mental health, diabetes, myocardial infarction, and heart failure.[5],[6],[7],[8],[9] Insomnia can also affect the occurrence of accidents[10],[11],[12]; insomnia or sleeping medication use increases the risk of road collisions by 1.2- and 1.5-fold, respectively.[11] Given the high prevalence of insomnia symptoms and insomnia, an increase in this risk could have a significant impact. A study of 8,625 community respondents in France has reported 8% of insomnia patients and 1% of non-insomniacs experienced an occupational accident in the past 12 months.[12] In addition, insomnia can negatively affect work productivity. Workers with sleep problems have decreased job performance and increased absenteeism when compared with workers without sleep problems.[13]
Noise is a major exposure risk factor for workers. In Korea, 16.0% of employees are exposed to occupational noise.[14] Noise exposure can cause many health problems, such as hearing impairments, hypertension, ischemic heart disease, annoyance, and injuries.[15],[16],[17],[18],[19],[20] Sleep disturbances can be caused by noise, especially from road traffic or environmental exposure.[21],[22],[23],[24] However, the link between occupational noise exposure and sleep disturbance is not fully understood. Workers in manufacturing factories are exposed to high levels of occupational noise that can affect sleep. Additionally, shift workers can be vulnerable to sleep problems; therefore, it is necessary to identify and manage sleep problems. This prospective 4-year follow-up study aimed to investigate whether occupational noise exposure is a risk factor for insomnia among male workers at a tire-manufacturing factory.
MethodsStudy participants
This prospective cohort study was conducted from 2014 to 2018 among 810 shift workers aged 25 to 60 years from a tire-manufacturing factory. The production process in this factory runs 24 hours a day. All participants performed the same type of shift (four-teams, 8 hours rotating shift). Individuals who were female (n = 4); with a history of sleep disorders (n = 18); with an insomnia severity index (ISI) score of ≥15 at baseline (n = 63); or with a work department change or retirement during the study period (n = 102) were excluded. Finally, 623 male workers were included [Figure 1]. The present study was conducted in accordance with the ethical guidelines of the Declaration of Helsinki and was approved by the Institutional Review Board of Chonnam National University Hwasun Hospital (IRB number CNUHH-2014-123 & 2016-150). Participants were informed of the method and purpose of this study. In addition, each participant provided written, informed consent before participating.
Data collection
Participants were interviewed by trained physicians using a structured questionnaire. We investigated the following parameters: age, working period, smoking status, alcohol drinking, physical activity, medical history, and use of hearing protection devices during work. The participants were divided into the “no or seldom” and “always” groups, depending on whether they used hearing protection devices. Participants who smoked at the time of study inclusion were defined as current smokers, those who had stopped smoking at the time of study inclusion were defined as ex-smokers, and those who had never smoked were defined as non-smokers. Participants who consumed alcohol more than once per week were included in the alcohol drinking group. Participants who exercised for ≥30 minutes at least twice a week were classified in the physical activity group. The height, weight, neck circumference, and blood pressure of all participants were measured. Neck circumference was measured only at follow-up to assess obstructive sleep apnea from the level just below the laryngeal prominence perpendicular to the long axis of the neck. The body mass index (BMI) was calculated by dividing the weight (kg) by the squared height (m2). Blood pressure was measured on the right arm in the sitting position using a digital blood pressure monitor under stable conditions. After 12 hours of fasting, blood samples from all participants were collected via venipuncture; total cholesterol, high-density lipoprotein (HDL) cholesterol, triglyceride, and fasting glucose levels were measured. Low-density cholesterol (LDL) was determined using the Friedewald formula (LDL = total cholesterol − HDL − [triglyceride/5], applied only at triglyceride level <350 mg/dL). Hypertension was defined as systolic blood pressure ≥140 mmHg, diastolic blood pressure ≥90 mmHg, or taking antihypertensive medications. Diabetes mellitus was defined as a fasting glucose level ≥126 mg/dL or taking antidiabetic medications. Dyslipidemia were defined as a total cholesterol level ≥240 mg/dL, LDL cholesterol level ≥160 mg/dL, triglyceride level ≥500 mg/dL, or receiving treatment for dyslipidemia.
Occupational noise exposure
According to the Occupational Safety and Health Act in Korea, measurements of the work environment must be performed every 6 months at workplaces with an 8-hour time-weighted average (TWA) exceeding 80 dB (A). The Occupational Safety and Health Research Institute (OSHRI) of the Korea Occupational Safety and Health Agency (KOSHA) classifies and manages workplaces with a noise exposure level of ≥80 dB (A) in 8-hour TWA as noise exposure workplaces. We reviewed the work environment noise data measured in 2014 provided by the company. The noise in the working environment was measured in 130-unit processes of the company. The mean noise level in the department to which the workers were assigned was regarded as the noise level of the worker.[25] Participants who worked in a workplace with noise levels ≥80 and <80 dB (A) were categorized in the higher and lower noise exposure groups, respectively.
Insomnia
Insomnia was evaluated using the ISI. This is a brief and validating instrument designed to assess the severity of insomnia.[26],[27] The Korean version of the ISI has also been proven to be a reliable and valid instrument for assessing the severity of insomnia in Korean populations.[28] It consists of seven items: (1) severity of sleep onset; (2) sleep maintenance; (3) early morning awakening problems; (4) sleep dissatisfaction; (5) interference of sleep difficulties with daytime functioning; (6) noticeability of sleep problems by others; and (7) distress caused by the sleep difficulties. A 5-point Likert scale is used to rate each item (0 = no problem, to 4 = very severe problem) and a total score ranging from 0 to 28. The total score is interpreted as follows: absence of insomnia (0–7); sub-threshold insomnia (8–14); moderate insomnia (15–21); and severe insomnia (22–28).[26] The presence of insomnia was defined as an ISI score of ≥15 in this study.
Excessive daytime sleepiness
The Epworth sleepiness scale (ESS) is a screening tool developed to assess daytime sleepiness in adults. It consists of eight questions that evaluate their likelihood of dozing off or falling asleep in eight situations. Each question is scored from 0 to 3, with a total score of 0 to 24; a score of ≥11 indicated abnormal daytime sleepiness.[29] The presence of excessive daytime sleepiness (EDS) was defined as an ESS score of ≥11.
STOP-Bang questionnaire
The STOP-Bang questionnaire is a screening tool developed for preoperative evaluation to prevent complications due to undiagnosed obstructive sleep apnea (OSA) in adult patients undergoing surgery.[30] This questionnaire consists of eight questions related to the clinical features of OSA, including snoring, fatigue, observed apnea, hypertension, BMI (≥35 kg/m2), age (≥50 years), neck circumference (≥43 cm), and male sex. Each question is answered with “Yes” or “No”; 1 point is given to any “Yes” response. Thus, the total score ranged from 0 to 8. According to the STOP-Bang questionnaire, scores of 0 to 2, 3 to 4, and 5 to 8 are classified as low, intermediate, and high risk for OSA, respectively.[30] A patient with a score of ≥5 has a high probability of moderate-to-severe OSA.[31] The presence of OSA was defined as a STOP-Bang score of ≥5.
Statistical analyses
We compared the differences in each variable according to the level of occupational noise exposure at baseline. Student’s t-tests and Pearson χ2 tests were used to examine the continuous and categorical variables, respectively. At the 4-year follow-up, we compared any significant differences in sleep problems according to the level of occupational noise exposure. To assess the effect of occupational noise exposure on changes in ISI scores at 4-year follow-up, we performed a repeated measures analysis of covariance (ANCOVA), controlling for age. Multiple logistic regression analysis was used to examine the relationship between sleep problems and occupational noise exposure in three different models. The models were as follows: (1) adjusted for age; (2) adjusted for age, BMI, hypertension, dyslipidemia, and diabetes mellitus; and (3) adjusted for age, BMI, hypertension, dyslipidemia, diabetes mellitus, smoking, alcohol drinking, physical activity, and hearing protection device use. Continuous and categorical variables are presented as mean ± standard deviation (SD) and numbers (%), respectively. Results are shown as odds ratios (ORs) with 95% confidence intervals (CIs). Statistical significance was set at P < 0.05. SPSS version 27.0 (IBM Corp., Armonk, NY, USA) was used for all statistical tests.
ResultsAll study participants were men. The mean age was 46.3 ± 5.6 years. The general characteristics at baseline of the participants were compared according to occupational noise exposure level. Of the total 623 participants, 362 (58.1%) and 261 (41.9%) were categorized into the higher and lower noise exposure groups, respectively. The age, working period, and workplace noise level were significantly higher in the higher noise exposure group. There were no significant differences in the physical examination results, blood test results, life-related factors, presence of hypertension, dyslipidemia, diabetes mellitus, ISI score, and proportion of hearing protection device use between groups [Table 1].
Table 1 General characteristics of the subjects according to the level of occupational noise exposure at baseline.The prevalence of sleep problems was compared according to the level of occupational noise exposure at the 4-year follow-up. The ISI score was significantly higher in the higher noise exposure group than in the lower noise exposure group (5.8 ± 4.4 vs. 5.0 ± 4.2, P = 0.029). In addition, more participants with insomnia were distributed in the higher noise exposure group than in the lower noise exposure group (4.4% vs. 1.5%, P = 0.044). ESS and STOP-Bang scores were higher in the higher noise exposure group; however, this did not reach significance. There were more participants with EDS and OSA in the higher noise exposure group; however, this also did not reach significance. Regardless of the noise exposure level, the ISI score decreased at follow-up when compared with baseline. However, at 4 years of follow-up, the change in ISI score was significantly more decreased in the group exposed to low occupational noise levels [Table 2].
Table 2 Prevalence of sleep disorders according to the level of occupational noise exposure at 4-year follow-up.In the multiple logistic regression analysis, the OR of higher occupational noise exposure for Insomnia was 3.08 (95% CI: 1.013–9.384, P = 0.047) after adjustment for age. After additional adjustment for BMI, hypertension, dyslipidemia, diabetes mellitus, smoking, alcohol drinking, physical activity, and hearing protection device use, the OR was 3.35 (95% CI: 1.081–10.390, P = 0.036) [Table 3].
Table 3 Odds ratios for insomnia (insomnia severity index score ≥15) in association with higher noise exposure at workplace in separate adjusted models. Discussion In this 4-year follow-up study, we assessed the effect of occupational noise exposure on sleep problems among male night-shift production workers. This study found that the prevalence of insomnia at 4-year follow-up was higher in the group exposed to high occupational noise. Moreover, higher levels of occupational noise exposure increased the risk of insomnia after adjusting for covariates (OR 3.36; 95% CI: 1.083–10.405, P = 0.036). Although not statistically significant, the prevalence of EDS and OSA was higher in the group exposed to high occupational noise. Our results suggested that daytime workplace noise exposure may be a risk factor for sleep problems, especially insomnia.
Previous studies assessing the relationship between occupational noise exposure and sleep disturbance have been insufficient. Sleep problems are a health risk and a major concern at the workplace. Previous studies have suggested that insomnia is a determinant of subsequent work disability and delayed return to work.[32],[33],[34],[35],[36],[37],[38] Although rare, recent studies on the relationship between occupational noise exposure and sleep disorders have suggested that occupational noise exposure may decrease sleep quality or increase the risk of insomnia.[39],[40] However, these studies were cross-sectional and had some limitations. For example, 30,827 male and female Korean workers have shown a dose–response relationship for noise exposure to a certain extent.[39] However, the level of exposure to noise was evaluated using a single question without objective noise evaluation. Additionally, reliable measures of assessment of sleep problems were not performed. In a Taiwanese study of 40 workers, participants with higher occupational noise exposure during the day were found to have a lower percentage of slow wave sleep and lower sleep efficiency.[40] However, this was a quasi-experimental study design; it has the limitations of a small number of participants and an insufficient exposure period considering the chronic effect of noise.
Noise is one of the most common stressors. It can increase the secretion of stress hormones, including corticosteroids, and act on the autonomic nerve system. This results in changes in autonomic cardiac and vascular regulation.[41],[42] Noise can have acute and chronic effects on the quality or pattern of sleep, leading to sleep disturbances, and ultimately negative health effects.[43],[44] Meaningful noise events during the night repeatedly induce cerebral and autonomic arousals. These immediate physiological changes interfere with normal sleep; they can impair sleep quality and recovery by altering sleep structure, including decreased sleep persistence, delayed sleep onset and early awakening, and decreased deep sleep and rapid eye movement (REM) sleep.[45],[46],[47] Sleep disturbances increase blood pressure or heart rate, cause vasoconstriction or arrhythmia, and adversely affect the cardiovascular system.[23],[48] This can lead to decreased sleep satisfaction, increased fatigue, poor mental health, and decreased productivity, even after waking up the next day.[49] The relationship between exposure to daytime noise and nighttime sleep quality, or the explanation for the delayed effects of noise on sleep remains unclear. One potential mechanism is the prolonged effects of parasympathetic withdrawal and sympathetic elevation by loud and chronic noise.[40] Another potential mechanism is the disruption of hormone balance, such as cortisol or growth hormone, which can be caused by repeated arousals during sleep, long-duration daytime exercise, or daytime noise exposure.[40],[50],[51]
The relationship between exposure to environmental noise, such as traffic noise, and sleep disturbance is well known. However, other noise sources (hospital noise or wind turbine noise) show conflicting results, although no dose–response relationship was estimated.[21] This inconsistency may be due to the complex and unclear dose–effect relationship of noise exposure on sleep because of multiple factors, such as depth of sleep phase, background noise level, and individual noise sensitivity.[47],[52],[53] Additionally, one study has examined the link between daytime environmental noise and sleep problems using noise mapping; however, no association was found.[54] This may be due to the difficulty of accurately estimating individual noise exposure levels using noise mapping.[55] Further, the association between occupational noise exposure and sleep disorder has not been fully explained but previous studies have suggested that daytime noise exposure may have a delayed effect on sleep.[39],[40] However, no study has confirmed this effect in shift workers.
Shift work is a risk factor for sleep disorders and cardiovascular disease (CVD).[56],[57],[58] In a recent systematic review and meta-analysis that included 21 studies and a total of 173,010 unique participants, the risk of CVD events and morbidity was 17% and 26% higher in shift workers than day workers, respectively.[56] The high prevalence of insomnia in shift workers has been reported in previous studies.[57] Additionally, shift workers have a higher risk of incident and fatal CVD, partly mediated through modifiable risk factors, such as sleep duration and quality.[58] Therefore, it is important to find and manage these controllable risk factors for sleep problems in shift workers, such as exposure to occupational noise.
Limitations
This study has some limitations. First, a study of workers in one workplace cannot be representative of the general population. Additionally, the association between noise exposure and insomnia in females was not studied because only male workers were included. Second, confounding factors, such as environmental noise exposure and personal stress levels; mental health conditions like anxiety or depression; and caffeine intake, were not fully considered. Third, the average working period of the participants was approximately 20 years; therefore, it was impossible to obtain a cumulative approach to the noise exposed before the study period. Further, it was not possible to determine dose–response relationships including noise exposure levels and work department prior to the study period. Fourth, there may be a healthy worker effect. Fifth, after a 4-year follow-up, it was confirmed that the ISI score decreased overall, indicating the possibility of response bias. In Korea, workers are given special health checkups every year for items determined by the working environment or conditions according to the Occupational Safety and Health Act.[25] The participants of this study, as night-shift workers, were evaluated annually for sleep problems using the ISI. In addition, if there is a sleep problem, additional counseling or an evaluation of whether to continue to perform shift work was conducted. Therefore, workers may have answered that they do not have any particular sleep problems. Nevertheless, the higher noise exposure group showed a lower reduction in ISI scores after 4 years when compared with the lower noise exposure group. Finally, sleep problems were assessed using a subjective sleep questionnaire rather than the gold standard, such as polysomnography. However, ISI and ESS have sufficient reliability and validity to be used for assessing sleep disorders in Korean shift workers.[59] The probability of moderate-to-severe OSA increases as the STOP-Bang score increases.[60],[61] In this study, we sought to properly evaluate OSA based on a STOP-Bang score of ≥5.
Strengths
First, this study is a 4-years follow-up survey of 623 workers without sleep disorders. The causal relationship of sleep disorders caused by noise can be partially explained. Second, the participants share the same working environment in one factory and have a similar socioeconomic status. Third, the noise exposure level was objectively confirmed in the participant’s workplace. Changes in departments and tasks were considered during the study period; therefore, the occupational noise exposure was appropriately estimated. This study investigated the effects of occupational noise exposure on sleep in male night-shift workers. The results of this study are meaningful in that shift workers are more vulnerable to sleep disorders. Male workers are more exposed to physical hazards, including noise, and chemical substances than female workers.[62] In addition, many studies have been conducted on sleep disorders caused by exposure to environmental noise; however, few studies have assessed the effects of occupational noise. The results of this study suggest that occupational noise exposure may increase the risk of insomnia.
ConclusionIn this study, occupational noise exposure affected insomnia in night-shift production workers. Sleep disorders or poor sleep quality negatively affect the health, quality of life, and performance of the workers. Additionally, night-shift workers are more prone to sleep problems; therefore, these adverse effects may be greater. Protecting workers from noise by lowering workplace noise levels may help prevent sleep problems for workers exposed to occupational noise. Alternatively, workers with severe insomnia should be considered for a move out of noisy departments. Future research is required to observe whether the insomnia pattern is improved after the workplace noise environment is improved.
Authors’ contributions
W-J Park and S. Cho designed this report, prepared the draft of this manuscript. WY Kang and C. Lee collected the data. D-Y Lim and S. Kim interpreted the data. H. Kim helped the clinical diagnosis and data analysis. Prof. W-J Park critically reviewed this manuscript. All authors read and approved the final manuscript.
Acknowledgments
The authors thank all the members of the Health Promotion Center at Chonnam National University Hwasun Hospital. They thank all the members of the Department of Occupational and Environmental Medicine at Chonnam National University Hospital. They also thank the patients for their voluntary participation.
Ethics statement
The study protocol was approved in advance by the Institutional Review Board of Chonnam National University Hwasun Hospital (IRB number CNUHH-2014-123 & 2016-150). In addition, each subject provided written, informed consent before participating.
Financial support and sponsorship
Nil.
Conflicts of interest
There are no conflicts of interest.
References
Correspondence Address:
Won-Ju Park
Department of Occupational and Environmental Medicine, Chonnam National University Medical School and Chonnam National University Hwasun Hospital, 322 Seoyang-ro, Hwa
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