Shift work in Korea is most prevalent in automobile system manufacturing industries. The subjective sleep complaints and individual chronotypes of shift worker was evaluated.
A cross-sectional study was conducted among single automobile system manufacturing industries in Korea. All participants completed questionnaires about the work schedule, duration of sustained employment and various division of work. And sleep questionnaires including: Epworth Sleepiness Scale, Insomnia Severity Index, Hospital Anxiety and Depression Scale, and Morningness-Eveningness Questionnaire.
This study enrolled 208 participants [shift workers (SWs)=142, day workers (DWs)=66]. Insomnia symptoms were significantly worse in the SW group (
Insomnia symptoms were significant in SWs compare to DWs. In DWs the evening types were sleepier and had more insomnia symptoms than other chronotypes. The sleep symptoms are prevalent in automobile industries workers, and in part it is related to work schedule and individual chronotype.
The globalization of society requires industries to optimize productivity. Shift work allows 24-h operation for effective use of available natural resources and manpower in terms of cost-effective production and round-the-clock services [
Shift workers are vulnerable to circadian misalignment by maintaining alertness during the night and trying to sleep during the day. Circadian rhythm synchronization is essential for optimal health [
Each individual has variable endogenous circadian rhythms. These inter-individual differences are often divided into two groups: morning types, and evening types. Morning types prefer rising early and achieving maximum mental and physical activity in the morning. In contrast, evening types prefer later-than-average bed and rise times and function at their peak later in the day than do morning types [
This study examined shift workers in automobile system manufacturing industries, particularly the YS Enterprise Corporation. The company operates 24 hours a day. This automobile industry would be representative sample of the most prevalent shift work force in Korea. In this study, we evaluated sleep disturbance including EDS, insomnia, depressive mood and anxiety, and individual chronotypes according to shift schedules.
A manufacturing company with both shift and day work schedules was selected for this questionnaire-based cross-sectional study. The questionnaires were distributed to the YS Enterprise Corporation, which is located in Chungnam, Korea, in 2015. This company manufactures parts for automobile systems, which employs 728 workers. The work schedule includes day work, two-shift work, three-shift work, and variable shift work. In this study, the shift workers (SWs) group included only two-shift schedule shift workers, which shift work starts either at 8 a.m. or 8 p.m. The day workers (DWs) group included workers with regular working hours from 9 a.m. to 5 p.m.
Each individual was evaluated using a questionnaire that assessed the employees’ work schedule, duration of sustained employment and various division of work. Any sleep disturbance was also evaluated with standardized questionnaires. Participants were assessed for EDS using the Epworth Sleepiness Scale (ESS), for insomnia symptoms using the Insomnia Severity Index (ISI), and for depression and anxiety with the Hospital Anxiety and Depression Scale (HADS). Individual chronotypes were further assessed with the Morningness-Eveningness Questionnaire (MEQ).
The ESS measures the impact of subjective daytime sleepiness [
The ISI questionnaire was used to screen symptoms of insomnia. A Korean version of the 7-item subjectively reported ISI that assessed the severity of insomnia symptoms was used. The ISI queried participant about the sleep problems in the past 2 weeks. Participants graded each question on a scale between minimal (0 points) to very severe (4 points). The final score ranged from 0–28 points, with higher numbers indicating the severity of insomnia. An ISI score ≥15 was considered as clinically significant insomnia [
The HADS is used to determine the levels of anxiety and depression that a patient is currently experiencing. The HADS is a 14-item scale that generates ordinal data and is divided into seven items related to anxiety (HADS-A) and seven items related to depression (HADS-D). Each item on the questionnaire is scored from 0–3, which means that a person could score from 0–21 for either anxiety or depression[
The MEQ is aimed at determining when the respondent’s natural propensity to be active lies during the daily temporal span, as well as one’s sleep and wake behavior and schedules. The Korean version of the MEQ consists of 19 items pertaining to habitual rising and bed times, preferred times of physical and mental performance and subjective alertness after rising and before going to bed [
Statistical analyses were conducted using the Statistical Package for the Social Sciences (SPSS) for Windows, Version 17.0 (SPSS Inc., Chicago, IL, USA). The alpha level for statistical significance was accepted for two-tailed p values <0.05. Continuous data are expressed as mean±standard deviation and categorical data as frequencies and percentages. All continuous variables were analyzed using the Mann-Whitney U test, and Kruskal-Wallis test due to an uneven distribution, and categorical variables were evaluated using either chi-square tests or Fisher’s exact test.
In all, 217 participants completed the questionnaire, and 9 participants were excluded (one with missing data on work schedule status, seven with an irregular shift work and daytime work schedule, and one with 3-shift system). The study finally enrolled 208 participants [SWs:DWs=142 (68.3%):66 (31.7%)] (
Insomnia as a sleep symptom was significantly worse in the SW group (ISI: SWs 16.05±5.98, DWs 12.41±6.29,
Individual chronotypes were evaluated using the MEQ (
The sleep disturbances according to chronotypes in SWs and DWs were evaluated (
We found significant sleep disturbances in terms of insomnia and possibly for EDS in two-shift schedule in single automobile enterprise in Korea. Previously, sleep disturbances were reported more than 50% of SWs, compared to only 5–20% of DWs [
Shift workers have been reported to be sleepier compared to day workers [
An individual chronotype is defined as the phase-relationship between one’s sleep-wake cycle and the formal clock time [
Regarding mood symptoms, we expected that SW would result in increased anxiety and depression. This was not supported by our data, as there was no remarkable increase in mood scale by HADS in both SWs and DWs and no significant group differences. Previous studies have investigated the significant relationship between depression, anxiety and SW in particular [
There were several limitations to this study. Our study was based on questionnaires. To assess independent effects of shift work, underlying sleep disorders, underlying medical and psychological disorders, use of sleep related medications or substance, other social and economic factors should be controlled. Especially, the presence of other underlying sleep disorders such as; obstructive sleep apnea syndrome, restless legs syndrome, and other sleep disorders may present symptoms of insomnia and daytime sleepiness. Without these assessment, we cannot clearly state that the results of our study represents independent shift work assessment. Further assessments for shift rotation schedule, rotation speed and direction have not been included. Further on use of shift work non-specific chronotype questionnaire, which MCTQShift have not been validated in Korea, may have affected the results. And small sample size has limited evaluation regarding specific work task related factors. However, this study largely overviews the sleep symptoms and chronotype in two-shift workers compared to day workers in the automobile industry, the most prevalent shift work force in Korea.
This research was supported by Basic Science Research Program through the National Research Foundation of Korea funded by the Ministry of Science, ICT & Future Planning, Republic of Korea (No. 2014 R1A1A3049510) and by Samsung Biomedical Research Institute grant (#OTX0002111).
Excessive daytime sleepiness and insomnia symptom severity. (A) ESS is clinically significant in both SWs and DWs (ESS in SWs and DWs). (B) Insomnia symptoms were more common in SWs (ISI in SWs and DWs). *
Chronotype and sleep symptoms in SWs and DWs. ESS and ISI according to chronotype (A: SWs, B: DWs). *
Demographic findings and sleep questionnaires of shift workers and day workers
Shift workers (n=142) | Day workers (n=66) | ||
---|---|---|---|
Age, years | 42.44±7.15 | 43.09±5.77 | 0.168 |
Sex-male, n (%) | 142 (100) | 55 (83.3) | <0.001 |
Sustained employment, years | 17.57±5.90 | 17.37±6.68 | 0.552 |
Division of work, n (%) | |||
Manufacture | 86 (60.6) | 12 (18.2) | |
Casting | 47 (33.1) | 18 (27.3) | |
Inspection | 1 (0.7) | 14 (21.2) | |
Maintenance | 1 (0.7) | 8 (12.1) | |
Polishing | 3 (2.1) | 5 (7.6) | |
Others | 4 (1.9) | 9 (4.3) | |
ESS | 13.70±4.97 | 12.59±4.74 | 0.124 |
ESS>10, n (%) | 106 (75.7) | 42 (63.6) | 0.052 |
ISI | 16.05±5.98 | 12.41±6.29 | <0.001 |
ISI>14, n (%) | 85 (60.3) | 26 (40.0) | 0.005 |
HADS-A | 9.88±4.52 | 10.20±4.69 | 0.485 |
HADS-D | 11.64±3.65 | 11.09±3.96 | 0.484 |
ESS: Epworth Sleepiness Scale, ISI: Insomnia Severity Index, HADS: Hospital Anxiety and Depression Scale, A: anxiety, D: depression
Shift workers and day workers characteristic by individual chronotype
Morning type | Intermediate | Evening type | ||
---|---|---|---|---|
Total | ||||
n (%) | 24 (11.5) | 159 (76.4) | 25 (12.0) | |
Age, years | 44.31±4.33 | 43.04±6.59 | 40.16±4.54 | 0.007 |
Sustained empolyment, years | 18.55±4.41 | 17.79±6.55 | 14.35±3.91 | 0.012 |
ESS, mean±SD | 11.66±5.58 | 13.15±4.66 | 16.16±4.90 | 0.008 |
ISI, mean±SD | 13.04±7.15 | 14.68±6.14 | 18.12±5.45 | 0.034 |
Shift workers | ||||
n (%) | 15 (10.6) | 110 (77.4) | 17 (12.0) | |
Age, years | 46.25±3.59 | 43.04±6.04 | 40.76±4.27 | 0.003 |
Sustained work, years | 20.13±3.97 | 17.69±6.17 | 14.52±4.15 | 0.009 |
ESS, mean±SD | 12.33±6.16 | 13.51±4.60 | 16.11±5.62 | 0.170 |
ESS>10, n (%) | 9 (60) | 82 (75.9) | 15 (88.2) | 0.177 |
ISI, mean±SD | 15.93±5.50 | 15.61±5.9 | 19.00±6.16 | 0.203 |
ISI>14, n (%) | 9 (60) | 63 (57.8) | 13 (76.5) | 0.343 |
HADS-A, mean±SD | 9.60±5.22 | 9.68±4.3 | 11.41±5.26 | 0.445 |
HADS-D, mean±SD | 10.86±4.29 | 11.54±3.55 | 12.94±3.59 | 0.302 |
Day workers | ||||
n (%) | 9 (13.6) | 49 (74.2) | 8 (12.1) | |
Age, years | 41.83±6.08 | 43.15±7.54 | 38.90±5.12 | 0.392 |
Sustained empolymnet, years | 16.02±4.87 | 18.22±7.22 | 13.97±3.57 | 0.381 |
ESS, mean±SD | 10.55±4.58 | 12.36±4.73 | 16.25±3.19 | 0.034 |
ESS>10, n (%) | 3 (25) | 31 (63.3) | 8 (100) | 0.017 |
ISI, mean±SD | 8.22±7.22 | 12.58±6.14 | 16.35±3.05 | 0.030 |
ISI>14, n (%) | 2 (22.2) | 18 (37.5) | 6 (75.0) | 0.067 |
HADS-A, mean±SD | 8.37±5.70 | 10.30±4.52 | 11.37±4.80 | 0.535 |
HADS-D, mean±SD | 10.25±5.23 | 10.89±3.71 | 13.12±4.01 | 0.298 |
ESS: Epworth Sleepiness Scale, ISI: Insomnia Severity Index, HADS: Hospital Anxiety and Depression Scale, A: anxiety, D: depression