ORIGINAL ARTICLE
Combined Effects of Shiftwork and Individual Working Time Control on Long-Term Sickness Absence A Prospective Study of Finnish Employees Jouko N¨atti, PhD, Tomi Oinas, PhD, Mikko H¨arm¨a, PhD, Timo Anttila, PhD, and Irja Kandolin, PhD
Objective: To investigate whether the effects of shiftwork on long-term sickness absence vary according to the level of individual working time control (WTC). Methods: A representative sample of Finnish employees (1447 men and 1624 women) was combined with a register-based follow-up. A negative binomial model was used in the analysis of long-term sickness absence days. The results were adjusted for various background and work-related factors. Results: Individual WTC decreased long-term sickness absence. The higher rate of sickness absences in shiftwork was mainly due to the lower level of WTC. Working time control decreased sickness absence equally in day work and shiftwork. Conclusions: The negative health effects of shiftwork may be decreased by offering sufficient WTC. Establishments that use WTC as a human resource instrument may benefit from reduced absenteeism.
E
very fifth employee in Europe works shifts.1 Shiftwork and overtime work are often seen as forms of company-controlled flexibility, which aim to respond to fluctuations in demand and services. Nevertheless, shiftwork is a potential risk factor for employees’ health.2–5 Shiftwork is associated with fatigue, insomnia, and accidents, but is also probably related to a wide range of chronic health problems such as coronary heart disease,6 obesity and type 2 diabetes,7 and breast cancer.8 Evidence of the relationship between shiftwork and absenteeism is still inconclusive. Some studies have found higher sickness absence or longer absence spells among shiftworkers,9,10 whereas several studies are inconclusive or have found even less sickness absence among shiftworkers than among day workers.11–13 Working time control (WTC) is defined as the possibility to meet the needs of employees, providing them with autonomy regarding factors in the start and end times of their shifts, breaks, days off, holidays, and the total number of work hours.14 Working time control is often associated with the better mental and social well-being of employees, especially in situations where they have to be flexible for the needs of their employer or company.15,16 On the basis of a recent Cochrane review,17 the evidence from 10 different studies tentatively suggests that flexible working time interventions that increase worker control and choice are likely to have a positive effect on health. Nijp et al18 have presented two separate mechanisms that potentially explain WTC’s favorable associations with well-being. The first relates to the time regulation potential of WTC because control over working time helps people align their work commitments with their private life. The second mechanism is based on recovery regulation. Working time control can prevent work From the School of Social Sciences and Humanities (Dr N¨atti), University of Tampere; Department of Social Sciences and Philosophy (Drs Oinas and Anttila), University of Jyv¨askyl¨a; and Finnish Institute of Occupational Health (Dr H¨arm¨a, Dr Kandolin), Helsinki, Finland. This study was supported by the Academy of Finland (project 124456). The authors declare no conflicts of interest. Address correspondence to: Jouko N¨atti, PhD, School of Social Sciences and Humanities, University of Tampere, Linna 33014, Finland ([email protected]). C 2014 by American College of Occupational and Environmental Copyright Medicine DOI: 10.1097/JOM.0000000000000176
732
overload and sustain a favorable effort–recovery balance. Working time control facilitates both on the job recovery (breaks during the day) and external recovery (in-between working periods). There are several possible reasons why the possible positive effects of WTC on health could be most useful among shiftworkers and workers with irregular working hours. First, shifts rosters are often planned at short notice, decreasing the predictability of free time, although this depends on the type of shift schedule. Second, the work shifts of 24/7 work places such as hospitals may not suit the scheduling of family and social activities. Partly related to the decreased predictability and increased variability in shift timing, shiftwork is also related to less overall job control19 and lack of control over working hours20 as well as a higher likelihood of conflict between the demands of work and the needs of the family.21 Because of the irregularity of and disturbances to sleep and wakefulness, shiftwork is also related to a higher need for recovery after work.21,22 The social and health benefits of individual work time control may stem from advantages in combining shiftwork and nonwork demands and thereby improving the possibilities for mental recovery and stress reduction.16,23,24 A large amount of evidence and several theoretical work stress models indicate that decreased job control and insufficient recovery are among the main mechanisms linking stressful work to adverse health.25,26 Improved mental health and recovery, including better sleep, may be ways for psychophysiological unwinding and the prevention of chronic health conditions after irregular or abnormal working hours.26 Both low WTC and sleep disturbances predict work disability in the long run-–the work disability usually arises from mental and musculoskeletal problems.20,27 For musculoskeletal disorders, the opportunity to have additional breaks or days off in situations of increasing pain due to an acute or chronic disorder may be critical for the prevention and management of musculoskeletal problems. The positive influence of individual WTC on psychosocial and health outcomes may also be indirect. Hughes and Parkes28 found that WTC was not only directly related to reduced work– family interference, but high control also buffered the negative effect of longer hours on work–family relations. Similarly, Geurts et al29 found that workers who could take days off and vacations according to their own needs (leave control) were less likely to be required to neglect family activities because of work duties. In addition, control regarding starting and finishing times (flexitime) buffered the adverse impact of long contractual hours on work–family relations. The significance of WTC for the chronic health effects of shiftworkers is unclear. There is some evidence for the moderating role of WTC in the relationship between long work hours and health outcomes. Nabe-Nielsen et al30 found that moderate or low WTC among shiftworkers was associated with lower vitality, worse mental health, and more somatic stress symptoms. The work–time influence did not have a similar effect among day workers. Olsen and Dahl31 found that flexibility in the work schedule when working regular hours had no effect on sickness absence or on the work–family balance, but the situation was different among those with irregular hours. Men who worked irregular hours and had no control over when to work had a significantly higher sickness absence than other men. JOEM r Volume 56, Number 7, July 2014
Copyright © 2014 Lippincott Williams & Wilkins. Unauthorized reproduction of this article is prohibited.
JOEM r Volume 56, Number 7, July 2014
In addition, working irregular hours, particularly in cases without flexibility, resulted in adverse effects on the balance between work and family, for both women and men. The role of WTC for the health and sickness absence of shiftworkers may also be related to sex. Women clearly report higher self-certified as well as medically confirmed spells for sickness absence and adjusting for, for example, psychosocial working conditions. Family-related factors or the sex composition of a workplace does not explain the differences in long-term sickness absence.32 The longer sickness absence spells among women are mainly explained by the heavier burden of ill-health and, to a lesser extent, by higher physical work demands.32 Furthermore, women do more housework, even in countries like Finland where sex roles are not so firmly defined.33 Thus, increased WTC may be more important for women, if they are to avoid work–family conflicts because of shiftwork. To study the possible combined effects of shiftwork and WTC on chronic health problems, one aim of this study is to investigate whether shiftwork more strongly predicts long-term sickness absence among male and female employees with low WTC than among with those with high WTC.
STUDY POPULATION AND METHODS Sample and Participants The data consist of the Finnish Quality of Work Life Survey (FQWLS) from 2003 linked to a register follow-up on long-term sickness absence over 5 years. The sample is based on 5270 employees, of whom 4104 participated in face-to-face interviews in 2003, the response rate being 78%. The FQWLS is a representative sample of Finnish employees covering all sectors and occupations. The sample in 2003 was based on respondents in the October and November Labour Force Survey who were 15 to 64 years old, with a normal weekly working time of at least 5 hours. According to Statistics Finland, nonresponse does not seriously undermine the representativeness of FQWLS data.33 This study was restricted to 15- to 64-year-old employees, who worked day shifts or had shiftwork (1447 men and 1624 women), and who had worked in their current job at least 1 year. Statistics Finland conducted the survey on a face-to-face interview basis using a standard questionnaire. The sample was gathered from those employed persons who participated in the regular monthly labor force surveys for the first time.33 The questionnaire comprised a comprehensive set of questions dealing with various features of work life.
Register-Based Follow-Up Data The survey in 2003 was cross-sectional. To study long-term sickness absence in relation to shiftwork and WTC, panel data were required. Therefore, the FQWLS was merged with register-based follow-up data held and maintained by Statistics Finland. The combination of the survey and register-based data was approved and performed by Statistics Finland. Information on the participants in the FQWLS was obtained by following the information logged against their personal social security number by Statistics Finland. The survey and the combined material did not include any identification data. Accordingly, the ethical standards of Statistics Finland were followed when conducting the study.
Long-Term Sickness Absence Long-term sickness absence information was drawn from the register data. The Finnish Social Insurance Institution keeps records on sickness allowances paid for medically certified sickness absences of more than 10 days for the entire population. Sickness allowance is payable for a maximum of 300 working days, after which one can apply for disability pension. In 2008, the most common causes for sickness allowance were musculoskeletal diseases (eg, dorsopathies),
Shiftwork, Individual WTC, and Long-Term Sickness Absence
mental and behavioral disorders, and external causes (eg, fractures). Maternity leave and absence from work to care for a sick child are not included in the sickness absences. Long-term sickness absence in 2002 was treated as baseline absenteeism, and the accumulated number of days on long-term sick leave between 2004 and 2008 was used as an outcome measure. For descriptive purposes, sickness absence days were categorized in four groups. The cutoff values are based on zero sickness absences and tertiles of sickness absence days for those who had at least one medically certified sickness absence longer than 10 days.
Shiftwork and WTC In the 2003 survey, employees were asked their working hours: regular day work (between 6 AM and 6 PM, n = 2378), regular evening work (n = 25), regular night work (n = 33), two-shift work without night work (n = 338), two-shift work with night work (n = 91), three-shift work (n = 264), or other types of working hours (n = 306). Because of the small amount of those doing regular evening and regular night work, we excluded them from the analysis. Furthermore, because of the large heterogeneity of the other types of working hours, we excluded them from the analysis. Consequently, we compared employees who worked in shifts (n = 693) with daytime workers (n = 2378). Individual WTC was measured with four items by asking respondents how much they could influence their working time on a scale of 1 to 4 (1 = not at all; 4 = much), to what extent they agreed with the statement “I can use flexible working hours sufficiently for my own needs” (1 = untrue; 4 = true), to what extent they had the possibility for brief absences from work in the middle of the working day to run personal errands (1 = never; 4 = always when necessary), and whether they could change the starting and ending times of a workday (yes, no). All individual variables were rescaled to range from 0 to 1 before constructing the index. A four-item composite variable was constructed by summing up the response scores to the rescaled questions (Cronbach α = 0.74; mean = 0.59; and standard deviation = 0.30). The participants were classified into quartiles to indicate lower and higher levels of individual WTC. This index is very similar to the index developed in previous studies,24 excluding questions about control over vacations and days off.
Background and Work-Related Factors Other information relevant from the viewpoint of sickness absence, and therefore controlled for in the analyses, was drawn from the survey. This information consisted of the major backgroundrelated and work-related factors that are generally known to influence sickness absence. The four background factors were age, sex, marital status, dependent children (living with a partner and children, living with a partner without children, single parent, single), and the level of education (tertiary education, secondary, primary). Work-related factors consist of weekly working hours and job tenure. Weekly working hours were examined by asking participants how many hours they usually worked in their main job. Job tenure was measured by asking participants how many years they had worked for their current employer.
Statistical Analysis The relationships between individual WTC and the background factors and the working time model were examined by using analyses of variance. The effects of shiftwork and WTC on long-term sickness absence were analyzed using the negative binomial model. Long-term sickness absence days were clearly overdispersed (ie, the variance was higher than the mean and there was an excess of zeroes), which made a simple Poisson model unsuitable for the analysis. In addition, the negative binomial model is more appropriate than the Poisson when the events of interest are not independent,34 which is the case for sickness absence days. To take into account the unequal
C 2014 American College of Occupational and Environmental Medicine
Copyright © 2014 Lippincott Williams & Wilkins. Unauthorized reproduction of this article is prohibited.
733
JOEM r Volume 56, Number 7, July 2014
N¨atti et al
lengths of follow-up time, we included the natural logarithm of the follow-up time as an offset variable. The results are represented as incidence rate ratios and their 95% confidence intervals. To study the joint effects of individual WTC and shiftwork, we constructed combination variables by cross-tabulating the working time model (day work, shiftwork) and the level of individual WTC (low, high). The effects of the resulting four combinations on long sickness absence were studied using the hypothetically least adverse conditions (day work with high control) as the reference category. The significance of these joint effects was studied using appropriate cross-product terms (working time model multiplied by WTC) with the Wald test for interaction. All analyses were adjusted for background (age, sex, marital status, dependent children, level of education) and work-related factors (weekly working hours, job tenure). In addition, earlier longterm sickness absence (no or yes in 2002) before the 2003 survey was treated as baseline absenteeism, and added to the model.
RESULTS Table 1 shows descriptive information of the study participants. Slightly over half of the participants were female (53%), 76% were married or cohabiting, 41% had dependent children, and 41% had tertiary education. The mean age of participants was 43 years, the average weekly working time was 37 hours and job tenure 12 years, and on average participants had approximate 3 days of medically certified long-term sickness absences over 10 days. About one quarter of the sample worked shifts (23%), and the average score of WTC was 59 on a scale of 0 to 100. Tables 2 and 3 show the relationships between the control variables, WTC, and the working time model. All background and work characteristics, except marital status, were associated with the accumulated sickness absence days. Sickness absence was more common for females, older participants, employees without children, employees with a lower level of education, employees with normal working hours, long job tenure, and previous sickness absence. Similarly, all background and work characteristics, except age, marital status, and dependent children, were associated with WTC. Employees in day work, males, those with tertiary education, long work hours, medium job tenure, and no previous sickness absence had a higher level of WTC. Again, all controls except sex and job tenure were associated with working time. Shiftwork was more common for employees with low WTC, younger, married, with dependent children, low education level, and employees with previous sickness absence.
TABLE 1. Descriptive Statistics of the Sample Participants (N = 3071) Mean (SD) or Percentage
Variable Sex (female), % Age, yrs, mean (SD) Married or cohabiting, % Dependent children, % Level of education (tertiary), % Weekly working hours, mean (SD) Job tenure, yrs, mean (SD) Long-term sickness absence days in 2002, mean (SD) Working time control, mean (SD) Working time model (shiftwork), % SD, standard deviation.
734
53 43.4 (10.6) 76 41 41 37.5 (6.1) 12.2 (10.0) 2.7 (12.9) 58.8 (29.7) 23
Table 4 shows the unadjusted and adjusted coefficients of WTC and the working time model from negative binomial models predicting long-term sickness absence days. In the unadjusted model, both WTC and the working time model predicted sickness absence. Employees in the two highest quartiles of WTC had 0.6 and 0.4 times lower rates of long-term sickness absences than those in the lowest quartile. The sickness absence rate was 1.5 times higher for shiftwork than for day work. Adjusting for background and workrelated factors reduced the effects only slightly (to 0.7, 0.5, and 1.3, respectively). When WTC and the working time model were added in the same model together with controls, the effect of the working time model lost its significance. The effect of WTC however remained significant and virtually the same. We also tested whether there were interaction effects between sex and WTC or sex and the working time model, but neither of them was significant. Table 5 shows the unadjusted and adjusted combined effect of the working time model and WTC on long sickness absence. There was no statistically significant interaction between the working time model and WTC in either the unadjusted or adjusted models. Thus, high WTC reduces long-term sickness absence days equally in day and shiftwork. This is also evident from the effects reported in Table 5. Employees in day work with low control or shiftwork with low control had a 1.7 and a 2.1 times higher rate of long-term sickness absences than those in day work with high control. There was no difference between the accumulated long-term sickness absence days between day work with high WTC and shiftwork with high WTC. Adjusting for background and work-related factors did not change the results substantially.
DISCUSSION This study indicates that high WTC reduced sickness absence days equally for shiftwork and day work. Thus, there was no evidence that WTC moderates the effect of shiftwork on sickness absence. Nevertheless, the higher incidence of sickness absence in shiftwork compared with day work was found to be mainly due to the lower overall level of WTC. Earlier research has found some evidence for the moderating role of WTC in the relationships between long working hours,35 shiftwork,31 shiftwork and irregular working hours29,36 in relation to self-reported health, blood pressure, and work–family balance outcomes. In Kandolin and Huida’s intervention study,36 the increase in the participation of the employees in shift planning was associated with decreased mental stress and strain among female midwives. In another intervention study, Viitasalo et al37 found that the use of a flexible shift system, which allowed some individual flexibility and control over the work hours in exchange for variability, was followed by a decrease in blood pressure compared with the control group with no changes in shifts. Ala-Mursula et al24 found that the adverse effects of work stress (high job demands, low control, high strain, and a high effort–reward imbalance) on medically certified sickness absence were reduced by a higher level of employee control over daily working hours and days off, especially among females, although they did not focus on shiftwork. Nevertheless, none of these studies have studied the combined effects of shiftwork and WTC in relation to sickness absence. In this study, sick leave periods were medically certified but we only considered those over 10 days to be long term. Working time control was measured by four items: indicating the extent to which employees could influence their working time, the extent to which employees could use flexible working hours for their personal needs, the extent to which they had the possibility for brief absences from work to run personal errands, and whether the respondents had an influence on the starting and ending times of a workday. Thus, our measure did not include information on control over the choice of shifts or the scheduling of holidays and days off. To search for best
C 2014 American College of Occupational and Environmental Medicine
Copyright © 2014 Lippincott Williams & Wilkins. Unauthorized reproduction of this article is prohibited.
JOEM r Volume 56, Number 7, July 2014
Shiftwork, Individual WTC, and Long-Term Sickness Absence
TABLE 2. Relationships Between the Background Variables, Working Time Model, and Working Time Control With Accumulated Long-Term Sickness Absence Days (2004 to 2008) Accumulated Long-Term Sickness Absence Days (2004 to 2008)
Working time control 1 (low) 2 3 4 (high) Working time model Day work Shiftwork Sex Male Female Age, yrs 15–34 35–49 50–64 Married or cohabiting Yes No Dependent children Yes No Level of education Primary Secondary Tertiary Weekly working hours 1–34 35–40 ≥41 Job tenure, yrs 0–2 3–9 ≥10 Long-term sickness absence in 2002 Yes No
n
None, %
1–15, %
15–54, %
≥55, %
724 736 775 836
51 57 63 64
15 13 12 13
17 14 14 12
16 16 11 10
2378 693
62 50
12 16
14 16
12 17
1447 1624
66 53
12 14
11 17
11 15
716 1337 1018
66 59 54
14 14 12
13 14 15
6 13 19
735 2336
60 58
13 13
14 15
13 14
1275 1796
60 58
13 13
15 13
11 15
518 1287 1266
48 54 68
14 14 12
17 16 11
21 16 8
367 2472 232
59 58 71
12 14 8
16 14 9
13 14 12
500 1084 1487
65 63 54
12 13 14
14 12 15
8 11 17
344 2727
31 63
15 13
23 13
31 11
Difference Chi-Square Test (P)
Combined Effects of Shiftwork and Individual Working Time Control on Long-Term Sickness Absence A Prospective Study of Finnish Employees Jouko N¨atti, PhD, Tomi Oinas, PhD, Mikko H¨arm¨a, PhD, Timo Anttila, PhD, and Irja Kandolin, PhD
Objective: To investigate whether the effects of shiftwork on long-term sickness absence vary according to the level of individual working time control (WTC). Methods: A representative sample of Finnish employees (1447 men and 1624 women) was combined with a register-based follow-up. A negative binomial model was used in the analysis of long-term sickness absence days. The results were adjusted for various background and work-related factors. Results: Individual WTC decreased long-term sickness absence. The higher rate of sickness absences in shiftwork was mainly due to the lower level of WTC. Working time control decreased sickness absence equally in day work and shiftwork. Conclusions: The negative health effects of shiftwork may be decreased by offering sufficient WTC. Establishments that use WTC as a human resource instrument may benefit from reduced absenteeism.
E
very fifth employee in Europe works shifts.1 Shiftwork and overtime work are often seen as forms of company-controlled flexibility, which aim to respond to fluctuations in demand and services. Nevertheless, shiftwork is a potential risk factor for employees’ health.2–5 Shiftwork is associated with fatigue, insomnia, and accidents, but is also probably related to a wide range of chronic health problems such as coronary heart disease,6 obesity and type 2 diabetes,7 and breast cancer.8 Evidence of the relationship between shiftwork and absenteeism is still inconclusive. Some studies have found higher sickness absence or longer absence spells among shiftworkers,9,10 whereas several studies are inconclusive or have found even less sickness absence among shiftworkers than among day workers.11–13 Working time control (WTC) is defined as the possibility to meet the needs of employees, providing them with autonomy regarding factors in the start and end times of their shifts, breaks, days off, holidays, and the total number of work hours.14 Working time control is often associated with the better mental and social well-being of employees, especially in situations where they have to be flexible for the needs of their employer or company.15,16 On the basis of a recent Cochrane review,17 the evidence from 10 different studies tentatively suggests that flexible working time interventions that increase worker control and choice are likely to have a positive effect on health. Nijp et al18 have presented two separate mechanisms that potentially explain WTC’s favorable associations with well-being. The first relates to the time regulation potential of WTC because control over working time helps people align their work commitments with their private life. The second mechanism is based on recovery regulation. Working time control can prevent work From the School of Social Sciences and Humanities (Dr N¨atti), University of Tampere; Department of Social Sciences and Philosophy (Drs Oinas and Anttila), University of Jyv¨askyl¨a; and Finnish Institute of Occupational Health (Dr H¨arm¨a, Dr Kandolin), Helsinki, Finland. This study was supported by the Academy of Finland (project 124456). The authors declare no conflicts of interest. Address correspondence to: Jouko N¨atti, PhD, School of Social Sciences and Humanities, University of Tampere, Linna 33014, Finland ([email protected]). C 2014 by American College of Occupational and Environmental Copyright Medicine DOI: 10.1097/JOM.0000000000000176
732
overload and sustain a favorable effort–recovery balance. Working time control facilitates both on the job recovery (breaks during the day) and external recovery (in-between working periods). There are several possible reasons why the possible positive effects of WTC on health could be most useful among shiftworkers and workers with irregular working hours. First, shifts rosters are often planned at short notice, decreasing the predictability of free time, although this depends on the type of shift schedule. Second, the work shifts of 24/7 work places such as hospitals may not suit the scheduling of family and social activities. Partly related to the decreased predictability and increased variability in shift timing, shiftwork is also related to less overall job control19 and lack of control over working hours20 as well as a higher likelihood of conflict between the demands of work and the needs of the family.21 Because of the irregularity of and disturbances to sleep and wakefulness, shiftwork is also related to a higher need for recovery after work.21,22 The social and health benefits of individual work time control may stem from advantages in combining shiftwork and nonwork demands and thereby improving the possibilities for mental recovery and stress reduction.16,23,24 A large amount of evidence and several theoretical work stress models indicate that decreased job control and insufficient recovery are among the main mechanisms linking stressful work to adverse health.25,26 Improved mental health and recovery, including better sleep, may be ways for psychophysiological unwinding and the prevention of chronic health conditions after irregular or abnormal working hours.26 Both low WTC and sleep disturbances predict work disability in the long run-–the work disability usually arises from mental and musculoskeletal problems.20,27 For musculoskeletal disorders, the opportunity to have additional breaks or days off in situations of increasing pain due to an acute or chronic disorder may be critical for the prevention and management of musculoskeletal problems. The positive influence of individual WTC on psychosocial and health outcomes may also be indirect. Hughes and Parkes28 found that WTC was not only directly related to reduced work– family interference, but high control also buffered the negative effect of longer hours on work–family relations. Similarly, Geurts et al29 found that workers who could take days off and vacations according to their own needs (leave control) were less likely to be required to neglect family activities because of work duties. In addition, control regarding starting and finishing times (flexitime) buffered the adverse impact of long contractual hours on work–family relations. The significance of WTC for the chronic health effects of shiftworkers is unclear. There is some evidence for the moderating role of WTC in the relationship between long work hours and health outcomes. Nabe-Nielsen et al30 found that moderate or low WTC among shiftworkers was associated with lower vitality, worse mental health, and more somatic stress symptoms. The work–time influence did not have a similar effect among day workers. Olsen and Dahl31 found that flexibility in the work schedule when working regular hours had no effect on sickness absence or on the work–family balance, but the situation was different among those with irregular hours. Men who worked irregular hours and had no control over when to work had a significantly higher sickness absence than other men. JOEM r Volume 56, Number 7, July 2014
Copyright © 2014 Lippincott Williams & Wilkins. Unauthorized reproduction of this article is prohibited.
JOEM r Volume 56, Number 7, July 2014
In addition, working irregular hours, particularly in cases without flexibility, resulted in adverse effects on the balance between work and family, for both women and men. The role of WTC for the health and sickness absence of shiftworkers may also be related to sex. Women clearly report higher self-certified as well as medically confirmed spells for sickness absence and adjusting for, for example, psychosocial working conditions. Family-related factors or the sex composition of a workplace does not explain the differences in long-term sickness absence.32 The longer sickness absence spells among women are mainly explained by the heavier burden of ill-health and, to a lesser extent, by higher physical work demands.32 Furthermore, women do more housework, even in countries like Finland where sex roles are not so firmly defined.33 Thus, increased WTC may be more important for women, if they are to avoid work–family conflicts because of shiftwork. To study the possible combined effects of shiftwork and WTC on chronic health problems, one aim of this study is to investigate whether shiftwork more strongly predicts long-term sickness absence among male and female employees with low WTC than among with those with high WTC.
STUDY POPULATION AND METHODS Sample and Participants The data consist of the Finnish Quality of Work Life Survey (FQWLS) from 2003 linked to a register follow-up on long-term sickness absence over 5 years. The sample is based on 5270 employees, of whom 4104 participated in face-to-face interviews in 2003, the response rate being 78%. The FQWLS is a representative sample of Finnish employees covering all sectors and occupations. The sample in 2003 was based on respondents in the October and November Labour Force Survey who were 15 to 64 years old, with a normal weekly working time of at least 5 hours. According to Statistics Finland, nonresponse does not seriously undermine the representativeness of FQWLS data.33 This study was restricted to 15- to 64-year-old employees, who worked day shifts or had shiftwork (1447 men and 1624 women), and who had worked in their current job at least 1 year. Statistics Finland conducted the survey on a face-to-face interview basis using a standard questionnaire. The sample was gathered from those employed persons who participated in the regular monthly labor force surveys for the first time.33 The questionnaire comprised a comprehensive set of questions dealing with various features of work life.
Register-Based Follow-Up Data The survey in 2003 was cross-sectional. To study long-term sickness absence in relation to shiftwork and WTC, panel data were required. Therefore, the FQWLS was merged with register-based follow-up data held and maintained by Statistics Finland. The combination of the survey and register-based data was approved and performed by Statistics Finland. Information on the participants in the FQWLS was obtained by following the information logged against their personal social security number by Statistics Finland. The survey and the combined material did not include any identification data. Accordingly, the ethical standards of Statistics Finland were followed when conducting the study.
Long-Term Sickness Absence Long-term sickness absence information was drawn from the register data. The Finnish Social Insurance Institution keeps records on sickness allowances paid for medically certified sickness absences of more than 10 days for the entire population. Sickness allowance is payable for a maximum of 300 working days, after which one can apply for disability pension. In 2008, the most common causes for sickness allowance were musculoskeletal diseases (eg, dorsopathies),
Shiftwork, Individual WTC, and Long-Term Sickness Absence
mental and behavioral disorders, and external causes (eg, fractures). Maternity leave and absence from work to care for a sick child are not included in the sickness absences. Long-term sickness absence in 2002 was treated as baseline absenteeism, and the accumulated number of days on long-term sick leave between 2004 and 2008 was used as an outcome measure. For descriptive purposes, sickness absence days were categorized in four groups. The cutoff values are based on zero sickness absences and tertiles of sickness absence days for those who had at least one medically certified sickness absence longer than 10 days.
Shiftwork and WTC In the 2003 survey, employees were asked their working hours: regular day work (between 6 AM and 6 PM, n = 2378), regular evening work (n = 25), regular night work (n = 33), two-shift work without night work (n = 338), two-shift work with night work (n = 91), three-shift work (n = 264), or other types of working hours (n = 306). Because of the small amount of those doing regular evening and regular night work, we excluded them from the analysis. Furthermore, because of the large heterogeneity of the other types of working hours, we excluded them from the analysis. Consequently, we compared employees who worked in shifts (n = 693) with daytime workers (n = 2378). Individual WTC was measured with four items by asking respondents how much they could influence their working time on a scale of 1 to 4 (1 = not at all; 4 = much), to what extent they agreed with the statement “I can use flexible working hours sufficiently for my own needs” (1 = untrue; 4 = true), to what extent they had the possibility for brief absences from work in the middle of the working day to run personal errands (1 = never; 4 = always when necessary), and whether they could change the starting and ending times of a workday (yes, no). All individual variables were rescaled to range from 0 to 1 before constructing the index. A four-item composite variable was constructed by summing up the response scores to the rescaled questions (Cronbach α = 0.74; mean = 0.59; and standard deviation = 0.30). The participants were classified into quartiles to indicate lower and higher levels of individual WTC. This index is very similar to the index developed in previous studies,24 excluding questions about control over vacations and days off.
Background and Work-Related Factors Other information relevant from the viewpoint of sickness absence, and therefore controlled for in the analyses, was drawn from the survey. This information consisted of the major backgroundrelated and work-related factors that are generally known to influence sickness absence. The four background factors were age, sex, marital status, dependent children (living with a partner and children, living with a partner without children, single parent, single), and the level of education (tertiary education, secondary, primary). Work-related factors consist of weekly working hours and job tenure. Weekly working hours were examined by asking participants how many hours they usually worked in their main job. Job tenure was measured by asking participants how many years they had worked for their current employer.
Statistical Analysis The relationships between individual WTC and the background factors and the working time model were examined by using analyses of variance. The effects of shiftwork and WTC on long-term sickness absence were analyzed using the negative binomial model. Long-term sickness absence days were clearly overdispersed (ie, the variance was higher than the mean and there was an excess of zeroes), which made a simple Poisson model unsuitable for the analysis. In addition, the negative binomial model is more appropriate than the Poisson when the events of interest are not independent,34 which is the case for sickness absence days. To take into account the unequal
C 2014 American College of Occupational and Environmental Medicine
Copyright © 2014 Lippincott Williams & Wilkins. Unauthorized reproduction of this article is prohibited.
733
JOEM r Volume 56, Number 7, July 2014
N¨atti et al
lengths of follow-up time, we included the natural logarithm of the follow-up time as an offset variable. The results are represented as incidence rate ratios and their 95% confidence intervals. To study the joint effects of individual WTC and shiftwork, we constructed combination variables by cross-tabulating the working time model (day work, shiftwork) and the level of individual WTC (low, high). The effects of the resulting four combinations on long sickness absence were studied using the hypothetically least adverse conditions (day work with high control) as the reference category. The significance of these joint effects was studied using appropriate cross-product terms (working time model multiplied by WTC) with the Wald test for interaction. All analyses were adjusted for background (age, sex, marital status, dependent children, level of education) and work-related factors (weekly working hours, job tenure). In addition, earlier longterm sickness absence (no or yes in 2002) before the 2003 survey was treated as baseline absenteeism, and added to the model.
RESULTS Table 1 shows descriptive information of the study participants. Slightly over half of the participants were female (53%), 76% were married or cohabiting, 41% had dependent children, and 41% had tertiary education. The mean age of participants was 43 years, the average weekly working time was 37 hours and job tenure 12 years, and on average participants had approximate 3 days of medically certified long-term sickness absences over 10 days. About one quarter of the sample worked shifts (23%), and the average score of WTC was 59 on a scale of 0 to 100. Tables 2 and 3 show the relationships between the control variables, WTC, and the working time model. All background and work characteristics, except marital status, were associated with the accumulated sickness absence days. Sickness absence was more common for females, older participants, employees without children, employees with a lower level of education, employees with normal working hours, long job tenure, and previous sickness absence. Similarly, all background and work characteristics, except age, marital status, and dependent children, were associated with WTC. Employees in day work, males, those with tertiary education, long work hours, medium job tenure, and no previous sickness absence had a higher level of WTC. Again, all controls except sex and job tenure were associated with working time. Shiftwork was more common for employees with low WTC, younger, married, with dependent children, low education level, and employees with previous sickness absence.
TABLE 1. Descriptive Statistics of the Sample Participants (N = 3071) Mean (SD) or Percentage
Variable Sex (female), % Age, yrs, mean (SD) Married or cohabiting, % Dependent children, % Level of education (tertiary), % Weekly working hours, mean (SD) Job tenure, yrs, mean (SD) Long-term sickness absence days in 2002, mean (SD) Working time control, mean (SD) Working time model (shiftwork), % SD, standard deviation.
734
53 43.4 (10.6) 76 41 41 37.5 (6.1) 12.2 (10.0) 2.7 (12.9) 58.8 (29.7) 23
Table 4 shows the unadjusted and adjusted coefficients of WTC and the working time model from negative binomial models predicting long-term sickness absence days. In the unadjusted model, both WTC and the working time model predicted sickness absence. Employees in the two highest quartiles of WTC had 0.6 and 0.4 times lower rates of long-term sickness absences than those in the lowest quartile. The sickness absence rate was 1.5 times higher for shiftwork than for day work. Adjusting for background and workrelated factors reduced the effects only slightly (to 0.7, 0.5, and 1.3, respectively). When WTC and the working time model were added in the same model together with controls, the effect of the working time model lost its significance. The effect of WTC however remained significant and virtually the same. We also tested whether there were interaction effects between sex and WTC or sex and the working time model, but neither of them was significant. Table 5 shows the unadjusted and adjusted combined effect of the working time model and WTC on long sickness absence. There was no statistically significant interaction between the working time model and WTC in either the unadjusted or adjusted models. Thus, high WTC reduces long-term sickness absence days equally in day and shiftwork. This is also evident from the effects reported in Table 5. Employees in day work with low control or shiftwork with low control had a 1.7 and a 2.1 times higher rate of long-term sickness absences than those in day work with high control. There was no difference between the accumulated long-term sickness absence days between day work with high WTC and shiftwork with high WTC. Adjusting for background and work-related factors did not change the results substantially.
DISCUSSION This study indicates that high WTC reduced sickness absence days equally for shiftwork and day work. Thus, there was no evidence that WTC moderates the effect of shiftwork on sickness absence. Nevertheless, the higher incidence of sickness absence in shiftwork compared with day work was found to be mainly due to the lower overall level of WTC. Earlier research has found some evidence for the moderating role of WTC in the relationships between long working hours,35 shiftwork,31 shiftwork and irregular working hours29,36 in relation to self-reported health, blood pressure, and work–family balance outcomes. In Kandolin and Huida’s intervention study,36 the increase in the participation of the employees in shift planning was associated with decreased mental stress and strain among female midwives. In another intervention study, Viitasalo et al37 found that the use of a flexible shift system, which allowed some individual flexibility and control over the work hours in exchange for variability, was followed by a decrease in blood pressure compared with the control group with no changes in shifts. Ala-Mursula et al24 found that the adverse effects of work stress (high job demands, low control, high strain, and a high effort–reward imbalance) on medically certified sickness absence were reduced by a higher level of employee control over daily working hours and days off, especially among females, although they did not focus on shiftwork. Nevertheless, none of these studies have studied the combined effects of shiftwork and WTC in relation to sickness absence. In this study, sick leave periods were medically certified but we only considered those over 10 days to be long term. Working time control was measured by four items: indicating the extent to which employees could influence their working time, the extent to which employees could use flexible working hours for their personal needs, the extent to which they had the possibility for brief absences from work to run personal errands, and whether the respondents had an influence on the starting and ending times of a workday. Thus, our measure did not include information on control over the choice of shifts or the scheduling of holidays and days off. To search for best
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JOEM r Volume 56, Number 7, July 2014
Shiftwork, Individual WTC, and Long-Term Sickness Absence
TABLE 2. Relationships Between the Background Variables, Working Time Model, and Working Time Control With Accumulated Long-Term Sickness Absence Days (2004 to 2008) Accumulated Long-Term Sickness Absence Days (2004 to 2008)
Working time control 1 (low) 2 3 4 (high) Working time model Day work Shiftwork Sex Male Female Age, yrs 15–34 35–49 50–64 Married or cohabiting Yes No Dependent children Yes No Level of education Primary Secondary Tertiary Weekly working hours 1–34 35–40 ≥41 Job tenure, yrs 0–2 3–9 ≥10 Long-term sickness absence in 2002 Yes No
n
None, %
1–15, %
15–54, %
≥55, %
724 736 775 836
51 57 63 64
15 13 12 13
17 14 14 12
16 16 11 10
2378 693
62 50
12 16
14 16
12 17
1447 1624
66 53
12 14
11 17
11 15
716 1337 1018
66 59 54
14 14 12
13 14 15
6 13 19
735 2336
60 58
13 13
14 15
13 14
1275 1796
60 58
13 13
15 13
11 15
518 1287 1266
48 54 68
14 14 12
17 16 11
21 16 8
367 2472 232
59 58 71
12 14 8
16 14 9
13 14 12
500 1084 1487
65 63 54
12 13 14
14 12 15
8 11 17
344 2727
31 63
15 13
23 13
31 11
Difference Chi-Square Test (P)
