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Keywords:

  • binge drinking;
  • follow-up;
  • food habits;
  • heavy drinking;
  • physical activity;
  • smoking

Summary

  1. Top of page
  2. Summary
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Acknowledgements
  9. Disclosure Statement
  10. References

It has been suggested that there are associations among insomnia symptoms and unhealthy behaviours. However, previous studies are sparse and mainly cross-sectional, and have not been focused on several key unhealthy behaviours. The aim of this study was to examine whether the associations are bidirectional, i.e. whether insomnia symptoms are associated with subsequent unhealthy behaviours, and whether unhealthy behaviours are associated with subsequent insomnia symptoms. The data were derived from the Helsinki Health Study prospective cohort study. The baseline data were collected in 2000–02 (n = 8960, response rate 67%) among 40–60-year-old employees of the City Helsinki, Finland. The follow-up data were collected in 2007 (n = 7332, response rate 83%). Logistic regression analysis was used to examine the associations among insomnia symptoms and unhealthy behaviours, including smoking, heavy and binge drinking, physical inactivity and unhealthy food habits. Frequent insomnia symptoms at baseline were associated with subsequent heavy drinking [odds ratio (OR): 1.34; 95% confidence interval (CI): 1.07–1.68] and physical inactivity (OR: 1.27; 95% CI: 1.08–1.48) after full adjustment for gender, age, corresponding unhealthy behaviour at baseline, marital status, occupational class, sleep duration and common mental disorders. Additionally, heavy drinking (OR: 1.48; 95% CI: 1.22–1.80) and binge drinking (OR: 1.26; 95% CI: 1.08–1.46) at baseline were associated with subsequent insomnia symptoms at follow-up after full adjustment. In conclusion, insomnia symptoms were associated with subsequent heavy drinking and physical inactivity, and heavy and binge drinking were also associated with subsequent insomnia symptoms.


Introduction

  1. Top of page
  2. Summary
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Acknowledgements
  9. Disclosure Statement
  10. References

Insomnia symptoms are prevalent, but their associations with unhealthy behaviours are poorly understood. In particular, prospective studies are few and several key unhealthy behaviours have not been examined. A previous Swedish study reported cross-sectional associations between insomnia symptoms and unhealthy behaviours. That study suggested that physical inactivity and having symptoms of alcohol dependence were associated with insomnia symptoms (Janson et al., 2001). Cross-sectional studies have found associations of alcohol consumption (Fabsitz et al., 1997; Härmäet al., 1998; Tachibana et al., 1996) and smoking (Kageyama et al., 2005) with insomnia symptoms, and also suggest that physical activity is associated with reduced risk of insomnia symptoms (Fabsitz et al., 1997; Sherrill et al., 1998). There is only one cross-sectional study on the association between sleep and food habits among motor freight workers, but that study concerned sufficient sleep, and insomnia symptoms were not included. The study found that sufficient sleep was associated with healthier food habits (Buxton et al., 2009).

Whether insomnia symptoms are associated with subsequent unhealthy behaviours or whether unhealthy behaviours are associated with subsequent insomnia symptoms is poorly understood. Most previous studies of insomnia symptoms and unhealthy behaviours have been cross-sectional (Fabsitz et al., 1997; Fogelholm et al., 2007; Härmäet al., 1998; Kageyama et al., 2005; Phillips and Danner, 1995; Sherrill et al., 1998; Strine et al., 2005; Tachibana et al., 1996;). One prospective study assessed the association of insomnia symptoms among individuals with chronic and remitted alcohol dependence and found that chronic alcohol dependence is associated with subsequent insomnia symptoms (Crum et al., 2004). Another 8-year prospective study (Morgan and Clarke, 1997) found that physical inactivity in elderly general practice patients was associated with subsequent insomnia symptoms.

We lack longitudinal studies of the associations of sleep with key unhealthy behaviours, including alcohol consumption, smoking, poor food habits and physical inactivity, as well as studies examining the direction of the association. It is therefore important to study the bidirectional nature of the associations.

The aim of this longitudinal study was, first, to examine the associations of insomnia symptoms with subsequent unhealthy behaviours among women and men. The second aim was to examine the corresponding associations between unhealthy behaviours and subsequent insomnia symptoms.

Methods

  1. Top of page
  2. Summary
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Acknowledgements
  9. Disclosure Statement
  10. References

The data were derived from the Helsinki Health Study baseline and follow-up mail questionnaire surveys. The baseline data were collected in 2000–02 (n = 8960, response rate 67%, 80% women) (Lahelma et al., 2012) among 40–60-year-old employees of the City Helsinki, Finland. The follow-up survey data were collected in 2007 (n = 7332, response rate 83%). Department of Public Health, University of Helsinki and the City of Helsinki health authorities gave ethical approval for the Helsinki Health Study.

Insomnia symptoms

Insomnia symptoms were measured using the four-item Jenkins Sleep Questionnaire (Jenkins et al., 1988), which measures insomnia symptoms during the previous 4 weeks. This questionnaire includes four items, which were asked by the following questions: ‘How often in the past 4 weeks did you have the following symptoms? (i) Trouble falling asleep, (ii) Waking up several times per night, (iii) Trouble staying asleep and (iv) Waking up after the usual amount of sleep, feeling tired and worn out?’. The response categories were ‘not at all’, ‘1–3 nights’, ‘4–7 nights’, ‘8–14 nights’, ‘15–21 nights’ and ‘22–28 nights’. Based on all four insomnia symptoms, we formed a summary measure, which was dichotomized. The analyses included those with responses to at least three of the four items, except that if the response to any of the four items was 15–28 nights, the respondent was classified as having frequent insomnia symptoms. Others served as a reference group (no or occasional insomnia symptoms).

Unhealthy behaviours

Heavy drinking included questions on the consumption of bottles of ‘beer or cider’ and ‘wine or other mild beverages’ per week and bottles of ‘spirits’ per month: ‘How much of the following alcoholic drinks do you consume on average?’. Alcohol consumption was measured per week by the sum of portions of beer, wine and hard liquor. Each portion contained approximately 12 g of pure alcohol. A cutoff point of 140 g of alcohol for women (approximately two portions a day) and 280 g for men (approximately four portions a day) per week was used. These amounts are considered as heavy drinking in The Finnish Current Care Guideline (2011). Binge drinking was defined as six or more portions of any alcoholic drinks at a single occasion at least once a week in men and at least once a month in women. Smoking was divided into current smoking and non-smoking, including ex-smokers.

Food habits were asked by a food frequency questionnaire (FFQ). Healthy food habits included (i) using mainly vegetable-based fat on bread, (ii) using mainly vegetable-based fat in cooking, (iii) dark bread at least once per day, (iv) fresh vegetables at least once per day, (v) cooked vegetables at least five times per week, (vi) fresh fruits and/or berries at least once per day, (vii) fish at least twice per week and (viii) low-fat or skimmed (but not high-fat) milk daily. Food habits were categorized into healthy and unhealthy, based on the response alternatives: ‘not during the past 4 weeks’, ‘1–3 times per month’, ‘once per week’, ‘2–4 times per week’, ‘5–6 times per week’ or ‘once per day or two or more times per day’. Similar measurements of food habits have been used in previous studies (Helldán et al., 2011; Seiluri et al., 2011). Food habits were classified as unhealthy if two or fewer of the eight recommended habits were met. Healthy and unhealthy food habits were defined based on Finnish national dietary guidelines (The National Nutrition Council 2005).

The respondents were asked about their average weekly hours of physical activity during leisure time or commuting within the previous 12 months in four grades of intensity: walking, brisk walking, jogging and running or equivalent activities. Each intensity grade had five response alternatives: ‘no activity’, ‘0–½ h per week’, ‘½–1 h per week’, ‘2–3 h per week’ and ‘4 h or more per week’. Total physical activity was measured by approximate metabolic equivalent tasks (METs) (Kujala et al., 1998). One MET is defined as the energy expenditure of a person sitting quietly. The total MET-hours per week for physical activity were calculated by multiplying the average time used by the estimated MET value of each physical activity (Kujala et al., 1998) and adding the four values together (Ainsworth et al., 2000). Physical activity was dichotomized into physical inactivity (fewer than 14 MET-hours per week) or physical activity (14 or more MET-hours per week).

We excluded those with missing data on all alcohol use questions, smoking, each of the four physical activity items, two or more of the eight food habit items [if one item of the eight examined healthy food habits was missing (6%), the missing response was coded as ‘zero’ and thus was classified as ‘unhealthy’] or insomnia symptoms both at baseline and follow-up. We also excluded those with missing data at baseline on marital status, occupational class or sleep duration. Those with missing data at baseline on common mental disorders were categorized into one group. In all variables the numbers of the excluded varied from 874 to 1010 (12–14%). The final data used in the analyses varied from 5131 to 5248 in women and 1191 to 1210 in men, depending on the outcome measure.

Covariates

All examined covariates were derived from the baseline questionnaire surveys. Marital status was categorized as unmarried, cohabiting/married and divorced/widowed. Occupational class was categorized as professionals, semi-professionals, routine non-manual employees and manual workers. Sleep duration was categorized as 5 h or less and 6, 7, 8 and 9 h or more. Common mental disorders were measured by the 12-item General Health Questionnaire, GHQ-12 [the summary score varied from 0 to 12 and was dichotomized into those without (scores 0–2) and those with (scores 3–12) common mental disorders; this is in line with previous procedures; Goldberg, 1972]. We examined whether these covariates contributed to the associations among insomnia symptoms and unhealthy behaviours.

Statistical methods

Prevalence percentages for heavy drinking, binge drinking, smoking, physical inactivity and unhealthy food habits by insomnia symptoms were calculated first followed by the prevalence percentages for insomnia symptoms by unhealthy behaviours. Logistic regression analysis [odds ratios (OR) and their 95% confidence intervals (CI)] was used to examine the associations among insomnia symptoms and unhealthy behaviours over time. Associations of baseline insomnia symptoms with unhealthy behaviours at follow-up were adjusted for gender, baseline age, marital status, occupational class, sleep duration and common mental disorders, as well as baseline unhealthy behaviours. Associations of baseline unhealthy behaviours with insomnia symptoms at follow-up were also adjusted for baseline insomnia symptoms. Gender did not interact with insomnia symptoms or any of the examined unhealthy behaviours (data not shown). Thus the analyses were conducted in the pooled data adjusting for gender as a covariate. These analyses were conducted using the spss program package version 18.1 (SPSS Inc., IBM, Chicago, IL, USA).

Results

  1. Top of page
  2. Summary
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Acknowledgements
  9. Disclosure Statement
  10. References

At baseline, 21% of women and 17% of men reported frequent insomnia symptoms. At follow-up, the corresponding figures were 26 and 20%. Heavy drinking, binge drinking and physical inactivity both at baseline and at follow-up, as well as smoking at follow-up, were more prevalent among women who reported frequent insomnia symptoms at baseline than among those who reported no or occasional insomnia symptoms (Table 1). Among men, physical inactivity at baseline and follow-up was more prevalent among those who reported frequent insomnia symptoms at baseline than among those who reported no or occasional insomnia symptoms.

Table 1. Prevalence (%) of unhealthy behaviours at baseline and follow-up by insomnia symptoms at baseline among women and men
 Baseline insomnia symptoms
Women (= 5248)Men (= 1210)
No or occasional (%)Frequent (%)No or occasional (%)Frequent (%)
  1. Chi2-test **< 0.01, ***< 0.001, no or occasional versus frequent insomnia symptoms (separate tests at baseline and follow-up).

Heavy drinking
 Baseline912**78
 Follow-up1114**1013
Binge drinking
 Baseline1722***2430
 Follow-up1721**2732
Smoking
 Baseline22232524
 Follow-up1720**2222
Unhealthy food habits
 Baseline10122221
 Follow-up891819
Physical inactivity
 Baseline2328***2330**
 Follow-up2027***2330**

Frequent insomnia symptoms at baseline and follow-up were more prevalent among women who reported heavy and binge drinking, as well as physical inactivity at baseline (Table 2). Among men, frequent insomnia symptoms at follow-up were more prevalent among those who reported binge drinking at baseline and frequent insomnia symptoms at baseline were more prevalent among those who reported physical inactivity at baseline.

Table 2. Prevalence (%) of frequent insomnia symptoms at baseline and follow-up by unhealthy behaviours at baseline among women and men
 Baseline health behaviours
Heavy drinkingBinge drinkingSmokingUnhealthy food habitsPhysical inactivity
NoYesNoYesNoYesNoYesNoYes
  1. Chi2-test *< 0.05, **< 0.01, ***< 0.001, healthy behaviours versus unhealthy behaviours (separate tests at baseline and follow-up).

Women (= 5248)
 Insomnia symptoms
  Baseline2126**2025**212221242025***
  Follow-up2636***2533***262927272630**
Men (= 1210)
 Insomnia symptoms
  Baseline17191621171617171621*
  Follow-up19251824*192119221823

Logistic regression analysis was next used to examine the associations among insomnia symptoms and unhealthy behaviours. First, we examined whether insomnia symptoms at baseline were associated with subsequent unhealthy behaviours (Table 3). Frequent insomnia symptoms were associated with subsequent heavy drinking after adjusting for gender and age (model 1, OR: 1.42; 95% CI: 1.18–1.70). Adjusting for all covariates had minor effects on the examined associations (model 4, OR: 1.34; 95% CI: 1.07–1.68). Frequent insomnia symptoms were associated with subsequent binge drinking after adjusting for gender and age (model 1, OR: 1.37; 95% CI: 1.18–1.60). Frequent insomnia symptoms were also associated with subsequent smoking after adjusting for gender and age (model 1, OR: 1.23; 95% CI: 1.05–1.43), and this association strengthened slightly after model 1 was adjusted additionally for baseline smoking (model 2, OR: 1.37; 95% CI: 1.06–1.76). After mutually adjusting for all other covariates, the association remained (model 3, OR: 1.32; 95% CI: 1.02–1.71). After model 3 was adjusted additionally for common mental disorders (model 4, OR: 1.21; 95% CI: 0.93–1.59), the association lost statistical significance. Frequent insomnia symptoms were also associated with subsequent physical inactivity after adjusting for gender and age (model 1, OR: 1.46; 95% CI: 1.27–1.67). After mutually adjusting for all covariates, the association between frequent insomnia symptoms and physical inactivity attenuated, but remained (model 4, OR: 1.27; 95% CI: 1.08–1.48).

Table 3. Associations between insomnia symptoms at baseline and unhealthy behaviours at follow-up. Odds ratios (OR) and their 95% confidence intervals (CI) from logistic regression models
 Women and men (= 6458)
Model 1 OR (95% CI)Model 2 OR (95% CI)Model 3 OR (95% CI)Model 4 OR (95% CI)
  1. Model 1: age, gender.

  2. Model 2: age, gender, corresponding unhealthy behaviour at baseline.

  3. Model 3: age, gender, corresponding unhealthy behaviour at baseline, marital status, occupational class, sleep duration.

  4. Model 4: age, gender, corresponding unhealthy behaviour at baseline, marital status, occupational class, sleep duration, common mental disorders.

Health behaviours at follow-up
 Heavy drinking1.42 (1.18–1.70)1.35 (1.09–1.66)1.33 (1.07–1.66)1.34 (1.07–1.68)
 Binge drinking1.37 (1.18–1.60)1.17 (0.97–1.41)1.13 (0.94–1.37)1.13 (0.92–1.38)
 Smoking1.23 (1.05–1.43)1.37 (1.06–1.76)1.32 (1.02–1.71)1.21 (0.93–1.59)
 Unhealthy food habits1.16 (0.95–1.41)1.10 (0.90–1.37)1.08 (0.87–1.34)1.05 (0.84–1.32)
 Physical inactivity1.46 (1.27–1.67)1.37 (1.18–1.58)1.34 (1.15–1.55)1.27 (1.08–1.48)

Secondly, we examined whether unhealthy behaviours at baseline are associated with subsequent insomnia symptoms at follow-up (Table 4). Heavy drinking (model 1, OR: 1.57; 95% CI: 1.31–1.88) and binge drinking (model 1, OR: 1.42; 95% CI: 1.23–1.63) were associated with subsequent insomnia symptoms after adjusting for gender and age and the associations remained after full adjustment (model 4, OR: 1.48; 95% CI: 1.22–1.80; and model 4, OR: 1.26; 95% CI: 1.08–1.46). Physical inactivity was also associated with subsequent insomnia symptoms after adjusting for gender and age (model 1, OR: 1.27; 95% CI: 1.11–1.44). After adjustment for other covariates the association slightly attenuated (model 3, OR: 1.16; 95% CI: 1.01–1.33), but remained. However, when model 3 was adjusted additionally for common mental disorders (model 4, OR: 1.13; 95% CI: 0.98–1.30), the association weakened and statistical significance was lost.

Table 4. Association between unhealthy behaviours at baseline and insomnia symptoms at follow-up. Odds ratios (OR) and their 95% confidence intervals (CI) from logistic regression models
 Women and men (= 6458)
Baseline
Heavy drinking OR (95% CI)Binge drinking OR (95% CI)Smoking OR (95% CI)Unhealthy food habits OR (95% CI)Physical inactivity OR (95% CI)
  1. Model 1: age, gender.

  2. Model 2: age, gender, baseline insomnia symptoms.

  3. Model 3: age, gender, baseline insomnia symptoms, marital status, occupational class, sleep duration.

  4. Model 4: age, gender, baseline insomnia symptoms, marital status, occupational class, sleep duration, common mental disorders.

Follow-up insomnia symptoms
 Model 11.57 (1.31–1.88)1.42 (1.23–1.63)1.13 (0.99–1.30)1.09 (0.91–1.29)1.27 (1.11–1.44)
 Model 21.51 (1.24–1.83)1.29 (1.11–1.50)1.11 (0.96–1.28)1.03 (0.86–1.24)1.17 (1.02–1.34)
 Model 31.51 (1.24–1.85)1.29 (1.11–1.50)1.08 (0.94–1.25)1.00 (0.83–1.20)1.16 (1.01–1.33)
 Model 41.48 (1.22–1.80)1.26 (1.08–1.46)1.06 (0.91–1.23)0.98 (0.81–1.18)1.13 (0.98–1.30)

Discussion

  1. Top of page
  2. Summary
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Acknowledgements
  9. Disclosure Statement
  10. References

This study sought to examine, firstly, the associations between insomnia symptoms and subsequent unhealthy behaviours, and secondly, the associations between unhealthy behaviours and subsequent insomnia symptoms. Overall, insomnia symptoms had only few associations with subsequent unhealthy behaviours. Unhealthy behaviours also contributed little to subsequent insomnia symptoms. The associations between insomnia symptoms and heavy drinking were bidirectional, while binge drinking at baseline was associated with subsequent insomnia symptoms at follow-up. Additionally, frequent insomnia symptoms at baseline were associated with subsequent physical inactivity.

Interpretation

The associations between insomnia symptoms and heavy alcohol drinking were bidirectional. This suggests that heavy drinking may lead to difficulties in sleep maintenance (Gann et al., 2004), but insomnia symptoms may also lead to heavy drinking, because drinking can be used as a sleep aid (Stein and Friedmann, 2005). A previous prospective study of the association of insomnia symptoms among those with chronic and remitted alcohol dependence reported that chronic alcohol dependence is associated with subsequent insomnia symptoms (Crum et al., 2004). Although we lacked information on alcohol dependence, our results on heavy and binge drinking may point in the same direction. The association between binge drinking and subsequent insomnia symptoms was only one-way. A review suggested that two or three drinks before sleep could promote sleep onset, but that the effect diminishes in 3 days of continued use (Stein and Friedmann, 2005), which suggests why alcohol use, including binge drinking, may lead to insomnia symptoms. Our findings are in line with this review. Heavy and binge drinking represent different drinking habits and can also have different effects depending on the insomnia symptoms.

Our study showed that insomnia symptoms may be associated with unhealthy behaviours. Frequent insomnia symptoms at baseline predicted subsequent physical inactivity. This association may be accounted for partly by tiredness, which can affect the motivation for exercise. Also, physical inactivity at baseline predicted subsequent insomnia symptoms, but the association disappeared when we adjusted for common mental disorders. This finding lent support for the result of an 8-year prospective study among elderly general practice patients, showing that lower levels of physical activity at baseline were associated with subsequent insomnia symptoms (Morgan and Clarke, 1997), suggesting that physical activity may improve sleep (Lira et al., 2011).

A retrospective study showed that smoking was associated with increased risk of insomnia symptoms (Wallander et al., 2007), which is consistent with our results, except that when we adjusted additionally for common mental disorders the association disappeared. Some studies have suggested that the association between smoking and insomnia symptoms may be bidirectional (Kageyama et al., 2005; Patten et al., 2000), but our study did not find such an association. Our study does not disclose the mechanisms through which insomnia symptoms could contribute to smoking, but suffering from insomnia symptoms may be stressful and enhance or lead to smoking.

We could not confirm the bidirectional associations after full adjustment among insomnia symptoms and binge drinking, smoking, unhealthy food habits or physical inactivity. We lack previous studies among insomnia symptoms and unhealthy food habits. Covariates did not have major effects on the examined associations.

Methodological considerations

Our survey questions were all self-reported, which could have led to either under- or over-reporting of unhealthy behaviours and insomnia symptoms. However, insomnia symptoms were measured by a commonly used and validated measure (Jenkins et al., 1988), although we were unable to examine daytime impairment due to insomnia, which is considered to be a limitation (Haaramo et al., 2012). Alcohol consumption and physical activity were also examined, using standard questions (Aalto et al., 1999; Kujala et al., 1998). Food habits were examined by a FFQ, which is a common dietary assessment tool used in epidemiological studies (Erkkola et al., 2001). We were unable to confirm any associations among insomnia symptoms and unhealthy food habits, due possibly to our somewhat general questions on food habits. We lacked also information on the actual amount of food consumption. Healthy food habits can also be overestimated and unhealthy food consumption underestimated. Our categorization of food habits as healthy or unhealthy was only suggestive. Furthermore, dichotomization may reduce the statistical power of analyses.

The analyses were stratified additionally by gender. The associations among insomnia symptoms and unhealthy behaviours were similar among men and women (data not shown). However, the associations reached statistical significance only among women.

There were some limitations to this study. We lacked data about additional sleep disorders, such as sleep apnoea and excessive daytime sleepiness, and these also may have bidirectional associations with unhealthy behaviours. Our measure of insomnia symptoms is not specific and it is possible that it also reflects different sleep disorders, which could not be assessed in this study. Mental health can contribute to the association between insomnia symptoms and unhealthy behaviours. It may be a linking mechanism, but the relationships between insomnia symptoms and mental health are complex (Roth, 2009; Roth and Roehrs, 2003). In this study, the associations attenuated slightly after adjusting for common mental disorders, but mainly remained. Because the follow-up time was 5–7 years, several other factors may have affected the results over time; for example, physical inactivity and excessive alcohol consumption may have led to weight gain (Sayon-Orea et al., 2011; WHO, 2000), and contributed to the associations among insomnia symptoms and unhealthy behaviours.

We were unable to confirm any associations between smoking, unhealthy food habits and subsequent insomnia symptoms. There was also no association between insomnia symptoms and subsequent unhealthy food habits. In the future, it would be important to examine whether the different insomnia symptoms have different predictive power to unhealthy behaviours and vice versa.

The strengths of this study were a large data set and a follow-up design. Our non-response analyses suggest that the data are broadly representative of the target population (Laaksonen et al., 2008). However, our results cannot be generalized to the adult population, because we studied middle-aged people employed at baseline. We used a large number of covariates and all key unhealthy behaviours. The questions at baseline and follow-up were identical.

Conclusions

  1. Top of page
  2. Summary
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Acknowledgements
  9. Disclosure Statement
  10. References

Associations among insomnia symptoms and unhealthy behaviours were mainly weak and non-existent. However, insomnia symptoms were associated with subsequent heavy drinking and physical inactivity, and heavy and binge drinking were also associated with subsequent insomnia symptoms.

Acknowledgements

  1. Top of page
  2. Summary
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Acknowledgements
  9. Disclosure Statement
  10. References

The Helsinki Health Study is supported by the Academy of Finland (grants 1129225, 1121748, 1135630, 1257362) and the Finnish Work Environment Fund (grants 107187, 107281). P. H. is supported by the Juho Vainio Foundation (grant 4702432), the Finnish Cultural Foundation (grant 4702578), the Doctoral Programs in Public Health, and T. L. is supported by the Academy of Finland (grant 133434).

Disclosure Statement

  1. Top of page
  2. Summary
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Acknowledgements
  9. Disclosure Statement
  10. References

None of the authors have any conflicts of interest to declare.

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  2. Summary
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Acknowledgements
  9. Disclosure Statement
  10. References
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