title: Long working hours, socioeconomic status, and the risk of incident type 2 diabetes: a meta-analysis of published and unpublished data from 222 120 individuals.
abstract: BACKGROUND
  Working long hours might have adverse health effects, but whether this is true for all socioeconomic status groups is unclear. In this meta-analysis stratified by socioeconomic status, we investigated the role of long working hours as a risk factor for type 2 diabetes.
  
  
  METHODS
  We identified four published studies through a systematic literature search of PubMed and Embase up to April 30, 2014. Study inclusion criteria were English-language publication; prospective design (cohort study); investigation of the effect of working hours or overtime work; incident diabetes as an outcome; and relative risks, odds ratios, or hazard ratios (HRs) with 95% CIs, or sufficient information to calculate these estimates. Additionally, we used unpublished individual-level data from 19 cohort studies from the Individual-Participant-Data Meta-analysis in Working-Populations Consortium and international open-access data archives. Effect estimates from published and unpublished data from 222 120 men and women from the USA, Europe, Japan, and Australia were pooled with random-effects meta-analysis.
  
  
  RESULTS
  During 1·7 million person-years at risk, 4963 individuals developed diabetes (incidence 29 per 10 000 person-years). The minimally adjusted summary risk ratio for long (≥55 h per week) compared with standard working hours (35-40 h) was 1·07 (95% CI 0·89-1·27, difference in incidence three cases per 10 000 person-years) with significant heterogeneity in study-specific estimates (I(2)=53%, p=0·0016). In an analysis stratified by socioeconomic status, the association between long working hours and diabetes was evident in the low socioeconomic status group (risk ratio 1·29, 95% CI 1·06-1·57, difference in incidence 13 per 10 000 person-years, I(2)=0%, p=0·4662), but was null in the high socioeconomic status group (1·00, 95% CI 0·80-1·25, incidence difference zero per 10 000 person-years, I(2)=15%, p=0·2464). The association in the low socioeconomic status group was robust to adjustment for age, sex, obesity, and physical activity, and remained after exclusion of shift workers.
  
  
  CONCLUSIONS
  In this meta-analysis, the link between longer working hours and type 2 diabetes was apparent only in individuals in the low socioeconomic status groups.
  
  
  BACKGROUND
  Medical Research Council, European Union New and Emerging Risks in Occupational Safety and Health research programme, Finnish Work Environment Fund, Swedish Research Council for Working Life and Social Research, German Social Accident Insurance, Danish National Research Centre for the Working Environment, Academy of Finland, Ministry of Social Affairs and Employment (Netherlands), Economic and Social Research Council, US National Institutes of Health, and British Heart Foundation.
authors.affiliation: Department of Epidemiology and Public Health, University College London, London, UK; Hjelt Institute, Medical Faculty, University of Helsinki, Helsinki, Finland. Electronic address: m.kivimaki@ucl.ac.uk.
authors.forename: Mika
authors.initials: M
authors.lastname: Kivimäki
authors.affiliation: Finnish Institute of Occupational Health, Helsinki, Tampere, and Turku, Finland.
authors.forename: Marianna
authors.initials: M
authors.lastname: Virtanen
authors.affiliation: Harvard School of Public Health, Department of Society, Human Development and Health, Boston, MA, USA.
authors.forename: Ichiro
authors.initials: I
authors.lastname: Kawachi
authors.affiliation: Finnish Institute of Occupational Health, Helsinki, Tampere, and Turku, Finland.
authors.forename: Solja T
authors.initials: ST
authors.lastname: Nyberg
authors.affiliation: Centre for Occupational and Environmental Medicine, Stockholm County Council, Sweden; Institute of Environmental Medicine, Karolinska Institutet, Stockholm, Sweden.
authors.forename: Lars
authors.initials: L
authors.lastname: Alfredsson
authors.affiliation: Department of Epidemiology and Public Health, University College London, London, UK; Centre for Cognitive Ageing and Cognitive Epidemiology, University of Edinburgh, Edinburgh, UK; Alzheimer Scotland Dementia Research Centre, University of Edinburgh, Edinburgh, UK.
authors.forename: G David
authors.initials: GD
authors.lastname: Batty
authors.affiliation: National Research Centre for the Working Environment, Copenhagen, Denmark.
authors.forename: Jakob B
authors.initials: JB
authors.lastname: Bjorner
authors.affiliation: Department of Occupational and Environmental Medicine, Bispebjerg University Hospital, Copenhagen, Denmark.
authors.forename: Marianne
authors.initials: M
authors.lastname: Borritz
authors.affiliation: Department of Epidemiology and Public Health, University College London, London, UK.
authors.forename: Eric J
authors.initials: EJ
authors.lastname: Brunner
authors.affiliation: Federal Institute for Occupational Safety and Health, Berlin, Germany.
authors.forename: Hermann
authors.initials: H
authors.lastname: Burr
authors.affiliation: Institute for Medical Sociology, Medical Faculty, University of Düsseldorf, Düsseldorf, Germany.
authors.forename: Nico
authors.initials: N
authors.lastname: Dragano
authors.affiliation: Department of Epidemiology and Public Health, University College London, London, UK; School of Social and Community Medicine, University of Bristol, Bristol, UK.
authors.forename: Jane E
authors.initials: JE
authors.lastname: Ferrie
authors.affiliation: Institute of Environmental Medicine, Karolinska Institutet, Stockholm, Sweden; School of Health Sciences, Jönköping University, Jönköping, Sweden; Stress Research Institute, Stockholm University, Stockholm, Sweden.
authors.forename: Eleonor I
authors.initials: EI
authors.lastname: Fransson
authors.affiliation: Department of Epidemiology and Public Health, University College London, London, UK.
authors.forename: Mark
authors.initials: M
authors.lastname: Hamer
authors.affiliation: Finnish Institute of Occupational Health, Helsinki, Tampere, and Turku, Finland; School of Medicine, University of Tampere, Tampere, Finland.
authors.forename: Katriina
authors.initials: K
authors.lastname: Heikkilä
authors.affiliation: Department of Health Sciences, Mid Sweden University, Sundsvall, Sweden.
authors.forename: Anders
authors.initials: A
authors.lastname: Knutsson
authors.affiliation: Department of Public Health, University of Helsinki, Helsinki, Finland.
authors.forename: Markku
authors.initials: M
authors.lastname: Koskenvuo
authors.affiliation: National Research Centre for the Working Environment, Copenhagen, Denmark.
authors.forename: Ida E H
authors.initials: IEH
authors.lastname: Madsen
authors.affiliation: Department of Occupational and Environmental Medicine, Bispebjerg University Hospital, Copenhagen, Denmark.
authors.forename: Martin L
authors.initials: ML
authors.lastname: Nielsen
authors.affiliation: Department of Psychology, Umeå University, Umeå, Sweden.
authors.forename: Maria
authors.initials: M
authors.lastname: Nordin
authors.affiliation: Finnish Institute of Occupational Health, Helsinki, Tampere, and Turku, Finland.
authors.forename: Tuula
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authors.lastname: Oksanen
authors.affiliation: Danish National Centre for Social Research, Copenhagen, Denmark.
authors.forename: Jan H
authors.initials: JH
authors.lastname: Pejtersen
authors.affiliation: Finnish Institute of Occupational Health, Helsinki, Tampere, and Turku, Finland.
authors.forename: Jaana
authors.initials: J
authors.lastname: Pentti
authors.affiliation: National Research Centre for the Working Environment, Copenhagen, Denmark; Department of Public Health and Department of Psychology, University of Copenhagen, Copenhagen, Denmark.
authors.forename: Reiner
authors.initials: R
authors.lastname: Rugulies
authors.affiliation: Finnish Institute of Occupational Health, Helsinki, Tampere, and Turku, Finland; Department of Psychology, University of Turku, Turku, Finland.
authors.forename: Paula
authors.initials: P
authors.lastname: Salo
authors.affiliation: Institute for Medical Sociology, Medical Faculty, University of Düsseldorf, Düsseldorf, Germany.
authors.forename: Johannes
authors.initials: J
authors.lastname: Siegrist
authors.affiliation: Department of Epidemiology and Public Health, University College London, London, UK.
authors.forename: Andrew
authors.initials: A
authors.lastname: Steptoe
authors.affiliation: Folkhälsan Research Center, Helsinki, Finland; Nordic School of Public Health, Göteborg, Sweden; Department of Public Health, University of Turku, Turku, Finland.
authors.forename: Sakari
authors.initials: S
authors.lastname: Suominen
authors.affiliation: Stress Research Institute, Stockholm University, Stockholm, Sweden.
authors.forename: Töres
authors.initials: T
authors.lastname: Theorell
authors.affiliation: Finnish Institute of Occupational Health, Helsinki, Tampere, and Turku, Finland; Department of Public Health, University of Turku, Turku, Finland; Turku University Hospital, Turku, Finland.
authors.forename: Jussi
authors.initials: J
authors.lastname: Vahtera
authors.affiliation: Occupational and Environmental Medicine, Uppsala University, Uppsala, Sweden.
authors.forename: Peter J M
authors.initials: PJM
authors.lastname: Westerholm
authors.affiliation: Stress Research Institute, Stockholm University, Stockholm, Sweden.
authors.forename: Hugo
authors.initials: H
authors.lastname: Westerlund
authors.affiliation: Department of Epidemiology and Public Health, University College London, London, UK; Inserm U1018, Centre for Research in Epidemiology and Population Health, Villejuif, France.
authors.forename: Archana
authors.initials: A
authors.lastname: Singh-Manoux
authors.affiliation: Institute of Behavioral Sciences, University of Helsinki, Helsinki, Finland.
authors.forename: Markus
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authors.lastname: Jokela
mesh_terms: D000328:Adult; D003924:Diabetes Mellitus, Type 2 / Q000453:epidemiology / Q000209:etiology*; D005260:Female; D006801:Humans; D015994:Incidence; D008297:Male; D008875:Middle Aged; D012307:Risk Factors; D012923:Social Class; D014940:Work Schedule Tolerance