Long working hours and cancer risk: a multi-cohort study

Short Communication
BJC Open article

British Journal of Cancer (2016) 114, 813–818. doi:10.1038/bjc.2016.9 www.bjcancer.com
Published online 18 February 2016
Long working hours and cancer risk: a multi-cohort study

Katriina Heikkila1,2, Solja T Nyberg2, Ida E H Madsen3, Ernest de Vroome4, Lars Alfredsson5,6, Jacob J Bjorner3, Marianne Borritz7, Hermann Burr8, Raimund Erbel9, Jane E Ferrie10,11, Eleonor I Fransson6,12,13, Goedele A Geuskens4, Wendela E Hooftman4, Irene L Houtman4, Karl-Heinz Jöckel14, Anders Knutsson15, Markku Koskenvuo16, Thorsten Lunau17, Martin L Nielsen18, Maria Nordin13,19, Tuula Oksanen2, Jan H Pejtersen20, Jaana Pentti2, Martin J Shipley10, Andrew Steptoe10, Sakari B Suominen21,22,23, Töres Theorell13, Jussi Vahtera2,21,24, Peter J M Westerholm25, Hugo Westerlund13, Nico Dragano17, Reiner Rugulies3,26, Ichiro Kawachi27, G David Batty10,28, Archana Singh-Manoux10,29, Marianna Virtanen2 and Mika Kivimäki2,10,30 for the IPD-Work Consortium

Correspondence: Dr K Heikkila, E-mail: katriina.heikkila@lshtm.ac.uk

Received 22 September 2015; Revised 10 December 2015; Accepted 26 December 2015
Advance online publication 18 February 2016


Abstract


Background:

Working longer than the maximum recommended hours is associated with an increased risk of cardiovascular disease, but the relationship of excess working hours with incident cancer is unclear.
Methods:

This multi-cohort study examined the association between working hours and cancer risk in 116 462 men and women who were free of cancer at baseline. Incident cancers were ascertained from national cancer, hospitalisation and death registers; weekly working hours were self-reported.

Results:

During median follow-up of 10.8 years, 4371 participants developed cancer (n colorectal cancer: 393; n lung cancer: 247; n breast cancer: 833; and n prostate cancer: 534). We found no clear evidence for an association between working hours and the overall cancer risk. Working hours were also unrelated the risk of incident colorectal, lung or prostate cancers. Working greater than or equal to55 h per week was associated with 1.60-fold (95% confidence interval 1.12–2.29) increase in female breast cancer risk independently of age, socioeconomic position, shift- and night-time work and lifestyle factors, but this observation may have been influenced by residual confounding from parity.
Conclusions:

Our findings suggest that working long hours is unrelated to the overall cancer risk or the risk of lung, colorectal or prostate cancers. The observed association with breast cancer would warrant further research.


Epidemiological research suggests that working long hours has a detrimental effect on health. Extended working hours have been reported as being associated with an increased incidence of coronary heart disease and stroke (Kang et al, 2012; Virtanen et al, 2012; Kivimaki et al, 2015a) pre-term delivery (van Melick et al, 2014) and, in manual occupations, type 2 diabetes (Kivimaki et al, 2015b), as well as a high prevalence of anxiety, depression, sleeping difficulties and accidental injuries at work. (Dembe et al, 2005; Bannai and Tamakoshi, 2014). The relationship between long working hours and cancer, however, is unclear.

Long working hours could impact on cancer risk via their association with lifestyle-related exposures. Observational evidence suggests that working longer than recommended hours is linked to many behavioural cancer risk factors, such as excessive alcohol intake (Virtanen et al, 2015) and physical inactivity (Kirk and Rhodes, 2011; Angrave et al, 2015), possibly because individuals feel that they lack time to exercise because they spend extensive time at work (Escoto et al, 2012). As far as we are aware, the association between long working hours and incident cancer has been examined in only one previous investigation, which had inconclusive findings: in that prospective cohort study the association between working 45 h or longer per week and breast cancer was imprecisely estimated (hazard ratio (HR): 0.93, 95% confidence interval (CI): 0.54, 1.58) and no other cancer outcomes were examined (Nielsen et al, 2008).

To address this evidence gap, we examined the relationship between weekly working hours and the overall incident cancer as well as incident colorectal, lung, breast and prostate cancers using individual participant data from 116 000 men and women from 12 prospective cohort studies from six European countries.

Materials and Methods


The 12 studies in our analyses were conducted between 1992 and 2004 in Denmark, Finland, Germany, Sweden, The Netherlands and UK. All were a part of the Individual-Participant-Data Meta-analysis of Working Populations (IPD-Work) Consortium, a collaborative research effort to investigate the health impact of work-related exposures (Kivimaki et al, 2012). Details of each study’s design, recruitment of participants, data collection and ethics committee approval are provided in Supplementary eAppendix 1.
Participants

Our analyses were based on 116 462 men and women who were working and free of cancer at study baseline, whose records were linked to register-based information on incident cancers and who had complete data available on covariates (Supplementary eAppendix 1 and Supplementary Table S1).


Exposures and outcomes

Weekly working hours were ascertained from baseline self-report questions on usual weekly working hours and defined as the total number of hours in the main job and any secondary jobs (Supplementary eAppendix 2 and Supplementary Table S2).

Cancer events were identified from national cancer, hospitalisation and death registers in all studies apart from one (for details, see Supplementary eAppendix 2). The date of the cancer event was defined as the date of diagnosis or hospital admission due to cancer, whichever was earlier. Cancer cases were categorised according to the type and time of diagnosis of their first cancer. We excluded individuals whose first cancer record came from their death certificate (n=10), as the date of diagnosis for these cancers was uncertain. Codes for the incident cancer events were harmonised using ICD-10 (International Classification of Diseases, version 10) as any cancer (ICD-10 codes C00-C97), colorectal (C18-C20), lung (C34), female breast (C50) and prostate (C61) cancers.


Potential confounders and mediators

Details of the selection and ascertainment of the covariates included in our models are provided in Supplementary eAppendix 2. Briefly, potential confounders were age, sex, socioeconomic position, shift work and night-time work. Potential mediators were smoking, alcohol intake and body mass index (BMI). All covariates, measured at baseline, were harmonised across the studies as reported previously (Heikkila et al, 2012; Heikkilä et al, 2012; Nyberg et al, 2012, 2014).


Statistical analysis

Weekly working hours were analysed as a categorical exposure: <35 h, 35–40 h (reference category: standard working hours for the majority of the workforce in Europe), 41–48 h (the upper limit for the European Union Working Time Directive), 49–54 h and greater than or equal to55 h. Incident cancers (any cancer, colorectal, lung, female breast and prostate cancers) were analysed as binary outcomes. Each participant was followed-up from the date of their baseline assessment to the earliest of the following: incident cancer, death or the end of the registry follow-up. We modelled the associations between working hours and each cancer outcome in each study using Cox proportional hazards regression with the participant's age (i.e., time since birth) as the time scale in the model. Study-specific results were combined using random effects meta-analyses. All statistical analyses were conducted using Stata MP 13 (Stata Corporation, College Station, TX, USA) bar the study-specific analyses in the Danish studies, which were conducted using SAS 9.3 (SAS Institute Inc., Cary, NC, USA) and POLS, which were conducted using SPSS 20.0 (SPSS Inc., Chicago, IL, USA). Results



The characteristics of the 116 462 participants are summarised in Table 1. Overall, these men and women were aged 15–73 at baseline and the majority worked a standard 35–40 h per week, with the study-specific proportions varying from 31 to 71%. During a follow-up ranging from 4 to 22 years (median of study-specific medians: 10.8), 4371 individuals were diagnosed with cancer. Of these, 393 men and women had colorectal cancer and 247 had lung cancer; 833 women developed breast cancer and 534 men prostate cancer.