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To the Editor:
Conway et al conducted a retrospective cohort study to examine the presence of a dose-response relationship between working hours and incident cardiovascular disease (CVD).1 The authors clarified that compared with working 45 hours per week, working an additional 10 hours per week or more for at least 10 years increased CVD risk by at least 16%. I have some concerns about their study.
In past research, sleep disturbance was associated with long working hours,2 and long working hours and short sleep duration were independent risks for CVD.3 There is a trade-off relationship between working hours and sleep duration,4,5 and sleep parameters should be considered as adjusting factors for the risk assessment of incident CVD.
Relating to the first query, Hoevenaar-Blom et al6 conducted a long-term cohort study with number of events, and clarified that short sleepers, especially those with poor sleep quality, had an increased risk of incident CVD. They recommended that the combination of sleep duration and its quality should be considered for the analysis. In addition, Cappuccio et al7 conducted a systematic review by meta-analysis of prospective studies and concluded that both short and long duration of sleep were predictors for morbidity and mortality from CVD. A causal relationship between long sleep duration and incident CVD should be specified by further study.
Finally, Nakata8 reported that workplace depression from long work hours was significantly associated with sleep deprivation. Namely, mental status is also an important factor to know the effect of long working hours on incident CVD. In any case, there are other risk factors for CVD in the workplace such as shift work and personal relationships, and the relationship between working hours and CVD risk should be adequately adjusted.
1. Conway SH, Pompeii LA, Roberts RE, et al. Dose-response relation between work hours and cardiovascular disease risk: findings from the panel study of income dynamics. J Occup Environ Med
2. Kim BH, Lee HE. The association between working hours and sleep disturbances according to occupation and gender. Chronobiol Int
3. Cheng Y, Du CL, Hwang JJ, et al. Working hours, sleep duration and the risk of acute coronary heart disease: a case-control study of middle-aged men in Taiwan. Int J Cardiol
4. Nakashima M, Morikawa Y, Sakurai M, et al. Association between long working hours and sleep problems in white-collar workers. J Sleep Res
5. Virtanen M, Ferrie JE, Gimeno D, et al. Long working hours and sleep disturbances: the Whitehall II prospective cohort study. Sleep
6. Hoevenaar-Blom MP, Spijkerman AM, Kromhout D, et al. Sleep duration and sleep quality in relation to 12-year cardiovascular disease incidence: the MORGEN study. Sleep
7. Cappuccio FP, Cooper D, D’Elia L, et al. Sleep duration predicts cardiovascular outcomes: a systematic review and meta-analysis of prospective studies. Eur Heart J
8. Nakata A. Work hours, sleep sufficiency, and prevalence of depression among full-time employees: a community-based cross-sectional study. J Clin Psychiatry