Crazy Work Hours Do Not Drive Dementia


A look at the impact of shift work and long hours on cognitive function.

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Shift work and long work hours have their health detriments, but dementia risk does not appear to be one of them, according to a large population-based Danish study.1 The authors pointed out that even though life expectancy is increasing in Europe and the United States, the rate of dementia may be decreasing.

The decline may be due to improved living conditions and lifestyle habits-modifiable risk factors, that is. Shift work, particularly night work, and long working hours have been suspected of being modifiable risk factors of dementia in light of their effect on sleep, cortisol metabolism, and cardiovascular health. Studies have been few and the findings equivocal regarding the association between shift work/long work hours, compromised cognitive function, and dementia risk. The current study, which drew data from the Copenhagen Male Study (CMS), sought to at least clarify the issue of whether shift work affects dementia risk.

Study design
The CMS first captured population-based data for analysis in 1970-1971 and was inclusive of 5249 male employees age 40 to 59 years from 14 large companies in the Copenhagen area. Of the original cohort, 3387 surviving men participated in a follow-up study, which captured data via a survey conducted in 1985-1986. The current study reviewed data focused on exposure to shift work collected from 4766 participants, including 1597 from the follow-up population who met study inclusion criteria: current residence in Denmark and freedom from dementia for the first 5 years after participating in the 1985-1986 assessment. The association between shift work/long working hours and a later dementia diagnosis was made using Poisson regression, with results presented as incidence risk ratios (IRRs) and 95% confidence intervals (CIs).

Data analysis
Analysis of data from the 1970-1971 survey showed that 18% of participants engaged in long work hours, defined as 45 or more hours per week, and 21% were shift workers. At follow-up, 18% were shift workers.

Participants with long work hours, 41% of whom belonged to the 2 highest socioeconomic groups, were determined to be at greater risk for dementia in relation to their sleep, work stress, and leisure-time stress profiles and at lower risk in terms of smoking status, physical activity during leisure-time, age, and body mass index (BMI). Shift workers, many of whom were socioeconomically disadvantaged, were at greater risk for dementia in relation to their sleep, work stress, smoking, and BMI profiles and at lower risk in terms of leisure-time stress and cardiovascular disease profiles, physical activity during leisure-time, and age.

Dementia was ultimately diagnosed in 634 participants (13%), and the mean age at diagnosis was 80 years. When IRRs were examined and adjustments made for confounders, no statistical difference was seen between shift work (IRR, 0.86; 95% CI, 0.70-1.05) or long working hours (IRR, 0.97; 95% CI, 0.79-1.19) and dementia. The researchers did find that lower socioeconomic status (IRR, 1.11; 95% CI, 1.04-1.18), cardiovascular disease (IRR, 1.25; 95% CI, 1.03-1.53), and consumption of 3 or more units of alcohol per day (IRR, 1.52; 95% CI, 1.21-1.91) were significantly associated with a higher incidence of dementia in the follow-up cohort.

The bottom line
Although the study found no association between shift work or long work hours and dementia risk, the authors called for continued research in efforts to reach an evidence-based consensus on the impact of shift work on dementia risk.


1. Nabe-Nielsen K, Garde AH, Ishtiak-Ahmed K, et al. Shift work, long working hours, and later risk of dementia: a long-term follow-up of the Copenhagen Male Study. Scand J Work Environ Health. 2017 Jul 14. [Epub ahead of print]

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