BOSTON—Midlife shift work is associated with an increased risk of incident dementia in later life, according to research presented at the 31st Annual Meeting of the Associated Professional Sleep Societies. The association is stronger with longer duration of shift work, said Kathleen Bokenberger, a doctoral student in the Department of Medical Epidemiology and Biostatistics at the Karolinska Institutet in Stockholm.
“To the best of our knowledge, this is the first study to find a connection between shift work and greater dementia incidence,” said Ms. Bokenberger. “As such, we will need confirmation from other studies, especially if they have more nuanced measures of shift work, and perhaps repeated measures of shift work.”
An Analysis of Two Population-Based Cohorts
Shift work interferes with the circadian and homeostatic regulation of sleep and can lead to insomnia and excessive sleepiness. A prospective Danish study found that rotating and evening shift work were associated with increased dementia mortality, but the link between shift work and dementia is still relatively unexplored, said Ms. Bokenberger.
She and her colleagues followed two population-based cohorts from the Swedish Twin Registry. The first cohort, which the researchers called STR-1973, included 13,283 people who were age 30 or older when they received a mailed questionnaire in 1973 that elicited information about shift work status and duration. The Screening Across the Lifespan Twin (SALT) sample included 41,199 participants who participated in a telephone interview between 1998 and 2002 that asked about status and duration of night work history.
The primary outcome was all-cause dementia, and dementia diagnoses were obtained from Swedish national health registers. Investigators followed participants until the date of dementia diagnosis, death, or the end of follow-up in 2015.
A Dose-Response Relationship
The mean age at baseline was 38 in the STR-1973 cohort and 58 in the SALT cohort. Median follow-up time was 41 years in the STR-73 cohort and 14 years in the SALT cohort. The proportion of participants with shift work history was 17% in the STR-1973 cohort. The proportion of people who worked nights was 30% in the SALT cohort. Dementia incidence was 7.4% in the STR-1973 cohort and 5.0% in the SALT cohort.
The researchers used Cox proportional hazard models to analyze the data and adjust for age, sex, education, cardiovascular disease, stroke, and type 2 diabetes. Having shift work versus day work was associated with an increased hazard of dementia (hazard ratio [HR], 1.40), as was having night work versus day work (HR, 1.13). Spline models indicated modest dose-response relationships in which longer duration of shift work and night work predicted greater dementia risk.
Information about APOE e4 genotype was available for subsamples of both cohorts. APOE e4 seemed to be an effect modifier for the association of night work and dementia, but not for that of shift work and dementia. APOE e4 carriers with 20 or more years of shift work had a fourfold increased risk of dementia. APOE e4 carriers with 20 or more years of night work had a twofold increased risk of dementia. The sample sizes were small, however, and “it is possible that these findings are spurious,” said Ms. Bokenberger.
“Individuals in the labor force might consider minimizing shift work exposure or managing work scheduling practices, but this is dependent on individual differences and tolerances to certain types of shift work,” she concluded.
Bokenberger K, Ström P, Dahl Aslan AK, et al. Shift work and cognitive aging: a longitudinal study. Scand J Work Environ Health. 2017 Mar 31 [Epub ahead of print].
Jørgensen JT, Karlsen S, Stayner L, et al. Shift work and overall and cause-specific mortality in the Danish nurse cohort. Scand J Work Environ Health. 2017;43(2):117-126.