Developing Topics.

Background: Sleep disturbances have been suggested as potential risk factors for neurodegenerative diseases. However, longitudinal studies on insomnia and dementia have mostly had too short follow-up time to separate etiological risk factors from prodromal symptoms. Moreover, the impact of mortality as a competing risk has been overlooked. This study aims to clarify the relationship between insomnia symptoms and dementia incidence, considering the competing risk of death, within a large cohort from the Finnish population.

Method: The National FINRISK study, collected every five years from 1972 to 2012, included 63 013 Finns with baseline data. Approximately 12% (7638) developed dementia according to the health care registers. The follow-up time varied from 7 to 47 years (mean = 25 years). We studied association of self-reported insomnia (never, sometimes, often) at baseline with dementia incidence using in parallel Poisson model and Fine-Gray model accounting for the competing risk of death. Additional analysis directly examined this competing risk. Study year, follow-up time, sex, education, and cardiovascular risk factors were adjusted for. Sensitivity analyses included individuals with over 10 years of follow-up.

Result: In the Poisson model, insomnia symptoms "sometimes" (IRR = 1.10[1.05-1.16]) and "often" (IRR = 1.18[1.09-1.28]) showed significant association with incident dementia and a dose-response relationship. The results survived the sensitivity analyses. In the Fine-Gray model, insomnia "sometimes" (HR = 1.07[1.02-1.12) but not "often" (1.07[0.99-1.16]) showed significant associations. Significant associations with the competing risk of death surpassing that of dementia were detected for both categories with IRRs of 1.12[1.09-1.15]) and 1.32[1.27-1.37]), respectively.

Conclusion: The findings from the Poisson model suggest that insomnia may serve as an etiological risk factor for dementia. However, the Fine-Gray model, which focuses on the incidence of dementia in the presence of competing risk, did not show significant association with insomnia "often". This suggests that individuals with insomnia face a higher risk of death before potentially developing dementia, implying that insomnia might not increase the true dementia incidence, despite being a risk factor. These results highlight the significance of including the competing risk of death in dementia epidemiological studies to fully understand the associations with risk factors.