Background
In recent years, significant research efforts have focused on identifying modifiable risk factors to be leveraged for dementia prevention. Sleep is required for memory consolidation and normal brain functioning, and various mechanisms are proposed between poor sleep quality and cognitive impairment. Several studies have examined the role of sleep disturbances as a risk factor for dementia; however, most of these studies have involved smaller cohorts and short time intervals, and often rely on retrospective surveys and self-reported exposure data. Our aim was to examine the association between sleep disorders and late-onset dementia in an entire population.

Method
In a nationwide cohort with a 40-year follow up, we linked nationwide population data with information on hospital-based sleep disorder diagnoses and late-onset dementia from the extensive Danish health registers. Incidence rate ratios (IRR) were calculated using Poisson regression analysis. Analyses were adjusted for age, sex, calendar year, education, and somatic and psychiatric comorbidities.

Result
The cohort consisted of 1,491,276 people. Those with any hospital-diagnosed sleep disorder had a 17% higher risk of dementia (IRR 1.17, 95% CI 1.11-1.24) compared to people with no sleep disorder diagnosis. The highest risk was found for men with ‘other specific disorders’ (e.g. insomnia, hypersomnia, non-organic sleep disorders) with an IRR of 2.00 (95% CI 1.71-2.32). The risk of dementia was significantly increased 0 to 5 years after sleep disorder diagnosis (IRR 1.35, 95% CI 1.25-1.47), while the association after 5 years or more was non-significant (1.05, 95% CI 0.97-1.13).

Conclusion
Our findings indicate an increased short-term risk of dementia following a hospital-based sleep disorder diagnosis, while we found weaker evidence of a long-term risk. While this may indicate that sleep disorders could be early symptoms of dementia as well as a risk factor, further research is needed to evaluate sleep disorders as potential early symptoms of dementia, risk factors for dementia, or both, and characterize how this may change at different points along the dementia course. Increased knowledge of the role of sleep disorders in dementia could help inform efforts towards diagnosing dementia in the early stages of the disease or even in dementia prevention.