Association between sleep characteristics and incident dementia accounting for baseline cognitive status : A prospective population-based study

Background: While research has shown that sleep disorders are prevalent among people with dementia, the temporal relationship is unclear. We investigated whether atypical sleep characteristics were associated with incident dementia while accounting for baseline cognitive functioning. Methods: Screening Across the Lifespan Twin Study (SALT) participants were 11,247 individuals from the Swedish Twin Registry who were at least 65 years at baseline (1998-2002). Sleep and baseline cognitive functioning were assessed via the SALT telephone screening interview. Data on dementia diagnoses came from national health registers. Cox regression was performed to estimate hazard ratios (HR) for dementia. Results: After 17 years of follow-up, 1,850 dementia cases were identified. Short (≤ 6 hours) and extended (> 9 hours) time-in-bed (TIB) compared to the middle reference group (HR=1.40, 95% CI=1.06-1.85, HR=1.11, 95% CI=1.00-1.24, respectively) and rising at 8:00AM or later compared to earlier rising (HR=1.12, 95% CI=1.01-1.24) were associated with higher dementia incidence. Bedtime, sleep quality, restorative sleep, and heavy snoring were not significant predictors. Findings stratified by baseline cognitive status indicated that the association between short TIB and dementia remained in those cognitively intact at the start. Conclusions: Short and extended TIB as well as delayed rising among older adults predicted increased dementia incidence in the following 17 years. The pattern of findings suggests that extended TIB and late rising represent prodromal features whereas short TIB appeared to be a risk factor for dementia