Changes in cognition during middle-aged and older adulthood

Dementia is a syndrome in which there is deterioration in memory, thinking, behavior and the ability to perform everyday activities. Although dementia affects older people mostly, it is not a normal aging process. Alzheimer’s Disease (AD) and other related dementias contribute to 1 in 3 senior deaths, more than breast cancer and prostate cancer combined. Prevention, or early diagnosis can improve the quality of life for people with dementia and their families. It is expected that over the next 30 years the prevalence of AD will continue to rise and the number will double by 2050. Sleep has been shown to be an important target for intervention to improve health and specifically cognitive function. Both short and long sleep duration as well as poor sleep quality have been associated with cognitive impairment. However, there are limited studies that take a prospective approach, use objectively-measured sleep parameters, and include actual AD confirmed deaths to examine the relationship between sleep in middle age and dementia risk later in life. To address these limitations, this study investigated multiple sleep parameters from both self-report and objective measurement in relation to cognitive outcomes and AD mortality in two large samples of U.S. men and women from the Midlife in the United States (MIDUS) Study and the National Institutes of Health-American Association of Retired Persons-Diet and Health Study (NIH-AARP-Diet and Health Study). The main findings of our results for the MIDUS study demonstrated participants with self-reported long sleep duration (>8 hours/night) had lower composite cognitive scores (p=0.01), and the association was stronger among men and for weekend sleep duration. Additionally, episodic memory was negatively associated with longer sleep duration on both weekdays and weekends for cross-sectional results, while executive function was lower in the overall analysis in individuals sleeping >8 hours/night, particularly in men. There were no statistical significant prospective associations between self-reported sleep variables and changes in cognitive function. Using objectively measured sleep in the MIDUS, we found that <6 hours of total sleep time was associated with greater decline in episodic memory over 10 years of follow-up compared to referent group of 6-7 hours TST. In the NIH-AARP Diet and Health Study, results demonstrated that 9+ hours of sleep was associated with 50% increase in AD mortality risk when compared to the reference group (7-8 hours). In sensitivity analysis where AD deaths within the first 5 years were removed to avoid reverse causation, results remained significant in our main model and when including nap as a covariate. Sex specific analysis revealed that the association between 9+ hours of sleep and higher AD mortality was stronger in men than women. We also found that individuals reporting napping for 1+ hour/day had higher risk of AD death compared to those with no napping, and additionally adjusting for nighttime sleep duration and removing AD deaths within 5 years after baseline had little impact on this association. Results from this study demonstrate that there are many aspects of sleep associated with cognitive outcomes as well as increased risk in AD-specific death. The findings from this study suggest that sleep is a possible target for interventions to address in future studies in preventing or slowing the progression of cognitive decline and progression to AD death.