Research indicates that fragmented sleep experienced in one's 30s and 40s is associated with poorer cognitive performance a decade later, according to a study published in Neurology. Yue Leng, PhD, from the University of California San Francisco, and colleagues found that individuals in the highest tertile of a sleep fragmentation index had over two times the likelihood of performing poorly on three cognitive assessments compared to those in the lowest tertile.
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The three assessments included the Montreal Cognitive Assessment (MoCA), Letter Fluency and Category Fluency tests, and the Digit Symbol Substitution Test (DSST), revealing significant odds ratios for poor performance: MoCA (OR 2.29), Letter and Category Fluency tests (OR 2.42), and DSST (OR 2.97). Interestingly, neither the objective measure of sleep duration nor subjective sleep quality showed an association with cognitive performance.
Leng noted that Alzheimer's disease pathology can accumulate years before symptoms manifest, suggesting that late-life sleep disturbances might actually result from underlying pathology rather than being a direct cause of cognitive decline. This link raises public health concerns due to the high frequency of sleep issues among older adults.
Further complicating the relationship between sleep and dementia, an extensive 25-year study reported that individuals averaging six hours or less of sleep after age 50 were 30% more likely to be diagnosed with dementia later in life. Additional findings from the U.K. Biobank indicate that sleep regularity during midlife and older age may also be a risk factor for dementia.
The study analyzed data from 526 participants in the CARDIA cohort, with an average baseline age of 40.1 years; 58% were women and 44% were Black. Sleep duration was objectively measured using wrist actigraphy over three consecutive days on two occasions, while sleep quality was subjectively assessed with the Pittsburgh Sleep Quality Index (PSQI). Results showed a mean sleep duration of 6.1 hours, with 45.6% of participants reporting poor sleep quality (PSQI global score > 5).
The sleep fragmentation index—which assessed sleep restlessness—showed a mean of 19.2, with participants in the highest tertile exceeding 20.6. Cognitive performance was evaluated between 2015 and 2016, defining poor performance as scores one or more standard deviations below the mean. The impact of sleep fragmentation on cognitive performance did not significantly vary by race or sex, though no statistically significant relationships emerged for two additional cognitive tests, the Rey Auditory Verbal Learning Test and the Stroop test.
The authors acknowledged limitations, including the inability to control for baseline cognition and assess the relationship beyond the specified timeframe. They emphasized the need for future research to explore earlier life sleep patterns and their cognitive impacts.