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People who had fragmented sleep in their 30s and 40s were more likely to have worse cognition a decade later, longitudinal data showed.

Those in the highest versus the lowest tertile of the sleep fragmentation index had over twice the odds of performing poorly on three types of cognitive tests 11 years later, said Yue Leng, PhD, of the University of California San Francisco, and co-authors.

Cognitive performance was worse on the Montreal Cognitive Assessment (MoCA) test of global cognitive function (OR 2.29, 95% CI 1.06-4.94), Letter Fluency and Category Fluency tests (OR 2.42, 95% CI 1.17-5.02), and the Digit Symbol Substitution Test (DSST) of processing speed, executive function, and working memory (OR 2.97, 95% CI 1.34-6.56), the researchers reported in .

Neither objective sleep duration nor subjective sleep quality was associated with cognition.

"Since Alzheimer's disease pathology begins to accumulate in the brain many years before symptoms onset, it's possible that sleep disturbances identified in late life -- close to the time when memory loss becomes apparent -- is actually the consequence of this pathology that has been developing silently over the years," Leng said in a statement.

"Given the long, symptom-free window of Alzheimer's disease and the high prevalence of sleep problems, the understanding of midlife sleep disturbances has significant public health implications," she added.

The relationship between sleep and dementia is complex and as people age. In a study that spanned 25 years of follow-up, people who slept an average of 6 hours or less when they were 50 or older were 30% more later in life. And new data from the U.K. Biobank suggested that sleep regularity in midlife and older ages may be a dementia risk factor.

Leng and colleagues studied 526 participants in the cohort. Mean baseline age was 40.1; 58% were women, and 44% were Black.

From 2003 to 2005, the researchers assessed sleep duration objectively using wrist actigraphy and sleep quality subjectively using the Pittsburgh Sleep Quality Index (PSQI). Actigraphy was measured for 3 consecutive days on two occasions approximately 1 year apart.

Mean sleep duration was 6.1 hours, and 45.6% of participants indicated they had poor sleep quality, defined as a PSQI global score greater than 5.

The sleep fragmentation index -- a measure of restlessness during sleep -- was based on the percentage of time participants were moving and the percentage of time they were immobile. A higher fragmentation index indicated more disrupted sleep.

The mean sleep fragmentation index in the study was 19.2. The highest tertile of participants had an index greater than 20.6, and the lowest tertile had an index less than 15.3.

Cognitive testing occurred from 2015 to 2016, and poor cognitive performance was defined as a score one or more standard deviations below the mean.

The association between sleep fragmentation and cognition did not differ by race or sex. On two other cognitive tests that were administered -- the Rey Auditory Verbal Learning Test and the Stroop test -- sleep fragmentation did not have a statistically significant relationship with cognitive performance.

"Although we examined sleep in the mid-30s to late 40s and cognition 11 years later, we were unable to control for cognition at baseline or study the link between sleep and cognition beyond this period of life," Leng and co-authors acknowledged.

"We also could not assess whether cognition improved with an improvement in sleep quality," they added. "Future studies are needed to determine the impact of sleep on cognition even earlier in life and clarify the direction of this relationship."

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