Global cognitive function declined faster in people with either very short or very long sleep duration than in people who slept 7 hours a night, combined data from England and China showed.
Over 100,000 person-years of follow-up, cognitive z scores had a pooled β of -0.022 (95% CI -0.035 to -0.009 SD per year) with four or fewer hours of sleep a night and a pooled β of -0.033 (95% CI -0.054 to -0.011 SD per year) with 10 or more hours, in adjusted analyses, reported Wuxiang Xie, PhD, of the Peking University Clinical Research Institute in Beijing, and co-authors.
Extreme sleep duration also was associated with lower cognitive function at baseline, they wrote in JAMA Network Open.
Findings were consistent in English and Chinese cohorts, despite cultural differences. An inverted U-shaped association between sleep duration and change in global cognition score emerged, which also was seen in longitudinal analyses for memory scores, but not executive function or orientation scores.
How sleep duration and cognitive decline are linked is one of the most studied and controversial topics in recent years, observed Yue Leng, MD, PhD, and Kristine Yaffe, MD, both of the University of California San Francisco, in an accompanying editorial.
Earlier research has shown conflicting results: small studies have produced inconsistent findings, perhaps due to lack of statistical power, while large studies and meta-analyses have identified mostly inverted U-shaped associations, Leng and Yaffe said.
This pooled study from England and China presented a unique opportunity to investigate extreme categories of sleep duration: people who sleep four hours or less per night, and people who sleep 10 hours or more, they noted.
“The associations were small, especially for longitudinal analyses,” they wrote. “Worse cognitive function was also observed for 8 or 9 vs 7 hours of sleep in cross-sectional analyses but not longitudinal analyses.”
While the researchers “should be commended for the large sample size of their study and the ability to examine extreme sleep duration, the interpretation of its public health relevance has been limited by the small effect sizes and the low prevalence of extreme sleep duration among the general population,” they added.
In this analysis, Xie and collaborators followed 20,065 adults in two cohorts: 9,254 people from the English Longitudinal Study of Aging and 10,811 individuals from the China Health and Retirement Longitudinal Study.
In the English study, the average age of English participants was 64.6; 56% were women and median follow-up was 8 years. In the Chinese study, average age was 57.8, 50% were women, and median follow-up was four years.
Differences in findings between the two cohorts may be due to genetics, culture, environment, lifestyle, or a combination of variables, “yet almost nothing is known about how these factors might contribute to the association between sleep duration and cognition in different populations,” Leng and Yaffe said. Over time, Chinese participants with more extreme sleep duration had faster memory decline than their English counterparts.
The cause of a U-shaped association between sleep duration and cognitive outcomes is not clear, the editorialists noted. “Although ample evidence supports the negative impact of sleep deprivation on cognitive aging through the promotion of amyloid and tau deposition, endothelial dysfunction, and inflammation, there are no known biological mechanisms that explain why long sleep duration might cause cognitive impairment,” they wrote.
Subjective sleep reports have limitations and long sleep durations may reflect other sleep disorders, comorbidities, medication use, or frailty, Leng and Yaffe said.
Extreme sleep duration could be an early manifestation of brain impairment and reverse causality is possible, Xie and co-authors pointed out. Cognitive function was measured using isolated tasks, which may not have been sensitive enough to detect subtle declines, they added.