Extreme sleep duration may impair cognitive function

People who were deprived of sleep (≤4 hours per night) or who slept excessively (≥10 hours per night) saw a faster cognitive decline than those with normal sleep duration (7 hours per night), shows a large cohort study.

During 100,000 person-years of follow-up, individuals with ≤4 hours of sleep per night had significantly faster decline in global cognitive z scores (pooled β=−0.022; p=0.001) than the reference group, after adjusting for age, sex, BMI, systolic blood pressure, education level, smoking status, alcohol consumption, and comorbidities. [JAMA Netw Open 2020;doi:10.1001/jamanetworkopen.2020.13573]

Similar association was also seen in individuals with excessive sleep duration (≥10 hours per night; pooled β=−0.033; p=0.003). The relationship between extreme sleep duration and cognitive decline thus yielded an inverted U-shaped association between the two.

“The inverted U-shaped association indicates that cognitive function should be monitored in middle-aged and older individuals with insufficient or excessive sleep duration,” said the researchers.

The population-based study pooled data from two national ageing cohorts: the English Longitudinal Study of Ageing (ELSA; n=9,254, mean age 64.6 years, 55.9 percent women) and the China Health and Retirement Longitudinal Study (CHARLS; n=10,811, mean age 57.8 years, 50.2 percent men). Sleep duration was self-reported by the participants, who underwent a battery of cognitive tests on three aspects, namely memory, orientation, and executive function.

In fact, insufficient or excessive sleep duration was already associated with lower cognitive z scores across the three cognitive domains at baseline.

“Extreme sleep duration was associated not only with lower cognitive function at baseline but also with faster cognitive decline during the follow-up assessments,” observed the researchers. “In the present longitudinal study, a differential association was detected between sleep duration and distinct cognitive aspects, and memory was the main cognitive domain altered among the three domains measured.”

“These findings were consistent in both the ELSA and the CHARLS despite the substantial cultural and racial/ethnic differences,” they pointed out, indicating the generalizability of the data,” they pointed out.

Impaired memory is the key manifestation of dementia, and is considered a factor driving mild cognitive impairment to progress to dementia. In addition, acute sleep deprivation can affect memory encoding and consolidation. In fact, previous studies have shown that short sleep duration was associated with a greater risk of developing memory impairment.

While the mechanisms underlying the association remain unclear, there was evidence that sleep deprivation could lead to increased deposition of amyloid plaques, which represent a hallmark of Alzheimer’s disease. Indeed, a randomized trial has shown that even one night of sleep deprivation could lead to increased Aβ42 protein levels in cerebrospinal fluid of healthy middle-aged men.

“Future studies are needed to examine the mechanisms of the association between sleep duration and cognitive decline,” the researchers suggested.