Sleep duration linked to cognitive functioning in elderly finds JAMA study

The number of older adults with cognitive impairment and dementia is increasing rapidly. Dementia is one of the most common and serious disorders in later life. It is responsible for a large proportion of disability and mortality in older people and imposes a huge burden of long-term care for families and society.

The mechanisms underlying the association between sleep duration and cognitive decline remain unclear, although several plausible biological pathways have been identified. Although the cerebral cortex normally thins with age, a longitudinal study reported an association between sleep duration of more or less than 7 hours and increased cortical thinning in the frontotemporal areas among cognitively normal older adults. Further studies suggest that activity in the interleukin 6 and C-reactive protein inflammatory pathways could be elevated by excessive sleep duration, and they reported a linear association between interleukin 6, C-reactive protein, and sleep duration. Furthermore, inflammatory disorders have been shown to mediate age-related cognitive impairment. Brief periods of sleep deprivation have been associated with an increase in hippocampal synaptic plasticity, contributing to subsequent impaired cognitive function.

Previous studies have indicated that acute sleep deprivation impairs memory encoding and consolidation and that short sleep duration is associated with an increase in the risk of developing memory deficits.

As the association between sleep duration and the trajectory of cognitive decline has not been conclusively demonstrated in the past, investigators undertook this study to investigate the association between sleep duration and cognitive decline by a pooled analysis of 2 nationally representative age cohorts.

The study design consisted of a pooled cohort using data from waves 4 to 8 (2008-2009 to 2016-2017) in the English Longitudinal Study of Ageing and waves 1 to 3 (2011 to 2015) in the China Health and Retirement Longitudinal Study in a population-based setting. Participants were 2 randomly enrolled cohorts comprising 28 756 individuals living in England who were 50 years or older and those living in China who were 45 years or older.

Self-reported sleep duration per night according to face-to-face interviews were used for estimating sleep time.

Global cognitive z scores were calculated according to immediate and delayed recall test, an animal fluency test, the serial sevens test, an intersecting pentagon copying test, and a date orientation test.

On analysis, the following key facts emerged.

• During 100 000 person-years of follow-up, global cognitive z scores in individuals with 4 hours or less (pooled β = −0.022; 95% CI, −0.035 to −0.009 SD per year; P = .001) and 10 hours or more (pooled β = −0.033; 95% CI, −0.054 to −0.011 SD per year; P = .003) of sleep per night declined faster than in the reference group (7 hours per night) after adjusting for several covariates.
• An inverted U-shaped association between sleep duration and global cognitive decline was also observed.

"In this pooled cohort study, an inverted U-shaped association between sleep duration and global cognitive decline was found, indicating that cognitive function should be monitored in individuals with insufficient (≤4 hours per night) or excessive (≥10 hours per night) sleep duration. Future studies are needed to examine the mechanisms of the association between sleep duration and cognitive decline." concluded the team.

For the full article click on the link: DOI:10.1001/jamanetworkopen.2020.13573

Primary source: JAMA Network Open