Sleep disturbances are common in older age, and previous studies have shown associations between too much or too little sleep and increased risk of cognitive decline, but the ideal amount of sleep for preserving mental health has not been well described, according to the authors of the new paper.
In the study published in Nature Aging, the team of researchers from China and the United Kingdom reviewed data from the UK Biobank, a national database of individuals in the United Kingdom that includes cognitive assessments, mental health questionnaires, and brain imaging data, as well as genetic information.
Sleep is important for physical and psychological health, and also serves a neuroprotective function by clearing waste products from the brain, lead author Yuzhu Li of Fudan University, Shanghai, China, and colleagues wrote.
The study population included 498,277 participants, aged 38-73 years, who completed touchscreen questionnaires about sleep duration between 2006 and 2010. The average age at baseline was 56.5 years, 54% were female, and the mean sleep duration was 7.15 hours.
The researchers also reviewed brain imaging data and genetic data from 39,692 participants in 2014 to examine the relationships between sleep duration and brain structure and between sleep duration and genetic risk. In addition, 156,884 participants completed an online follow-up mental health questionnaire in 2016-2017 to assess the longitudinal impact of sleep on mental health.
Both excessive and insufficient sleep was associated with impaired cognitive performance, evidenced by the U-shaped curve found by the researchers in their data analysis, which used quadratic associations.
Specific cognitive functions including pair matching, trail making, prospective memory, and reaction time were significantly impaired with too much or too little sleep, the researchers said. “This demonstrated the positive association of both insufficient and excessive sleep duration with inferior performance on cognitive tasks.”
When the researchers analyzed the association between sleep duration and mental health, sleep duration also showed a U-shaped association with symptoms of anxiety, depression, mental distress, mania, and self-harm, while well-being showed an inverted U-shape. All associations between sleep duration and mental health were statistically significant after controlling for confounding variables (P < .001).
On further analysis (using two-line tests), the researchers determined that consistent sleep duration of approximately 7 hours per night was optimal for cognitive performance and for good mental health.
The researchers also used neuroimaging data to examine the relationship between sleep duration and brain structure. Overall, greater changes were seen in the regions of the brain involved in cognitive processing and memory.
“The most significant cortical volumes nonlinearly associated with sleep duration included the precentral cortex, the superior frontal gyrus, the lateral orbitofrontal cortex, the pars orbitalis, the frontal pole, and the middle temporal cortex,” the researchers wrote (P < .05 for all).
The association between sleep duration and cognitive function diminished among individuals older than 65 years, compared with those aged approximately 40 years, which suggests that optimal sleep duration may be more beneficial in middle age, the researchers noted. However, no similar impact of age was seen for mental health. For brain structure, the nonlinear relationship between sleep duration and cortical volumes was greatest in those aged 44-59 years, and gradually flattened with older age.