Long sleep had a negative effect on cognitive function
A cross-sectional study by British scientists, based on data from the UK Biobank, has found that increased sleep duration has a negative impact on cognitive health. As reported in BMJ Public Health, early chronotype also negatively affected cognitive function, while middle and late chronotypes were found to be associated with better cognitive function scores. In this analysis, gender, age, angina pectoris, high blood pressure, diabetes, alcohol consumption and smoking were also found to be significant factors affecting cognitive function.
Sleep is known to play a crucial role in optimising cognitive function by promoting recovery, memory consolidation, learning and emotion regulation. Sleep disorders, particularly prevalent in the elderly, are consistently associated with an increased risk of cognitive decline and dementia. Animal studies have also shown that mechanistically, sleep disturbance causes neuroinflammation, activation of the complement system, and affects hippocampus-dependent learning.
Population studies in humans have found a relationship between sleep duration and cognitive ability. Studies have demonstrated that severe sleep deprivation causes changes in synaptic plasticity and impairments in learning and memory, thereby affecting cognitive abilities. That said, there are significant gaps in understanding how chronotypes can affect cognitive function.
A research team led by Daqing Ma from Imperial College London used a large dataset from the UK Biobank to examine the relationships between sleep duration, quality and chronotype and their combined effects on cognitive function. The analyses included data from 26820 individuals who underwent all baseline examinations, including all demographics, lifestyle data and comorbidities. Of the 26820 participants, 10067 who completed all four cognitive assessments were assigned to cohort 1 (mean age 71 years), while 16753 who completed only two cognitive assessments were assigned to cohort 2 (mean age 72 years).
Regression analyses for both cohorts included several diagnostic tests to confirm the validity of the multivariate linear regression model. In cohort 1, normal (7-8 hours) sleep duration was associated with a slightly higher total cognitive score (p < 0.001) than those who slept little. In contrast, those who slept long hours had significantly lower cognitive scores (p < 0.001). A similar pattern was observed for those who slept long in the second cohort (p < 0.001), although normal sleep duration had no significant effect on cognitive performance. Chronotype had a significant effect on cognitive performance in both cohorts. Those with a medium chronotype had the highest cognitive performance scores in both cohorts (p < 0.001) compared to those who classified themselves as early and late chronotypes.
Analyses also showed that women had lower cognitive scores than men; age was inversely correlated with cognitive ability, diabetes mellitus was found to be associated with lower cognitive scores. Those who never or occasionally consumed alcohol had significantly lower cognitive scores compared to those who consumed it daily or almost daily. Participants with angina and high blood pressure performed significantly worse on the cognitive test only in cohort 1. Smokers in cohort 1 had lower scores on cognitive tests, while ex-smokers in cohort 2 scored higher than those who never smoked.
Predictive field analyses revealed an inverse U-shaped relationship between sleep duration and cognitive function. Thus, optimal cognitive function was observed in those who reported a sleep duration of 6-9 hours. In cohort 2, stable cognitive function was observed in those who slept 5-9 hours. Notably, those who reported sleep durations greater than 9 hours showed marked cognitive decline. Meanwhile, in the predictive model, the middle and late chronotypes were associated with higher cognitive function scores (the late chronotype even showed an advantage over the middle chronotype).
According to the authors of the paper, this study highlights the multifaceted relationship between sleep parameters, health and lifestyle and cognitive function. In this case, sleep duration and chronotypes act as strong predictors of cognitive performance. Future studies should take a longitudinal approach, include more diverse populations, include objective measures of sleep, and investigate the biological mechanisms linking sleep duration to cognitive decline.