Sleep duration, chronotype, and lifestyle affect cognitive decline

By Dr. Liji Thomas, MDReviewed by Benedette Cuffari, M.Sc.Jul 15 2024

A recent study published in the BMJ explores the impact of lifestyle and health conditions on the prevalence and incidence of cognitive decline.

Study: Sleep duration, chronotype, health and lifestyle factors affect cognition: a UK Biobank cross-sectional study. Image Credit: Stock-Asso / Shutterstock.com

How does sleep affect cognition?

As the aging population continues to grow throughout the world, cognitive decline represents a significant public health challenge.

Sleep is essential for human health and survival. In fact, adequate sleep is necessary for restoring normal metabolism to body tissues, consolidating memories, learning, regulating emotions, and immune function.

Sleep quality and duration influence the risk of cognitive decline, with both extended and shorter sleep durations associated with reduced cognitive ability. Reduced sleep quality is also associated with an increased risk of neurodegenerative disease, metabolic dysfunction, autoimmune disease, and vascular disease. These effects could be attributed to neuroinflammation, activation of the complement system, poor learning and memory function, and impaired hippocampal learning.

Chronotypes, or sleep patterns, indicate at what time an individual naturally prefers to sleep. Like sleep quality and duration, an individual’s chronotype is also a determining factor in their risk of cognitive impairment; however, these associations remain unclear.

About the study

Data for the current study were obtained from the United Kingdom Biobank database. Taken together, a total of 26,820 individuals between the ages of 53 and 86 years were included in the analysis.

The cognitive performance of all study participants was assessed and compared with sleep, health, and lifestyle factors including sex, age, cardiovascular disease, diabetes, drinking, smoking, and body mass index (BMI).

The study participants were divided into two cohorts that comprised about 10,000 and 16,700 individuals in cohorts one and two, respectively. The mean age of both cohorts was about 70 years.

Study participants in the first cohort completed four tests including the fluid intelligence/reasoning, pairs matching, reaction time, and prospective memory tests. The second cohort only completed the pairs matching and reaction time tests, and their cognitive performance was separately analyzed.

Educational status, physical activity, and other important parameters were not available for all participants, thus leading to their exclusion from the analysis. 

What did the study show?

Normal sleep duration is between seven and nine hours. In the current study, individuals in cohort one with normal sleep durations had higher cognitive scores than those who slept for shorter durations. However, abnormally long sleep durations were associated with lower cognitive scores in both cohorts as compared to those with normal sleep patterns.

Sleep provides a restorative and protective function on cognition by the removal of toxic metabolites from the central nervous system.”

Intermediate and evening chronotypes were associated with better cognitive function in both cohorts. However, the increase from intermediate to evening types was more notable in cohort two. No association was observed for insomnia.

Both age and gender significantly impacted cognitive function, with older people and women exhibiting lower cognitive scores than younger individuals and men, respectively. The presence of angina and hypertension was also associated with lower scores in cohort one, whereas diabetes was more likely to be present in individuals with lower cognitive scores in both cohorts.

Daily drinkers had lower cognitive scores as compared to never- or occasional drinkers, as well as those with intermediate levels of drinking. Likewise, current and former smokers had lower scores in cohort one and two, respectively, as compared to never-smokers.

Conclusions

Various factors contribute to how cognitive performance varies with age. Sleep duration appears to be among the most important variables, with both long and short sleep affecting cognition.

The study findings clarify how circadian rhythms, which are specific to each individual, have a significant effect on cognition. In adolescence, morning chronotypes are associated with better health. In contrast, evening and intermediate chronotypes in the current study were associated with superior cognition.

With age, chronotype plays a changing role in cognitive ability. This may be because the morning type of sleep pattern is not advantageous for cognition as one grows older.

Sleep quality was not related to cognitive performance, which does not agree with earlier studies. This may be due to the broad nature of the parameter used.

Taken together, chronotypes, the duration of sleep, health conditions, and lifestyle factors form a complex network of influences on cognitive ability over time. These associations should be further explored in larger and more diverse studies that use more objective sleep measures and consider other factors like educational status, physical activity, depression and isolation.

Further research and practices should focus on promoting interventions to improve sleep patterns in the general population.”