A recent analysis of over 20,000 middle-aged and older adults showed an association between a later chronotype (‘night owls’) and cognitive decline among highly educated people [1].
Early birds and night owls
One risk factor linked to dementia is a disrupted circadian rhythm, the natural 24-hour cycle of sleep and activity patterns. Common disruptions in that rhythm include shift work and frequent jet lag, which have been reported to contribute to neurodegeneration [2, 3].
Chronotype is a natural preference regarding when sleep and activity occur. For some people (‘early birds’), it is to wake up and go to sleep early. Others, ‘night owls’, experience more energy later in the day, which encourages them to stay up late.
In this study, the researchers aimed “to investigate the longitudinal association between chronotype and cognitive decline among adults of the general population.” It included data from 23,798 participants in the Lifelines population-based cohort study from the northern Netherlands; the participants were at least 40 years old with a median age of 49 years.
The researchers assessed the participants’ chronotypes and cognition, specifically non-verbal fluency and executive functioning, which were evaluated at the beginning of the study and again after a 10-year follow-up. Cognitive decline was calculated by subtracting the two values, resulting in a score between 0 and 175.
Bad news for ‘night owls’ with degrees
Based on previous research, the researchers hypothesized that the relationship between chronotype and cognitive health is U-shaped in nature; however, this was not the case, as the assessment revealed an association between a late chronotype and cognitive decline.
Such an association was not observed for the early birds. The authors explain that the lack of association between an early chronotype and cognitive decline may be due to the fact that individuals with extremely early chronotypes constituted only 0.11% of the study population. Future research would need to investigate it further.
Based on previously reported links between increased dementia risk and lower educational attainment, older age, and female sex, the researchers included these variables in the analysis of factors that moderate the association between chronotype and cognitive decline.
Among the tested variables, only educational attainment moderated the association between chronotype and cognitive decline. Therefore, the researchers divided the participants based on their education level and re-analyzed the data.
In the high educational attainment group, a negative association was found between chronotype and cognitive change. Specifically, “for every one-hour increase in chronotype, cognition declined by 0.80 points among the high-educational attainment group over a 10-year follow-up.”
In contrast, the middle-educational attainment group showed a borderline significant effect, and there was no association in the low-educational attainment group.
The study’s lead author, Ana Wenzler, thinks that this may be related to the types of work that people in those groups perform. When it comes to highly educated people, “that probably has to do with their sleep rhythm. They are often people who have to go back to work early in the morning and are therefore more likely to sleep too short, giving their brains too little rest.”
“We suspect that lower- or middle-educated people are more likely to have a job that allows them to take their sleep rhythm into account, such as a job in the hospitality industry or one with night shifts. If this is not possible, your brain does not get enough rest and you are more likely to adopt bad habits. It would be nice if more consideration was given to evening people who now have to work early: for example, by giving them the option of starting later, “ Wenzler continues.
There is also the possibility that this association was observed only in the highly educated group because more people in the low- and middle-educational attainment groups were lost to follow-up. As the researchers discuss, the baseline cognitive functions measured in the people who were lost to follow-up were lower, suggesting that they may not have continued participating due to cognitive difficulties. This would result in an underestimated cognitive decline in this group.
Better sleep, better brain
In further analysis, the researchers focused on highly educated people, aiming to understand the potential pathways linking chronotype to cognition. They focused their study on whether sleep quality and health behaviors such as alcohol intake, physical activity, and smoking mediate the association.
They learned that poorer sleep quality and current smoking status were partially mediating the association by 13.52% and 18.64%, respectively. Other lifestyle choices, such as physical activity, past smoking, and alcohol consumption, didn’t explain the association.
Those results go along with previous observations that linked a later chronotype to poorer sleep quality, including shorter sleep duration, which in itself is associated with a loss of brain volume [4] and disturbances in the various sleep phases [5], potentially disrupting the clearance of Aβ, which is associated with Alzheimer’s disease.
Late chronotype was also linked to increased smoking risk, and chronic smoking is linked to brain aging and white matter degeneration [6].
Social jetlag compared to chronotype
The researchers note that their study does not address whether the chronotype itself or social jetlag plays a role in cognitive decline. Social jetlag results from a misalignment between a person’s chronotype and daily activities, leading to a reduction in sleep duration and quality. For ‘night owls,’ it might mean needing to get up early to attend work in the morning, while ‘early birds’ might stay late due to social activities.
There are variations in the definition and measurement methods of social jetlag, making it more challenging to study and excluding it from this analysis. However, the study’s authors encourage future research to improve the standardization of social jetlag measurement methods and address the relationship between chronotype and cognitive decline, independent of social jetlag.
An open question
The authors summarize that current knowledge about the relationship between chronotype and cognition remains unsettled. They suggested that differences in study populations, chronotype definitions, cognitive measurements, and follow-up times, which vary across studies, contribute to the differences in results. Their study is a step towards a better understanding of this issue.
Literature
[1] Wenzler, A. N., Liefbroer, A. C., Voshaar, R. C. O., & Smidt, N. (2025). Chronotype as a potential risk factor for cognitive decline: The mediating role of sleep quality and health behaviours in a 10-year follow-up study. The journal of prevention of Alzheimer’s disease, 100168. Advance online publication.
[2] Lee, K. W., Yang, C. C., Chen, C. H., Hung, C. H., & Chuang, H. Y. (2023). Shift work is significantly and positively associated with dementia: A meta-analysis study. Frontiers in public health, 11, 998464.
[3] Musiek E. S. (2015). Circadian clock disruption in neurodegenerative diseases: cause and effect?. Frontiers in pharmacology, 6, 29.
[4] Montaruli, A., Castelli, L., Mulè, A., Scurati, R., Esposito, F., Galasso, L., & Roveda, E. (2021). Biological Rhythm and Chronotype: New Perspectives in Health. Biomolecules, 11(4), 487.
[5] Di, T., Zhang, L., Meng, S., Liu, W., Guo, Y., Zheng, E., Xie, C., Xiang, S., Jia, T., Lu, L., Sun, Y., & Shi, J. (2024). The impact of REM sleep loss on human brain connectivity. Translational psychiatry, 14(1), 270.
[6] Yu, R., Deochand, C., Krotow, A., Leão, R., Tong, M., Agarwal, A. R., Cadenas, E., & de la Monte, S. M. (2016). Tobacco Smoke-Induced Brain White Matter Myelin Dysfunction: Potential Co-Factor Role of Smoking in Neurodegeneration. Journal of Alzheimer’s disease : JAD, 50(1), 133–148.
