While the link between poor sleep and dementia has long been known, it was unclear whether poor sleep habits could cause dementia or whether poor sleep was an early symptom of dementia. However, new research has revealed that sleep quality may have a direct impact on the rate at which the brain ages.

"Our findings provide evidence that poor sleep may contribute to accelerated brain aging," explains Abigail Dove, a neuroepidemiologist at the Karolinska Institute in Sweden, "and point to inflammation as one of the underlying mechanisms."

High Correlation With Night-Owl Lifestyle and Snoring

The researchers assessed the quality of their sleep across five dimensions in 27,500 middle-aged and elderly people (average age 54.7 years) enrolled in the UK Biobank (a research institute conducting long-term follow-up studies of the effects of genetic predisposition and lifestyle on disease). Approximately nine years later, they scanned the participants' brains with MRI and used machine learning models to estimate their biological brain age.

The researchers quantified sleep quality based on chronotype (morningness or eveningness), duration of sleep, presence or absence of insomnia, presence or absence of snoring, and daytime sleepiness. Using these data, they categorized participants into three sleep patterns, finding that 41.2 percent had healthy sleep, 3.3 percent had clearly poor sleep, and 55.6 percent fell into the middle group.

The analysis showed that for every point decrease in the healthy sleep score, the difference between brain age and chronological age increased by approximately six months. The group with the poorest sleep quality showed that their brains were approximately one year older than their chronological age. This suggests that differences in sleep duration and sleep habits can significantly affect the rate at which the brain ages.

Researchers have found that a night-owl lifestyle, unhealthy sleep duration (beyond 7-8 hours), and snoring habits are particularly strongly associated with brain aging. They also found that the five factors that determine sleep quality interact with each other. For example, insomnia can lead to excessive daytime sleepiness, and a nocturnal lifestyle can lead to shorter sleep times.

Poor Sleep Leads to Chronic Inflammation in the Body

To understand the mechanism by which poor sleep affects the brain, the research team also measured the level of low-grade inflammation in the body. Specifically, they used a combination of biomarkers, such as C-reactive protein levels, white blood cell and platelet counts, and the ratio of granulocytes to lymphocytes (a type of white blood cell), to analyze the role of inflammation in the relationship between sleep patterns and brain aging.

The results confirmed that higher levels of inflammation in the body tend to increase brain age. Mediation analysis (a method of analyzing the influence of variables intermediate in the causal relationship between two variables) found that inflammation explained approximately 7 percent of the association between intermediate sleep patterns and brain aging, and more than 10 percent of the association with poor sleep patterns. In other words, it is highly likely that poor sleep quality makes it easier for chronic inflammation to occur in the body, which in turn accelerates brain aging.