How Slow-Wave Sleep Affects the Risk Of Dementia

The risk of dementia may increase as we age if we don’t get enough slow-wave sleep. People over 60 have a 27% higher risk of dementia if they lose just 1% of this deep sleep each year, according to a new study.

Slow-wave sleep or NREM (non-rapid eye movement) sleep is the third stage of a 90-minute human sleep cycle, lasting approximately 20-40 minutes. It is the most restful stage, in which brain waves and heart rate slow down, and blood pressure drops.

Deep sleep strengthens our muscles, bones and immune system and prepares our brain to absorb more information.

During this year, a research found that people with Alzheimer’s-related brain changes did better on memory tests when they had more slow-wave sleep.

“Slow-wave sleep, or deep sleep, supports the aging brain in many ways, and we know that sleep enhances the removal of metabolic waste from the brain, including facilitating the removal of proteins that aggregate in Alzheimer’s disease,” says neurologist Matthew Pase, from Monash University in Australia.

However, until now, researchers were unsure of the role of slow-wave sleep in the development of dementia.

Findings from this study suggest that loss of slow-wave sleep could be a modifiable risk factor for dementia.

In the study, an international team from Australia, Canada and the United States examined 346 participants who completed two overnight sleep studies between 1995 and 1998 and between 2001 and 2003, with an average of five years between testing periods.

This cohort, which had no records of dementia at the time of the study from 2001-2003 and was over 60 years old in 2020, gave the researchers the chance to examine the link between the two factors over time by comparing data sets from the two in-depth polysomnographic sleep studies and then monitoring dementia among participants through 2018.

“We used them to examine how slow-wave sleep changed with aging and whether changes in the percentage of slow-wave sleep were associated with dementia risk at age two up to 17 years later,” he Dr. Pase explained.

During the 17 years of follow-up, 52 cases of dementia were registered among the participants.

The slow-wave sleep levels of sleep study participants were also examined to establish a link with cases of dementia.

In general, the rate of slow-wave sleep has been found to decline starting at age 60, with this loss peaking between 75 and 80 and leveling off thereafter.

Comparing the participants’ first and second sleep studies, the researchers found a link between each percentage point decrease in slow-wave sleep per year and a 27% increased risk of developing dementia.

This risk increased to 32% when they focused on Alzheimer’s disease, the most common form of dementia.

The Framington Heart Study measures several health data points over time, including hippocampal volume loss (an early sign of Alzheimer’s disease) and common factors that contribute to cardiovascular disease.

Low levels of slow-wave sleep have been linked to a higher risk of cardiovascular disease, taking medications that can impact sleep, and the presence of the APOE ε4 gene, which is linked to Alzheimer’s disease.

“We found that a genetic risk factor for Alzheimer’s disease, but not brain volume, was associated with the accelerated decline of slow-wave sleep,” says Pase.

Although these are clear associations, the authors note that this type of study does not prove that slow-wave sleep loss causes dementia, and that it is possible that dementia-related brain processes cause sleep loss.

For these factors to be fully understood, more research is needed.

In the meantime, experts urge us to take steps to increase our chances of getting more slow-wave sleep, which is the bottom line, and to prioritize getting enough sleep, which is important for more than memory enhancement.

The study was recently published in the medical journal JAMA Neurology.

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2023-11-26 20:27:00
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