Study details

For the study published in JAMA Neurology, researchers looked at 346 adults who were enrolled in the Framingham Heart Study and who completed two in-home overnight polysomnography studies — the first between 1995 and 1998 and the other from 1998 to 2001. The participants also underwent an MRI to determine their hippocampal volume.

All participants were at least 60 years old and had no dementia during their second polysomnography study.

The researchers found that the average percentage of time a person was in slow-wave sleep dropped by 0.57 units per year.

"The rate of slow-wave sleep loss accelerated nominally from age 60 years onwards before peaking at ages 75 to 80 years and slowing thereafter," the researchers wrote. Total sleep time across both polysomnography assessments was stable.

The researchers found that those who saw a drop in slow-wave sleep were more likely to have cardiovascular disease, carry at least one APOE4 allele, or took medications that affected their sleep.

Over the course of 12 years, 52 cases of incident dementia emerged, 44 of which were consistent with Alzheimer's disease. The researchers found that, for each percentage decrease in slow-wave sleep per year, participants saw their risk of dementia increase by 27%, after adjusting for age, sex, cohort, and other factors.

Discussion

According to Matthew Pase from Monash University, who co-authored the study, deep sleep can help people remove metabolic waste from their brain in a process called glymphatic clearance.

"This includes waste that builds up in Alzheimer's disease," he said. "Waste builds up in the brain when we are awake, and then is flushed out when we are in slow-wave sleep."

Research from earlier this year found that less slow-wave sleep and severe sleep apnea were linked to an increased amount of white matter abnormalities in otherwise cognitively unimpaired adults.

However, a relationship between deep sleep and dementia risk is difficult to show, Pase said. "We previously found that individual differences in slow-wave sleep, assessed at one time point, were not associated with risk of dementia," he said.

"We extended this work by looking at repeated sleep studies, which enabled us to examine changes in sleep with aging," Pase added. "Results suggested that chronic declines in slow-wave sleep, rather than individual differences at any given time, are important for predicting dementia risk."

The researchers noted the study had several limitations, including the fact that while dementia was diagnosed through uninterrupted surveillance as part of the Framingham study, Alzheimer's disease biomarkers weren't available.

"As our study is observational, we are unable to determine whether slow-wave sleep loss causes dementia," the researchers wrote. "Although we conducted several analyses to investigate potential bidirectional associations, it is possible that preclinical dementia disrupts the homeostatic mechanisms that regulate slow-wave sleep."

The researchers added that more studies are necessary to determine whether improving slow-wave sleep can limit cognitive decline and neurodegeneration in certain patients.

"Overnight sleep studies are the gold standard for assessing sleep, but these are costly and time-consuming to perform," they wrote. "Further improving the ability of wearables to detect slow-wave sleep could improve our ability to track changes in slow-wave sleep over time and could be used to advance dementia research." (George, MedPage Today, 10/31)

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