Slow-Wave Sleep Loss May Raise Dementia Risk, Study Finds

The likelihood of developing dementia could rise with age when people do not get sufficient slow-wave sleep. A 2023 study found that adults over 60 were 27 percent more likely to develop dementia for every 1 percent of this deep sleep lost each year.

What slow-wave sleep does for the brain and body

Slow-wave sleep is the third phase of the human 90-minute sleep cycle and generally lasts around 20–40 minutes. It is the most restorative sleep stage: brain activity and heart rate slow, while blood pressure falls.

This deep sleep helps reinforce muscles, bones and the immune system, while readying the brain to take in further information. More recent research has also shown that people with Alzheimer's-related brain changes performed better in 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 augments the clearance of metabolic waste from the brain, including facilitating the clearance of proteins that aggregate in Alzheimer's disease," said neuroscientist Matthew Pase from Monash University in Australia.

"However, to date we have been unsure of the role of slow-wave sleep in the development of dementia. Our findings suggest that slow-wave sleep loss may be a modifiable dementia risk factor."

The Framingham Heart Study and dementia risk

Pase and researchers from Australia, Canada and the US analysed 346 participants in the Framingham Heart Study. They had undertaken two overnight sleep assessments, one between 1995 and 1998 and another between 2001 and 2003, separated by an average of five years.

The community-based group had no documented dementia when the 2001–2003 assessment took place, and all were aged over 60 by 2020. This enabled the researchers to compare data from the two detailed polysomnography sleep studies and track participants for dementia until 2018.

"We used these to examine how slow-wave sleep changed with aging and whether changes in slow-wave sleep percentage were associated with the risk of later-life dementia up to 17 years later," said Pase.

Over the 17-year follow-up period, 52 participants developed dementia. The team also assessed whether the slow-wave sleep levels measured during the sleep studies were associated with these dementia diagnoses.

In general, slow-wave sleep declined from the age of 60. The rate of loss was greatest between ages 75 and 80, before reaching a plateau thereafter.

When the first and second sleep-study results were compared, every percentage-point annual reduction in slow-wave sleep was associated with a 27 percent higher chance of developing dementia.

For Alzheimer's disease, the most common form of dementia, the increased risk was 32 percent.

Factors associated with reduced slow-wave sleep

The Framingham Heart Study collects a range of health measures over time, including loss of hippocampal volume, which is an early indicator of Alzheimer's, and common risk factors for cardiovascular disease.

Having lower levels of slow-wave sleep was associated with a greater cardiovascular disease risk, the use of medicines that may affect sleep, and the APOE ε4 gene, which is associated with Alzheimer's.

"We found that a genetic risk factor for Alzheimer's disease, but not brain volume, was associated with accelerated declines in slow wave sleep," Pase said.

While the associations were distinct, the researchers stressed that this kind of study cannot establish that loss of slow-wave sleep causes dementia. It is also possible that brain processes linked to dementia lead to sleep loss. Further work is needed to clarify these relationships.

In the meantime, making adequate sleep a priority may be worthwhile, as its benefits extend beyond supporting memory. There are also steps people can take to improve their chances of getting more of this essential slow-wave sleep.

The study was published in JAMA Neurology.

An earlier version of this article was published in November 2023.