Slow-Wave Sleep Loss Raises Dementia Risk by 27 Percent

Slow-wave sleep and dementia risk

A good night’s sleep is widely recognised as essential for keeping the body healthy, yet sleep matters in more ways than many people may appreciate.

Research indicates that, with increasing age, too little slow-wave sleep could raise the risk of dementia.

A study published in 2023 reported that people aged over 60 were 27 percent more likely to develop dementia when they lost only 1 percent of this deep-sleep stage each year.

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

Deep sleep helps build stronger muscles and bones, supports the immune system and readies the brain to take in further information.

Separate recent research found that people with Alzheimer’s-related brain changes performed better in memory testing when they experienced 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 analysis

Pase and fellow researchers in Australia, Canada and the US assessed 346 participants in the Framingham Heart Study. Each had taken part in two overnight sleep assessments, first between 1995 and 1998 and again between 2001 and 2003, with an average interval of five years between the tests.

The community-based group had no documented dementia when the 2001–2003 assessment took place and were aged over 60 by 2020. By comparing data from the two detailed polysomnography sleep studies, then tracking participants for dementia through to 2018, the researchers were able to investigate how the two factors related over time.

"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.

During 17 years of follow-up, 52 participants were diagnosed with dementia. The team also assessed whether the slow-wave sleep measurements taken during the sleep studies were associated with those dementia diagnoses.

In general, the proportion of slow-wave sleep declined after the age of 60. The decline was greatest from 75 to 80, before becoming stable 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 prevalent form of dementia, the associated risk rose to 32 percent.

Factors associated with reduced deep sleep

The Framingham Heart Study tracks a range of health measures over time, including reductions in hippocampal volume-an early indicator of Alzheimer’s-and factors commonly linked with cardiovascular disease.

Lower amounts of slow-wave sleep were associated with an increased cardiovascular disease risk, the use of medicines that may affect sleep and carrying 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 links are clear, the researchers stress that this study design cannot demonstrate that loss of slow-wave sleep causes dementia. It is also possible that brain processes related to dementia lead to sleep loss. Further studies are needed to fully clarify these factors.

In the meantime, people can certainly make sufficient sleep a priority-it is valuable for far more than reinforcing memory.

There are also measures people can take to improve their chances of gaining 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.