Slow-Wave Sleep Loss May Raise Dementia Risk by 27 Per Cent

A sound night’s sleep is widely recognised as essential for the body to function healthily, yet sleep may matter in more ways than many people appreciate.

Research suggests that, with increasing age, insufficient slow-wave sleep could raise the risk of dementia.

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

Slow-wave sleep and the ageing brain

Slow-wave sleep, the third phase of the human 90-minute sleep cycle, lasts roughly 20–40 minutes. It is the most restorative phase, during which 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.

A separate recent study found that people with Alzheimer’s-related brain changes performed better in memory assessments if 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."

Framingham Heart Study findings on dementia

Pase and fellow researchers in Australia, Canada and the US studied 346 participants in the Framingham Heart Study. The participants had undergone two overnight sleep assessments, first between 1995 and 1998 and again between 2001 and 2003, with an average five-year interval between the tests.

The community-based group had no dementia diagnosis at the time of the 2001–2003 assessment and were aged over 60 in 2020. By comparing data from the two detailed polysomnography sleep studies and tracking participants for dementia until 2018, the researchers could investigate how the two factors were linked 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.

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

In general, slow-wave sleep was found to decline after the age of 60. The reduction was greatest between ages 75 and 80, before stabilising thereafter.

Comparing the first and second sleep assessments showed that every percentage-point annual fall in slow-wave sleep was associated with a 27 per cent greater chance of developing dementia.

When the researchers focused specifically on Alzheimer’s disease, the most common type of dementia, the risk rose to 32 per cent.

Factors linked with reduced deep sleep

The Framingham Heart Study tracks a range of health measures over time, including loss of hippocampal volume, an early indicator of Alzheimer’s, and common contributors to cardiovascular disease.

Lower levels of slow-wave sleep were 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.

Although the associations are evident, the authors stress that this study design cannot establish that losing slow-wave sleep causes dementia. It is also possible that dementia-related processes in the brain lead to sleep loss. Further research is needed to fully clarify these factors.

In the meantime, making enough sleep a priority is worthwhile, as its benefits extend beyond reinforcing memory.

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