One stage of sleep that is critical for reducing dementia risk every night

A typical night's sleep consists of four to six sleep cycles, each lasting about 90 to 120 minutes and can vary on a number of factors

We're all guilty of not getting enough sleep at the best of times, but a new study has found one particular stage is critical in preventing the risk of dementia. The 2023 study, from Monash University in Australia, found that over-60s are 27 percent more likely to develop dementia if they lose just one percent of deep sleep each year.

Officially called the slow-wave sleep cycle, this is the third stage of a human 90-minute sleep cycle, lasting around 20 to 40 minutes. It's said to be the most restful stage, where brain waves and heart rates slow and blood pressure drops.

Deep sleep strengthens our muscles, bones, and immune system, and prepares our brains to absorb more information. Neuroscientist and researcher Matthew Pase from Montash University told Science Alert : "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."

Together with a team of international colleagues, Pase studied roughly 350 subjects who participated in two overnight sleep studies, one conducted between 1995 and 1998 and another between 2001 and 2003. Researchers compared datasets from the two in-depth polysomnography sleep studies.

Subjects over 60 as of 2020 who had no record of dementia during their 2001-2003 tests, were monitored for signs of cognitive decline until 2018. This community-based cohort, who had no record of dementia at the time of the 2001-2003 study, and were over 60 years old in 2020, gave researchers a chance to look into the link between two factors over time by comparing the datasets from the two in-depth polysomnography sleep studies, and then monitoring for dementia among participants up until 2018.

Pase explained: "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. Our findings suggest that slow-wave sleep loss may be a modifiable dementia risk factor."

Low levels of slow-wave sleep were linked to a higher risk of cardiovascular disease, taking medications that can impact sleep, and having the APOE ε4 gene, which is linked to 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.

So what are sleep waves? Sleep waves are characterised by different patterns of brain waves that occur during each stage of sleep.

They can be measured using an electroencephal-ogram (EEG) which records fluctuations in brain electrical activity.

A 2021 study also found that sleep deprivation in middle age increases the risk of cognitive decline, with results showing those who averaged less than six hours of sleep a night at ages 50, 60 and 70 were associated with a 30 percent higher dementia risk.