You might have read about a new and depressing study that has found a gradual decrease in slow-wave sleep raises the risk of dementia.

By a lot.

The study, from Monash University, found that “as little as one per cent reduction in deep sleep per year for people over 60 years of age translates into a 27 per cent increased risk of dementia”.

This suggests that “enhancing or maintaining deep sleep, also known as slow-wave sleep, in older years could stave off dementia”.

Associate Professor Matthew Pase, who led the study, said slow-wave sleep, or deep sleep, supports the ageing brain in many ways.

Associate Professor Matthew Pase.

“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,” he said.

“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.”

These findings are supported by previous research.

Read more about the study in a Monash statement.

The staged descent into deep sleep

There are four stages in the sleep cycle.

The first stage, known as N1, is fairly short: it’s when your breathing and heartbeat begin to slow down, and your muscles relax.

During the second stage, N2, your heartbeat and breathing slow down further. There is no eye movement. Your body temperature drops. And  brain activity produces ‘sleep spindles’.

Empirical evidence suggests that sleep spindles are a pattern of brain waves that facilitate the plasticity which supports learning, memory consolidation, declarative learning, motor skills, and overall intellectual performance.

Sleep spindles are believed to dilute the effects of noise in the brain: the more sleep spindles generated, the better you can sleep through noise. Taken together, stages N1 and N2 are preparing the brain for deep or slow-wave sleep, which is known as N3.

N1, N2 and N3 are known as non-REM sleep stages. The fourth stage, REM (rapid eye movement) is when we have our most vivid dreams.

For more on the stages of sleep, see here.

What happens during slow-wave sleep

N3 is slow-wave sleep, the deepest sleep stage, when your heartbeat and breathing are at their slowest rate. There are no eye movements and the body is fully relaxed.

Brain activity slows down and changes, with delta waves being present, signalling that the body is in complete relaxation, and cortisol levels (stress hormones) drop.

During slow-wave sleep, tissue is repaired and grown, and cell regeneration occurs. The immune system is strengthened.

It can be hard to rouse a person in slow-wave sleep. This is especially the case for children, who need more slow-wave sleep, which will decline as they age.

By the time you’re in old age, you may have very little or even no slow-wave sleep. Which is bad news for the brain. Because it’s during slow-wave sleep that the brain is cleaned in a process only discovered 13 years ago.

It’s this cleaning that protects the brain against the toxic waste that can lead to dementia.

The glymphatic system

In 2012 neuroscientists at the University of Rochester Medical Center in upstate New York discovered a system that “drains waste from the brain at a rapid clip”.

Dr Maiken Nedergaard.

The researchers describe what they’ve called “the glymphatic system”, as acting like “a series of pipes that piggyback on the brain’s blood vessels, sort of a shadow plumbing system that seems to serve much the same function in the brain as the lymph system does in the rest of the body – to drain away waste products”.

It was already known that cerebrospinal fluid (CSF) plays an important role cleansing brain tissue, “carrying away waste products and carrying nutrients to brain tissue through a process known as diffusion”.

The glyphatic system circulates CSF to every corner of the brain much more efficiently.

Rochester’s Dr Maiken Nedergaard, who led the research, is co-director of the University’s Center for Translational Neuromedicine.

In a statement, she said: “Waste clearance is of central importance to every organ, and there have been long-standing questions about how the brain gets rid of its waste.

“This work shows that the brain is cleansing itself in a more organised way and on a much larger scale than has been realised previously.”

Dr Nedergaard and colleagues were hopeful that these findings “have implications for many conditions that involve the brain, such as traumatic brain injury, Alzheimer’s disease, stroke, and Parkinson’s disease”.

Indeed, glymphatic clearance is thought to wash away amyloid-beta and tau proteins that are associated with Alzheimer’s disease.

Since the 2012 discovery, it’s become known that glymphatic clearance moves tau proteins and amyloid-beta out of the brain.

This suggests that the glymphatic system is involved in modulating, or possibly protective against, Alzheimer’s disease.

However, during the waking hours, glymphatic clearance isn’t happening.

Sleep is the main driver. In fact, the system is mainly engaged when we’re in slow-wave sleep.

So the big question is, how can older people increase their slow-wave sleep and get these brain clearing benefits?

How to ride the slow waves

There are the usual remedies to keep in mind. Eat a healthy diet. Get regular exercise. Maintain regular sleep patterns and avoid long naps. Get a blue light-filter on your glasses. Do breathing exercises to blow off stress, inviting the body to totally relax.

A number of studies suggest that having a hot bath two hours before going to bed will improve your sleep. It’s thought to stimulate the thermoregulatory system which, for sleep, involves cooling the body.

Last year, Swiss researchers, in a novel study, found that pink noise during deep sleep increased slow waves in the brain for some participants. Pink noise includes sounds such as falling rain or rustling leaves.

A 2017 study found that playing sounds at certain frequencies enhanced slow-wave brain activity. It also improved sleep-dependent memory in older adults.