Dementia and sleep: What do we know about this link?

Dementia affects at least 55 million people worldwide and the number is increasing by about 10 million every year. In part, this is because we are living longer, but dementia is not an inevitable part of aging. So, are there ways to decrease our risk of developing dementia? Much research is currently focusing on the potential role of sleep.

According to the World Health OrganizationTrusted Source (WHO), “dementia is currently the seventh leading cause of death among all diseases and one of the major causes of disability and dependency among older people globally.”

The WHO states that around 55 million people have dementia, and by 2050 the number is likely to be almost 140 million. Between 60% and 70% of people with dementia have Alzheimer’s disease.

Dementia is primarily a disease of old age, although young-onset dementiaTrusted Source — where symptoms begin before the age of 65 — accounts for about 9%Trusted Source of cases. However, dementia is not an inevitable consequence of aging.

There is a genetic component to dementia — if you have a close relative with dementia, this might increase your risk. However, several studies have shown that even those with a hereditary risk can reduce it by adopting a healthy diet, exercising regularly, and avoiding smoking and too much alcohol.

One part of a healthy lifestyle is getting enough of the right sort of sleep. And many researchers are now seeing connections between sleep and dementia, as Dr. David Merrill, geriatric psychiatrist and director of the Pacific Brain Health Center at the Pacific Neuroscience Institute at Providence Saint John’s Health Center in Santa Monica, CA, told Medical News Today.

“Sleep,” he noted, “is a factor that can either be protective or risky for cognitive health. The effects of sleep on cognitive health depend on the attributes of an individual’s sleep, including the quality, quantity, frequency, and even the regularity of sleep.“

How long should we sleep for?

“It’s recommended — not only for brain health, but for overall health — that people get 7 to 9 hours of quality sleep per night.”

– Dr. Percy Griffin, Alzheimer’s Association director of scientific engagement

So, the optimum quantity for most people is somewhere between 7 and 9 hours, but is lack of sleep a risk factor?

Dr. Anton Porsteinsson, professor and director of the Alzheimer’s Disease Care, Research and Education Program (AD-CARE) at the University of Rochester Medical Center told MNT this might be the case.

According to him, “[i]nadequate sleep duration may increase risk of dementia. This pattern holds even when you look at sleep patterns years or decades before AD becomes clinically apparent.”

So perhaps we should get more sleep? Not according to a large-cohort study from Boston University. This study found that those who regularly slept for more than 9 hours a night had double the dementia risk of those who slept between 6 and 9 hours. They also had lower brain volumes.

However, it may be that excessive sleep was a symptom of early neuronal changes rather than the cause. The researchers of this study suggest that long sleep time could be a predictor of dementia risk.

Sleep quality

The National Sleep Foundation lists four key features of quality sleep for optimum health benefits:

• falling asleep within 30 minutes of going to bed
• waking no more than once in the night
• no more than 20 minutes awake during the night
• spending at least 85% of your time in bed asleep.

“The disrupted, poor-quality sleep seen in sleep disorders leads to both acute and chronically worsening changes in the brain. Normally, a good night’s sleep literally allows for repair and restoration of brain function to the levels seen at the beginning of the prior day.”

– Dr. David Merrill

Quality sleepTrusted Source includes periods of non-rapid eye movement sleep (NREM) and REM sleep. These cycle throughout the night, with the deepest sleep occurring during one of the stages of NREM sleep.

According to one study, low-frequency brain waves during the deep NREM sleep clear the brain of the Alzheimer’s-related toxins beta-amyloid and tau. These low-frequency brain waves give a pulse of cerebrospinal fluid, which carries the toxins away.

If sleep is disturbed, brain waste, such as beta-amyloid and tau, may start to build up, eventually forming the plaques and tangles characteristic of Alzheimer’s. Accumulation of beta-amyloid and tau may begin 10-20 years before dementia symptoms become noticeable.

Dr. Porsteinsson explained: “When you sleep, the brain ‘shrinks,’ which appears to open up [the] flow of cerebrospinal fluid that flushes out toxic byproducts such as [beta-amyloid] 42 and p-tau. The brain also resets its balance (homeostasisTrusted Source) during sleep. The quality of sleep and how much time you spend in deep-sleep matters here as well.”

Sleep apnea and dementia

Sleep apnea affects almost 1 billionTrusted Source people worldwide, the most common form being obstructive sleep apnea (OSA). The condition interrupts breathing during sleep and often wakes a person up.

People with sleep apnea are at increased risk of several health conditionsTrusted Source, such as asthma, cardiovascular problems, atrial fibrillation, and cancer. Recent studies have also suggested links between sleep apnea and dementia.

“Sleep apnea is one health condition increasingly known to be a risk factor for dementia. A person with sleep apnea stops breathing during sleep. […] This leads to potentially dangerous drops in the nocturnal oxygenation of the brain.”

– Dr. David Merrill

This hypoxiaTrusted Source is thought to cause brain changes. One study found that the temporal lobes — which are vital for memory — were reduced in thickness in those with sleep apnea, a change that is also seen in people with dementia.

Another study found that the hippocampus was reduced in volume in people with sleep apnea — hippocampal atrophy is a feature of Alzheimer’s diseaseTrusted Source.

This study also showed that two toxins — tau and beta-amyloid, which are thought to be responsible for many of the symptoms of dementia — build up in the brains of people with sleep apnea, probably because of a lack of oxygen in the blood.

Two more studies added to these findings. OneTrusted Source detected raised tau levels in those with sleep apnea; the otherTrusted Source found them to have amyloid plaques.

However, no study has yet proved a causative effect. And there are effective treatments for sleep apnea, as Dr. Merrill explained: “Luckily, we now have non-invasive peripheral oxygenation monitors used in-home sleep apnea tests that can detect these changes, and allow for effective treatments of OSA to restore nocturnal oxygenation.”

“The gold-standard treatment of OSA is [the] use of a continuous positive airway pressure (CPAP) device. Research studies have shown that even 4 hours per night using a CPAP device results in significantly less worsening of cognitive decline over time,” he added.

How does dementia affect sleep?

“Dementia disrupts sleep in a number of ways. Dementia is a neurodegenerative disorder, meaning that brain cells [experience] dysfunction and progressively die over time. As a person loses brain cells, the sleep centers of the brain start to [experience] dysfunction — we lose the ability to send signals to stay asleep. Oftentimes, sleep becomes fragmented or even inverted such that patients are awake all through the night, then sleep during most of the day.”

– Dr. David Merrill

A small study found that the daytime sleepiness characteristic of Alzheimer’s disease is linked to the death of key brain cells. The researchers suggested this is due to a build-up of tau protein and loss of neurons in areas of the brain that promote wakefulness.

A more recent study has also found that sleep disturbance in Alzheimer’s disease may increase the severity of symptoms. In this study, carried out in mouse cells, the researchers found that when phagocytosis of beta-amyloid plaques was interrupted, the plaques built up.

They identified a molecule — heparan — that inhibited this phagocytosis at high concentrations. Levels of heparan change throughout the day, so disturbances to circadian rhythmsTrusted Source affect these levels and may account for the build-up of plaques in Alzheimer’s disease.

Cause or symptom?

The same study suggested that improving sleep might be a way of alleviating dementia symptoms, but is it possible that treating sleep disorders might help prevent dementia?

A 2019 review of studies into sleep disorders and cognitive decline tried to answer this question. It found that sleep disorders, including sleep apnea, insomnia, inadequate or overlong sleep, and sleep disturbance were linked to cognitive decline and dementia.

There was also a link between sleep disorders and beta-amyloid and tau deposition. The review concluded that sleep management might be a promising target for dementia prevention.

However, no study has yet proved a causative link — or which way the relationship acted. Did sleep problems predispose to dementia, or were sleep problems a sign of the early stages of dementia?

The relationship is still not clear, as Dr. Porsteinsson explained: “Soluble [beta-amyloid] 42 may have [a] negative impact on sleep, and sleep quality and neurodegeneration associated with dementia damages the centers that control sleep and the sleep-wake cycles. Ιnterestingly, increased sleep need and excessive sleep in late life may also predict impending Alzheimer’s disease.”

Dr. Merrill also commented: “Dementia continues to be a disorder without a cure, and the available drug therapies are marginally effective at treating symptoms of dementia. So, it’s crucial to use all available strategies to treat sleep symptoms to alleviate dementia symptoms.”

“Unfortunately, as dementia progresses it can become increasingly challenging to have good sleep hygiene, especially in individuals who lose self-awareness about their deficits. In these cases, it’s important to have dedicated night-time caregivers, so patients can be looked after, kept safe, and allow daytime caregivers the chance to rest,” he added.

Maintaining a healthy lifestyle and ensuring you get enough quality sleep may reduce the risk of both dementia and many other health problems. But the hunt for cause and effect continues.

“More research is needed to fully understand the different characteristics of sleep and the brain, as well as the mechanisms for how sleep impacts the biology of the brain over time. We also need studies that look at sleep as an intervention for cognitive health.”

– Dr. Percy Griffin