Sleep may enhance the brain’s ability to remove harmful waste. Poor or disrupted sleep could raise dementia risk.
The brain possesses its own system for removing waste, known as the glymphatic system, which is believed to become more active during sleep.
When sleep is disrupted, this cleaning process may be impaired, slowing the removal of toxins and other waste products from the brain. Researchers suggest that the accumulation of such toxins due to inadequate sleep could contribute to a higher risk of dementia.
Uncertainty remains about how exactly the glymphatic system functions in humans, as much of the current evidence comes from mouse studies. Still, these findings raise the possibility that better sleep could enhance the clearance of harmful substances in the brain and thereby lower dementia risk.
Here is what researchers currently understand about this developing field of study.
Why waste matters
All cells in the body generate waste. Outside the brain, the lymphatic system removes this material by carrying it from the spaces between cells into the bloodstream through a network of vessels.
The brain, however, lacks its own lymphatic vessels. Until about 12 years ago, scientists did not know how the brain handled its waste. That changed with the discovery of the glymphatic system, which was described as a mechanism that “flushes out” toxins from the brain.
Cerebrospinal fluid, which surrounds both the brain and spinal cord, plays a central role in this process. The fluid flows around blood vessels in the brain, then moves into the spaces between brain cells, where it collects waste before draining out through large veins.
Studies in mice later demonstrated that the glymphatic system becomes most active during sleep, with waste removal markedly increased at that time.
One of the substances cleared through this system is amyloid beta (Aβ) protein. When Aβ builds up in the brain, it can form plaques. Alongside tangles of another protein, tau, found in neurons, these plaques are key features of Alzheimer’s disease, the most common form of dementia.
In humans and mice, studies have shown that levels of Aβ detected in the cerebrospinal fluid increase when awake and then rapidly fall during sleep.
But more recently, another study (in mice) showed pretty much the opposite – suggesting the glymphatic system is more active in the daytime. Researchers are debating what might explain the findings.
So we still have some way to go before we can say exactly how the glymphatic system works – in mice or humans – to clear the brain of toxins that might otherwise increase the risk of dementia.
Does this happen in humans too?
We know sleeping well is good for us, particularly our brain health. We are all aware of the short-term effects of sleep deprivation on our brain’s ability to function, and we know sleep helps improve memory.
In one experiment, a single night of complete sleep deprivation in healthy adults increased the amount of Aβ in the hippocampus, an area of the brain implicated in Alzheimer’s disease. This suggests sleep can influence the clearance of Aβ from the human brain, supporting the idea that the human glymphatic system is more active while we sleep.
This also raises the question of whether good sleep might lead to better clearance of toxins such as Aβ from the brain, and so be a potential target to prevent dementia.
How about sleep apnea or insomnia?
What is less clear is what long-term disrupted sleep, for instance if someone has a sleep disorder, means for the body’s ability to clear Aβ from the brain.
Sleep apnea is a common sleep disorder when someone’s breathing stops multiple times as they sleep. This can lead to chronic (long-term) sleep deprivation, and reduced oxygen in the blood. Both may be implicated in the accumulation of toxins in the brain.
Sleep apnea has also been linked with an increased risk of dementia. And we now know that after people are treated for sleep apnea more Aβ is cleared from the brain.
Insomnia is when someone has difficulty falling asleep and/or staying asleep. When this happens in the long term, there’s also an increased risk of dementia. However, we don’t know the effect of treating insomnia on toxins associated with dementia.
So again, it’s still too early to say for sure that treating a sleep disorder reduces your risk of dementia because of reduced levels of toxins in the brain.
So where does this leave us?
Collectively, these studies suggest enough good quality sleep is important for a healthy brain, and in particular for clearing toxins associated with dementia from the brain.
But we still don’t know if treating a sleep disorder or improving sleep more broadly affects the brain’s ability to remove toxins, and whether this reduces the risk of dementia. It’s an area researchers, including us, are actively working on.
For instance, we’re investigating the concentration of Aβ and tau measured in blood across the 24-hour sleep-wake cycle in people with sleep apnea, on and off treatment, to better understand how sleep apnea affects brain cleaning.
Researchers are also looking into the potential for treating insomnia with a class of drugs known as orexin receptor antagonists to see if this affects the clearance of Aβ from the brain.
If you’re concerned
This is an emerging field and we don’t yet have all the answers about the link between disrupted sleep and dementia, or whether better sleep can boost the glymphatic system and so prevent cognitive decline.
So if you are concerned about your sleep or cognition, please see your doctor.
Adapted from an article originally published in The Conversation.
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