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A growing body of research suggests poor sleep is tied to impaired thinking and dementia in older adults. Now a new study may shed light on why.

Researchers at the University of Toronto have found a potential explanation of what disrupted sleep does to the human brain. They studied 685 adults older than 65, who participated in two large U.S. studies, and looked at their sleep patterns, their performance on thinking tests and, later, their brain-tissue samples after the participants died.

The researchers’ findings, published Wednesday in the journal Science Advances, indicate disrupted sleep may contribute to changes in a type of immune cell in the brain called microglia, which in turn appear to be related to poorer cognitive functions, such as memory and the ability to reason.

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While further research is needed to determine whether fixing people’s sleep problems can prevent or reverse cognitive decline, Andrew Lim, one of the authors of the study, said fragmented sleep should not be ignored.

Many people believe “having bad sleep is just part of aging, and it’s something that’s annoying but to be tolerated, rather than aggressively managed or aggressively investigated,” said Dr. Lim, an associate professor of neurology at the University of Toronto and sleep neurologist at the Sunnybrook Health Sciences Centre. “This adds one more reason to take sleep [problems] seriously and to look for your treatable causes and to address them.”

This study builds on previous research, including studies on rodents and genetic studies, that suggest microglia play a role in the link between poor sleep and cognitive impairment and dementia. Microglia normally help fight infections and clear debris from the brain. But dysfunction of microglia appears to be involved in the development of Alzheimer’s disease, Dr. Lim said.

In the study, researchers recorded participants’ sleep disruptions by having them wear wristwatch-like actigraphy devices, which could detect subtle signs of them awakening at night, even when participants themselves were unaware. The participants were asked to wear these devices for 10 days, once a year, for a median period of two years.

Participants also performed a series of cognitive tests annually. They agreed to donate their brains for research after their death, and researchers were able to examine their microglia in two ways. First, since activated and resting microglia differ in appearance, researchers were able to determine the density and proportion of activated microglia by looking at participants’ brain tissue under a microscope, Dr. Lim said. Then, they examined patterns of gene expression to identify “older” versus “younger” microglia – that is, whether the microglia appeared as though they came from an older person from a genetic perspective.

The researchers found connections between all three variables. Individuals who had higher levels of sleep fragmentation had a higher proportion of activated microglia and gene expression characteristic of older microglia. This was the case both for participants who had Alzheimer’s disease and those who did not. Those with poorer sleep also performed worse on their cognitive tests. And participants who had a greater proportion of activated microglia or genetically older microglia also had poorer cognitive test results.

Professor Teresa Liu-Ambrose of the University of British Columbia, who was not involved in the study, said the findings align with what researchers know thus far about the importance of sleep and the possible contribution of poor sleep to Alzheimer’s disease.

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Dr. Liu-Ambrose, the Canada research chair in physical activity, mobility and cognitive neuroscience at UBC, said good-quality sleep is believed to allow the brain to clear itself of toxic beta-amyloid protein, the buildup of which is one of the characteristics of Alzheimer’s disease. And, she said, there is also good evidence to suggest an accumulation of beta-amyloid can further contribute to disrupted sleep.

“It’s a vicious cycle,” she said.

“What [the new study] really does show is that it’s important to protect your sleep overall," she said. "[Sleep quality] does seem to have very direct effects on the brain, both acutely, but also chronically.”

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