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"Sleep appears to be a missing piece in the Alzheimer's puzzle, and enhancing sleep may lessen the cognitive burden that Alzheimer's disease imparts," said study author Bryce Mander, a postdoctoral fellow at the University of California, Berkeley.
It's not clear how sleep and memory affect -- or are affected by -- the accumulation of beta amyloid plaques, believed to interfere with mental functioning. Still, the study findings hint at a major message regarding Alzheimer's, said Mander, who works at the university's Sleep and Neuroimaging Laboratory.
For the new study, Mander and colleagues recruited 26 mentally healthy adults ages 70 to 79. They underwent brain imaging to assess plaque buildup, and were asked to remember pairs of words before and after a night's sleep. Overnight, researchers measured their brain waves, and the next day they conducted MRI scans during the memory testing.
Those patients with the highest levels of amyloid plaques in one part of the brain -- the medial prefrontal cortex -- had lighter sleep and higher levels of memory problems, the researchers found.
"It is not so much that memory after sleep is important, but that sleep after initial learning is important to help us retain memory for a longer period of time," Mander said.
The study suggests -- but does not prove -- that insufficient deep sleep contributes to "a reduced ability to cement memories in the brain over the long-term, resulting in greater memory loss," he noted.
In particular, disrupted sleep can lead to impairment of "episodic memory," which helps people remember events, Mander said.
"For example, what we had for breakfast last Tuesday and who we were with, and what that person's name is. This is a critical form of memory that helps us navigate our daily lives. Without it, we quickly become lost, and our interaction with our world disjointed," Mander explained.
Sleep disorders are frequently reported in Alzheimer's patients, noted one expert.
Dr. Ricardo Osorio, research assistant professor of psychiatry with the Center for Brain Health at NYU Langone Medical Center in New York City, said sleep disorders "have a significant impact on caregivers and are a common cause for early institutionalization."
But which comes first, poor sleep or accumulation of the brain plaques? Mander thinks they contribute to each other, creating a "vicious cycle" that leads toward Alzheimer's disease.
Osorio said the study does point to this possibility.
Is it possible that elderly people don't sleep as well as younger people, boosting their risk of Alzheimer's? Maybe not. Osorio said that "in healthy elderly individuals, the rate of normal sleep is quite high."
But poorer sleep throughout life appears to boost the risk of Alzheimer's, he said, and better sleep lowers the risk.
"Insomnia has been shown to promote cognitive decline in the elderly, and sleep apnea both increases the risk for developing Alzheimer's and reduces the age of onset of Alzheimer's," Orsio said. (Sleep apnea is a sleep disorder characterized by repeated breathing disruptions.)
In the big picture, both experts agreed, sleep matters, and better sleep can likely help on the Alzheimer's front.
The study was published in the June 1 issue of Nature Neuroscience.
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SOURCES: Bryce Mander, Ph.D., postdoctoral fellow, University of California, Berkeley; Ricardo Osorio, M.D., research assistant professor, psychiatry, Center for Brain Health, NYU Langone Medical Center, New York City; June 1, 2015, Nature Neuroscience