They found that in older people with no signs of cognitive impairment, those who had a messy sleep-wake cycle were more likely to have amyloid protein deposits in their brains.
"It wasn't that the people in the study were sleep-deprived," study author Erik S. Musiek, said in a news release. "But their sleep tended to be fragmented. Sleeping for eight hours at night is very different from getting eight hours of sleep in one-hour increments during daytime naps."
Those amyloid protein deposits (or plaques) are considered sure signs of the disease. Amyloid levels typically decrease during sleep and increase when sleep is disrupted or when people don’t get enough good sleep, researchers said.
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In Alzheimer’s, the amyloid protein divides improperly to form a sticky beta amyloid, which is toxic to neurons in the brain and leads to the popular symptoms associated with the disease, including memory loss.
Alzheimer’s-associated memory loss can begin 15 or 20 years before symptoms of the disease are even noticed.
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"At the very least, these disruptions in circadian rhythms may serve as a biomarker for preclinical disease," senior author Yo-El Ju added. "We want to bring back these subjects in the future to learn more about whether their sleep and circadian rhythm problems lead to increased Alzheimer's risk or whether the Alzheimer's disease brain changes cause sleep/wake cycle and circadian problems."
The new findings are part of growing evidence of the link between sleep problems and Alzheimer’s.
In July 2017, scientists found that after adjusting for age and family history of the disease, poor sleep and daytime sleepiness were linked to spinal fluid indicators of Alzheimer's.
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"Not everyone with sleep problems is destined to develop Alzheimer's disease," senior author of the 2017 study, Barbara B. Bendlin, told the New York Times. "We're looking at groups of people, and over the whole group we find the association of poor sleep with the markers of Alzheimer's. But when you look at individuals, not everyone shows that pattern."