Alzheimer's disease, which is the sixth leading cause of death among adults in the US, is difficult to spot in the early stages because symptoms don't appear until later.
The study, published in JAMA Neurology, is the latest to link the sleep-wake cycle to the neurodegenerative disease and could help doctors identify people at-risk of developing the disease years in advance. 'In this new study we found people with pre-clinical Alzheimer's disease had more fragmentation in their [sleep-wake] activity patterns - with more periods of inactivity or sleep during the day and more periods of activity at night,' said senior author Dr Yo-El Ju of Washington University.
Symptoms of Alzheimer's disease, which includes inability to create new memories and forgetfulness, usually do not appear until after the age of 60, according to the Centers for Disease Control and Prevention.
One of the hallmarks of the disease is the accumulation of beta-amyloid plaques between nerve cells in the brain. Clumps of this toxic brain protein destroys memory and causing confusion.
For the study, researchers at Washington University in St Louis analyzed data from 189 cognitively normal adults with an average age of 66. Their sleep cycles were tracked for one to two weeks using devices similar to exercise trackers.
Some underwent PET scans to look for Alzheimer's-related amyloid plaques in their brains and others cerebro-spinal fluid tests. A third group had both.
Researchers found that those with beta-amyloid plaques were either nodded off during the day, frequently had their sleep disrupted, or both.
In other words, people who experienced short spurts of activity and rest during the day and night were more likely to have evidence of amyloid build-up in their brains.
Among the 50 whose results were abnormal - suggesting presence of the neuron-killing protein clumps - they were either waking up regularly during the night, nodding off in the day or both.
The 139 others had no evidence of the amyloid protein that signifies pre-clinical Alzheimer's. Most had normal sleep-wake cycles although several had circadian disruptions that were linked to advanced age, sleep apnea or something else.
The subjects from Washington University's Knight Alzheimer's Disease Research Centre wore devices similar to exercise trackers for one to two weeks.
Previous studies have linked sleep activity to the development of Alzheimer's disease.<!-- Global site tag (gtag.js) - Google Analytics -->
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Research published last year in the journal Brain found that a poor night's sleep was associated with higher levels of amyloid plaque, proteins associated with Alzheimer's disease.
A separate 2015 study in mice being published in The Journal of Experimental Medicine showed similar circadian disruptions speeded up the development of the beta-amyloid plaques.
The team's previous research in people and animals has found levels of the protein fluctuate fall during sleep - and rise when this is interrupted or when people don't get enough deep sleep.
However, the current study isn't just about lack of sleep, it's about how people sleep.
'It wasn't the people in the study were sleep-deprived. But their sleep tended to be fragmented,' said researcher Dr Erik Musiek of Washington University. 'Sleeping for eight hours at night is very different from getting eight hours of sleep in one-hour increments during daytime naps.'
Researchers said it's top early to answer the chicken-and-egg question of whether disrupted circadian rhythms put people at risk for Alzheimer's - or whether Alzheimer's-related changes in the brain disrupt circadian rhythms.
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