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Known as rapid eye movement (REM) sleep, this critical phase "occurs in intervals throughout the night, and is characterized by more dreaming and rapid eye movements," explained study author Matthew Pase. He is a senior research fellow with Swinburne University of Technology in Australia, and a visiting researcher in the department of neurology at Boston University School of Medicine.
"We found that persons experiencing less REM sleep over the course of a night displayed an increased risk of developing dementia in the future," Pase said. He noted that for every 1 percent drop in REM sleep, the seniors in his study saw their dementia and Alzheimer's disease risk go up by about 9 percent.
While prior research has pointed to this REM-dementia link, the current investigation is the first to link less REM sleep to a higher risk for developing dementia up to 18 years down the road. And that, said Pase, means that "changes in REM sleep may not simply be a consequence of dementia," but rather potentially a contributing cause.
Sleep moves through five distinct phases each night: light sleep; a pre-deep sleep preparatory phase; two subsequent stages of deep sleep; and finally REM sleep.
It is during REM -- which repeats itself several times over the course of a typical night -- that the eyes move quickly. REM is also characterized by greater brain activity, quicker breathing and pulse, and a relatively higher body temperature.
Pase's study focused on 321 men and women aged 60 and up (average age of 67) who had participated in the Framingham Heart Study (FHS) between 1995 and 1998. The FHS is a long-term study that includes residents of Framingham, Mass.
The research team first tracked the sleep cycle of the men and women over the course of a single night. All the patients were then tracked for signs of dementia for up to 19 years (12 years, on average). Ultimately, 32 participants developed dementia. Twenty-four of those people developed Alzheimer's.
The investigators concluded that deep sleep patterns had no bearing on dementia risk. But REM patterns did.
Specifically, those for whom REM comprised 20 percent of their total sleep did not go on to develop dementia. But that number fell to 17 percent of overall sleep among those who did develop dementia.
And even after adjusting for other factors -- including heart disease risk, depression and medication routines -- the team concluded that those who spent less time in REM and/or took longer to actually enter into the REM phase, faced a greater risk for dementia.
That said, Pase stressed that "our results do not test a cause-and-effect relationship," but rather show an association.
As to what might explain the link between REM and dementia, he noted that "more REM sleep may help protect connections in the brain, which become damaged with dementia."
The findings were published online Aug. 23 in the journal Neurology.
Dr. Yvette Sheline, director of the Center for Neuromodulation in Depression and Stress at the University of Pennsylvania's School of Medicine in Philadelphia, pointed to prior animal research that indicated mice that are deliberately disturbed during sleep have greater deposits of amyloid plaque in their brains. Such deposits are believed to boost Alzheimer's risk.
Meanwhile, Dr. Ricardo Osorio, a research assistant professor of psychiatry with the Center for Brain Health at NYU Langone Medical Center in New York City, noted that part of the difficulty with trying to better understand REM's influence on dementia risk "is that we don't have a good explanation of why we have REM sleep."
According to Osorio, "We know that [REM sleep] is important for the survival of most mammalian and avian species" and also appears to help facilitate memory function and infant brain development. "But that is about it," he added.
Still, he stressed that most sleep-dementia research has focused on the role of deep sleep rather than REM sleep.
"So, in my opinion, this study brings back REM sleep as a sleep stage that is important for brain function, and that neural activity occurring specifically during REM might be critical for normal memory consolidation and protective against Alzheimer's," Osorio said.
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