A new study suggests that insufficient 'dreaming sleep' may increase the risk of dementia.
It is known that people with dementia often have sleep disturbances. However, researchers do not yet know why these sleep troubles occur, nor is it clear whether different kinds of sleep and sleep phases influence the chances of developing dementia.
Put simply, sleep occurs in two large stages: REM and non-REM. In more detail, the first four stages are all non-REM. The fifth and last sub-phase of sleep, REM, typically occurs after 90 minutes of non-REM sleep, and this is when the brain does most of its dreaming.
The first author of the new study is Matthew Pase, Ph.D., of the Swinburne University of Technology in Melbourne, Australia. Dr. Pase and team examined data from the large population-based Framingham Heart Study (FHS), which started in 1971.
Studying REM sleep and dementia risk
Dr. Pase and colleagues examined a subset of FHS participants who had taken part in the Sleep Heart Health Study for 3 years between 1995 and 1998.
At the time of that study, the participants were at least 60 years old and half of them were male. Sleep stages were assessed using a home-based sleep study, or polysomnography, which is a widely used approach for assessing sleep phases and disorders.
The test can determine sleep cycles and different stages of sleep, such as REM or non-REM, by recording things such as physiological changes and brain waves.
The team followed up the participants for up to 19 years, all the while looking for cases of dementia. During the follow-up period, Dr. Pase and colleagues noted 32 diagnoses of dementia, 24 of which were Alzheimer's disease.
Those who developed dementia spent 17 percent of their entire sleep in REM, on average, whereas those who did not receive a dementia diagnosis spent 20 percent, on average, in REM.
Less REM correlates with dementia risk
The researchers adjusted for variables such as sex and age, and they found a strong correlation between higher dementia risk and a lower percentage of REM sleep and longer "REM sleep latency," which refers to how long it takes someone to reach the REM phase.
Specifically, with every percentage point that REM sleep decreased, the researchers found a 9 percent increase in dementia risk.
The correlation withstood adjustments for possible confounders such as cardiovascular risk factors, depression, and medication.
Dr. Pase and team excluded from their analysis those who had mild cognitive impairment - an early marker of Alzheimer's - at baseline, as well as those who developed dementia early in the study.
He spoke to Medical News Today about the significance of the findings.
"In our study, the association between lower REM sleep and dementia was not explained by those with cognitive impairment at baseline or by those who converted to dementia within the first 3 years, [which] suggests that reduced REM is not simply a consequence of early dementia."
Matthew Pase, Ph.D.
Speaking to MNT about the possible mechanisms that could explain the findings, he said, "We do not have the exact answers but we have a few hypotheses."
"Possible mechanisms may include (i) stress or anxiety, which may curtail REM sleep and increase risk for dementia, (ii) there may be a contributing role of sleep disorders such as sleep-disordered breathing, which may disrupt REM sleep and increase risk for dementia, and (iii) more REM sleep may help maintain brain integrity in the face of changes that occur with aging and early Alzheimer's disease."
Strengths and limitations of the study
The authors also highlight some strengths and limitations of their study. Strengths, they write, include the fact that the study is population-based. However, the research is limited by the small size of the sample and its predominantly Caucasian ethnic makeup.
As a consequence of the reduced study sample, there were also few incident events, which means that the researchers "had limited power to detect weaker but potentially important associations with other sleep-related exposures and dementia."
The long follow-up duration of the study and the use of polysomnography to examine sleep architecture are listed by the authors as further strengths of the research.
"Future research should confirm our findings and determine the mechanisms linking REM sleep to incident dementia," the researchers write.
Dr. Pase also spoke to MNT about the study and areas of future research. He said, "Although our study is small compared to the larger parent Framingham Heart Study cohort, it is still a large sample given that all participants underwent an overnight sleep study and follow-up for dementia."
"There are few other studies that have these data. We are currently investigating whether others have similar data and whether it may be possible to combine analysis and results."