Like so many other good things in life, sleep is best in moderation. A multi-year study of older adults found that both short and long sleepers
experienced greater cognitive decline than people who slept a moderate
amount, even when the effects of early Alzheimer’s disease were taken
into account. The study was led by researchers at Washington University
School of Medicine in St. Louis.
Poor sleep and Alzheimer disease are both associated with cognitive
decline, and separating out the effects of each has proven challenging.
By tracking cognitive function in a large group of older adults over
several years and analyzing it against levels of Alzheimer disease-related
proteins and measures of brain activity during sleep, the researchers
generated crucial data that help untangle the complicated relationship
among sleep, Alzheimer disease, and cognitive function. The findings could aid
efforts to help keep people’s minds sharp as they age.
The findings are published Oct. 20 in the journal Brain.
“It’s been challenging to determine how sleep and different stages of
Alzheimer disease are related, but that’s what you need to know to
start designing interventions,” said first author Brendan Lucey, MD, an associate professor of neurology and director of the Washington University Sleep Medicine Center.
“Our study suggests that there is a middle range, or ‘sweet spot,’ for
total sleep time where cognitive performance was stable over time. Short
and long sleep times were associated with worse cognitive performance,
perhaps due to insufficient sleep or poor sleep quality. An unanswered
question is if we can intervene to improve sleep, such as increasing
sleep time for short sleepers by an hour or so, would that have a
positive effect on their cognitive performance so they no longer
decline? We need more longitudinal data to answer this question.”
Alzheimer disease is the main cause of cognitive decline in older adults,
contributing to about 70% of dementia cases. Poor sleep is a common
symptom of the disease and a driving force that can accelerate the disease’s progression.
Studies have shown that self-reported short and long sleepers are both
more likely to perform poorly on cognitive tests, but such sleep studies
typically do not include assessments of Alzheimer disease.
To tease apart the separate effects of sleep and Alzheimer disease
on cognition, Lucey and colleagues turned to volunteers who participate
in Alzheimer disease studies through the university’s Charles F. and Joanne Knight Alzheimer Disease Research Center.
Such volunteers undergo annual clinical and cognitive assessments and
provide a blood sample to be tested for the high-risk Alzheimer disease
genetic variant APOE4. For this study, the participants also
provided samples of cerebrospinal fluid to measure levels of Alzheimer disease
proteins, and each slept with a tiny electroencephalogram (EEG) monitor
strapped to their foreheads for four to six nights to measure brain
activity during sleep.
In total, the researchers obtained sleep and Alzheimer disease data on 100
participants whose cognitive function had been monitored for an average
of 4 1/2 years. Most (88) had no cognitive impairments, 11 were very
mildly impaired, and one had mild cognitive impairment. The average age
was 75 at the time of the sleep study.
The researchers found a U-shaped relationship between sleep and
cognitive decline. Overall, cognitive scores declined for the groups
that slept less than 4.5 or more than 6.5 hours per night — as measured
by EEG — while scores stayed stable for those in the middle of the
range. EEG tends to yield estimates of sleep time that are about an hour
shorter than self-reported sleep time, so the findings correspond to
5.5 to 7.5 hours of self-reported sleep, Lucey said.
The U-shaped relationship held true for measures of specific sleep
phases, including rapid eye movement (REM), or dreaming, sleep; and
non-REM sleep. Moreover, the relationship held even after adjusting for
factors that can affect both sleep and cognition, such as age, sex,
levels of Alzheimer disease proteins, and the presence of APOE4.
“It was particularly interesting to see that not only those with
short amounts of sleep but also those with long amounts of sleep had
more cognitive decline,” said co-senior author David Holtzman MD, a professor of neurology. “It suggests that sleep quality may be key, as opposed to simply total sleep.”
Each person’s sleep needs are unique, and people who wake up feeling
rested on short or long sleep schedules should not feel compelled to
change their habits, Lucey said. But those who are not sleeping well
should be aware that sleep problems often can be treated.
“I ask many of my patients, ‘How’s your sleep?’” said co-senior author Beau M Ances MD PhD,
the Daniel J Brennan MD, Professor of Neurology. Ances treats
patients with dementia and other neurodegenerative conditions at
Barnes-Jewish Hospital. “Often patients report that they’re not sleeping
well. Often once their sleep issues are treated, they may have
improvements in cognition. Physicians who are seeing patients with
cognitive complaints should ask them about their quality of sleep. This
is potentially a modifiable factor.”
Source: News Release
Washington University School of Medicine
October 20, 2021