A reanalysis of data on more than one million people in three countries found that certain characteristics of sleep - duration, insomnia and daytime sleepiness - evolve as people age. A second study, which looked at whole-night electroencephalography from nearly 4,000 older adults, found that certain metrics of sleep macro- and micro-architecture are predictive of cognitive performance.
You can look at sleep as a window on brain activity, said the lead author of the brain-wave study, Shaun Purcell, an associate professor of psychiatry at Harvard Medical School and Brigham and Women's Hospital in Boston. "Sleep is a good time to look at brain activity, free from external stimuli," Purcell said.
"We found that it was not total amount of sleep and not even how people felt in the morning - rested or tired - but really the 'invisible' structures of sleep that mattered," Purcell said. "People who were younger or healthier cognitively, had more dream, or REM, sleep."
REM sleep involves the cholinergic system, and that may be why it's a marker for cognitive health, Purcell said. That system also appears to be implicated in Alzheimer's disease.
There were also differences in slow-wave sleep that seemed to correlate with cognitive health, Purcell said. "Relative to cognition, it's not so much the total time spent in slow-wave sleep, but more the precise quality of slow-wave sleep," he said, adding that in older people the slow-wave signal appeared to be blunted.
To take a closer look at a variety of sleep metrics, Purcell and his colleagues analyzed data from cognitive tests and EEG recordings gathered in an ancillary study from two existing cohorts: the Multi-Ethnic Study of Atherosclerosis (MESA) and the Osteoporotic Fractures in Men Study (MrOS).
Along with polysomnography, cognition was tested in the 1,595 men and women in MESA and the 2,224 men in MrOS. The MESA participants were evaluated with four measures: the Digit Symbol Coding Test, the Cognitive Abilities Screening Instrument and the Digit Span Test forward and backward. The MrOS participants were evaluated with three measures: Trails B, an expanded version of the Mini-Mental State Examination and the Digit Vigilance Test. The participants in MESA ranged in age from 54 to 93, while those in MrOs ranged in age from 67 to 96.
In all, 23 out of 173 metrics studied predicted cognitive performance, and processing speed in particular, with effects that were broadly independent of gross changes in sleep quality and quantity, the study found. "Taken together, our results point to multiple facets of sleep neurophysiology that track coherently with underlying, age-dependent determinants of cognitive and physical health trajectories in older adults," the authors conclude.
In the meta-analysis study, researchers also found that changes in self-reported sleep quality tracked with age. The authors analyzed studies published between 2000 and 2017 with data on sleep in the general population assessed with questionnaires. They focused on results from 200,358 Dutch participants aged 1 to 100 years, 471,759 participants in the UK aged 40 to 69, and 409, 617 U.S. participants aged 18 and older.
One in four participants slept less than age-specific recommendations, but only 5.8% slept outside of the 'acceptable' sleep duration. Teens tended to report total sleep times that were less than the recommended 8 to 10 hours, with 18% saying they experienced daytime sleepiness. In participants aged 18 and older, 13.3% reported poor sleep quality and 9.6% to 19.4% reported insomnia symptoms.
Overall, the time people spent in the sack declined with age, leveling out around age 65.
Insomnia symptoms were least frequent in 26-to-40-year-olds and most frequent in those older than 65. Women who were 41 and older reported sleeping shorter times or slightly less efficiently than men.
"Given that poor sleep is more common than short sleep . . . recommendations for improving sleep might need to focus more on sleep quality," the authors, led by Desana Kocevska of the Erasmus MC University Medical Center in Rotterdam, write.
The authors did not respond to a request for comments.
These are two exciting additions to the sleep literature, said Kristine Wilckens, an assistant professor of psychiatry at the University of Pittsburgh, who wasn't involved in the studies.
"What is particularly significant about (the) meta-analysis of sleep across the lifespan is the number of participants that made up this meta-analysis," Wilckens said in an email. "One major benefit of having such a huge sample to work with is that they were able to examine effects across the entire lifespan … This paper shows clear declines in time in bed and total sleep time with increasing age, as well as declines in sleep efficiency, which is the amount of time spent sleeping while in bed trying to sleep."
One major unique aspect of the brain wave study is the number of sleep variables they tested, Wilckens said. "Other sleep studies will often select only a few sleep measures based on specific hypotheses," she added. "But this study used a data-driven approach … meaning they let the data tell them what specific sleep measures most strongly related to cognitive performance."
"The findings of this study help us to pinpoint the specific physiological aspects of sleep that are most strongly associated with cognition," she added. "From there, the next step is to work on ways to target and improve those specific sleep features with the goal of improving cognition through sleep."
Dr. Makoto Kawai commended the authors of the polysomnography of older adults study for its size.
"Often the data from older people are not sensitive enough to prove that good sleep quality is needed for brain health," said Dr. Kawai, a clinical associate professor in the division of sleep medicine at the Stanford University School of Medicine in California, who also wasn't involved in the studies. "In the older adult study, because the sample size is huge, you start to see that if you can maintain the sleep patterns you have at a younger age as you get older it's good for your brain. I think their approach is very powerful."
A limitation of that study is that the cognitive testing is "crude," Dr. Kawai said. "It's clear that the tests used were chosen because "they are simpler and you can more easily administer them to a large number of people."
The epidemiological study "reinforces what we know," Dr. Kawai said. "It shows that even in our modern world, human beings have not changed much and still need seven to eight hours of sleep. You do see some interesting age effects. Eight hours is usually recommended for middle aged people and seven and a half for those who are older."
By Linda Carroll
SOURCE: https://go.nature.com/35BEgDO and https://go.nature.com/2Ux35KF Nature Human Behaviour, online November 16, 2020.
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