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In a longitudinal study, investigators found older adults who slept less than 4.5 hours or more than 6.5 hours a night reported significant cognitive decline over time, but cognitive scores for those with sleep duration in between that range remained stable.
“This really suggests that there’s this middle range, a ‘sweet spot,’ where your sleep is really optimal,” lead author Brendan Lucey, MD, MSCI, associate professor of neurology and director of the Washington University Sleep Medicine Center, St. Louis, said in an interview.
The study, published online Oct. 20, 2021, in the journal Brain, is part of a growing body of research that seeks to determine if sleep can be used as a marker of Alzheimer’s disease progression.
A complex relationship
Studies suggest a strong relationship between sleep patterns and Alzheimer’s disease, which affects nearly 6 million Americans. The challenge, Dr. Lucey said, is unwinding the complex links between sleep, AD, and cognitive function.
An earlier study by Dr. Lucey and colleagues found that poor sleep quality is associated with early signs of AD, and a report published in September found that elderly people who slept less than 6 hours a night had a greater burden of amyloid-beta, a hallmark sign of AD.
For this new study, researchers monitored sleep-wake activity over 4-6 nights in 100 participants who underwent annual cognitive assessments and clinical studies, including APOE genotyping, as part of a longitudinal study at the Knight Alzheimer Disease Research Center at Washington University.
Participants also provided cerebrospinal fluid (CSF) total tau and amyloid-beta 42 and wore a small EEG device on their forehead while they slept.
The majority of participants had a clinical dementia rating (CDR) score of 0, indicating no cognitive impairment. Twelve individuals had a CDR greater than 0, with most reporting mild cognitive impairment.
As expected, CSF analysis showed greater evidence of AD pathology in those with a baseline CDR greater than 0.
Changes in cognitive function were measured using a Preclinical Alzheimer Cognitive Composite (PACC) score, a composite of results from a neuropsychological testing battery that included the Free and Cued Selective Reminding Test, the Logical Memory Delayed Recall Test from the Wechsler Memory Scale–Revised, the Digit Symbol Substitution Test from the Wechsler Adult Intelligence Scale–Revised, and the Mini-Mental State Examination.
Researchers found an upside-down U-shaped relationship between PACC scores and sleep duration, with dramatic cognitive decline in those who slept less than 4.5 hours or more than 6.5 hours a night (P < .001 for both).
The U-shaped relationship was also found with measures of sleep phases, including time spent in rapid eye movement and in non-REM sleep (P < .001 for both).
The findings persisted even after controlling for confounders that can affect sleep and cognition, such as age, CSF total tau/amyloid-beta 42 ratio, apo E four-allele carrier status, years of education, and sex.
Understanding how sleep changes at different stages of AD could help researchers determine if sleep can be used as a marker of disease progression, Dr. Lucey said. That could lead to interventions to slow that process.
“We’re not at the point yet where we can say that we need to monitor someone’s sleep time and then do an intervention to see if it would improve their risk for cognitive decline,” said Dr. Lucey, who plans to repeat this sleep study with the same cohort to track changes in sleep patterns and cognitive function over time. “But that’s a question I’m very excited to try to answer.”
A component of cognitive health
Commenting on the findings for this news organization, Heather Snyder, PhD, vice president of medical and scientific relations for the Alzheimer’s Association, noted that the study adds to a body of evidence linking sleep and cognition, especially how sleep quality can optimize brain function.
“We’ve seen previous research that’s shown poor sleep contributes to dementia risk, as well as research showing sleep duration may play a role in cognition,” she said.
“We also need studies that look at sleep as an intervention for cognitive health,” Dr. Snyder said. “Sleep is an important aspect of our overall health. Clinicians should have conversations with their patients about sleep as part of standard discussions about their health habits and wellness.”
The study was funded by the National Institutes of Health, the American Sleep Medicine Foundation, the Roger and Paula Riney Fund, and the Daniel J. Brennan, MD Fund. Dr. Lucey consults for Merck and Eli Lilly. Dr. Snyder has disclosed no relevant financial relationships.
A version of this article first appeared on Medscape.com.