By Jeanna D. Smiley — Fact checked by Catherine Carver, MPH
China has the world’s largest population of people with dementia, a neurodegenerative disorder. At least 6% of older adults, or one in 20 people age 60 or over, live with dementia.
However, most studies of sleep and cognitive impairment have focused on Caucasian populations in North America and Europe.
A recent population-based study of older people in rural China linked prolonged sleep and early sleep timing with an increased risk of dementia.
The study also found that even in those who did not develop dementia during the study time, there was still a degree of cognitive decline associated with prolonged sleep and early bedtimes. However, this particular finding was evident only in older people ages 60–74 and men.
This clinical investigation appears in the Journal of the American Geriatrics Society.
Sleep and dementia risks
Medical News Today discussed this study with Dr. Verna Porter, a neurologist and director of the Dementia, Alzheimer’s Disease, and Neurocognitive Disorders at Providence Saint John’s Health Center in Santa Monica, CA, who was not involved in the current research.
“It is important for studies to evaluate populations other than white (Caucasian), largely urban populations from North America or Western Europe. This study evaluates rural adults from China […] with unique socioeconomic, cultural, education and lifestyle practices,” Dr. Porter said.
Older adults in rural China typically go to sleep earlier, rise earlier, and have lower quality sleep than their white counterparts or people in urban areas.
Research shows that dementia occurs more often in the country’s rural regions than in its developed parts.
The present study’s aim was to “examine the associations of self-reported sleep characteristics (e.g., TIB, sleep timing, sleep duration, sleep quality, and EDS) with incident dementia, Alzheimer’s disease (AD), and cognitive decline, while taking into account their potential interactions with demographic features and APOE genotype.”
The study’s methods
The current cohort study recruited participants in the Shandong Yanggu Study of Aging and Dementia, which involved rural older people in western Shandong province.
Over several months in 2014, the researchers performed clinical examinations, in-person interviews, and laboratory tests on 3,274 subjects ages 60 and older.
A total of 1,982 survivors of this baseline group participated in a follow-up examination in 2018. The scientists studied sleep patterns at both baseline and follow-up.
They noted characteristics including:
- mid-sleep time (the median between bedtime and rise-time, to represent circadian phase)
- sleep latency (amount of time in minutes taken to fall asleep at night)
- sleep efficiency (the proportion of time spent sleeping while in bed)
Assessing cognition, and dementia diagnosis
The study’s authors used the Mini-Mental State Exam (MMSE) to measure cognitive function. They used criteria from the Diagnostic and Statistical Manual of Mental Disorders (DSM-IV) to diagnose dementia.
Next, the researchers recorded results from a statistical model adjusted for age, sex, and education.
They used another model to adjust for body mass index, smoking, alcohol use, diabetes, cardiovascular diseases, and APOE genotype.
How sleeping over 8 hours affects risk
During the study’s average follow-up period of 3.7 years, 97 of the 1,982 participants were diagnosed with dementia.
The participants’ mean age at baseline was 70.05. Women made up 59.6% of the sample, 83% were ages 60–74, and 38.2% of the subjects had no formal education.
The dementia risk was 69% greater for individuals who slept over 8 hours, versus 7-8 hours. The risk was also twice as high for those who went to sleep before 9:00 p.m., versus 10:00 p.m. or later.
Among those who did not develop dementia during the study, baseline long TIB, early bedtime and mid-sleep time, and early and late rise time metrics were “significantly associated” with a greater reduction in cognitive decline as evidenced by MMSE scores.
Age and gender variances
Further, whilst the dementia results were the same in different demographic groups, the cognitive decline changes in those free from dementia were evident only among individuals ages 60–74 years, but not among subjects ages 75 years and older.
Likewise, the study found that early and late rise times corresponded with a greater MMSE score decline in men but not in women.
Dr. Porter offered possible reasons for the higher risk of cognitive decline in men:
“Cultural expectations [regarding] traditional gender roles, and [their] impact on job choice and socioeconomic engagement, may potentially differentially affect men in rural China given their frequent role as the primary ‘breadwinner’ and their traditional engagement in more physically demanding and potentially stressful employment.”
MNT also discussed this study with Dr. Michal Schnaider-Beeri, a psychiatry professor and director of the Sagol Neuroscience Research Center at The Sheba Medical Center in the Mount Sinai School of Medicine in New York.
Dr. Beeri noticed the strong association of sleep with cognitive decline in men. “Certain sleep disturbances such as sleep apnea, [which is] prevalent in men, might be part of the explanation,” she said.
However, as Dr. Porter pointed out to MNT that the study did not address the presence or absence of sleep apnea.
Insights and more questions
Dr. Beeri was impressed with the study’s broad scope in covering sleep problems. Its unique population, large sample size, and adjustment for factors including age, sex, education, and more made this a robust work, she said.
The results do not show causality, though. For instance, the researchers could not pinpoint exact reasons for the age-related differences with cognitive decline. The sex differences in cognitive outcomes are still “poorly understood” as well.
Neither did the study consider mood-related symptoms or daytime napping, which is common among older adults in rural China.
Another limitation of this study is the use of self-reporting, which carries the potential for recall bias. Multiple testing could have produced false positive associations, the study’s authors noted.
Since the study’s participants came from only one region in China, the researchers urged caution when generalizing the results to other populations.
Further, Dr. Porter mentioned that the follow-up period was short.
Bridging the gap
The study’s authors hope that their findings “may partly bridge the knowledge gap” regarding people with low socioeconomic status.
They say that their results should encourage monitoring of older adults “who report prolonged TIB and advanced sleep timing, especially in older individuals [ages] 60–74 years and men.”
Future work may explore how reducing TIB and adjusting sleep timing might hold back the onset of cognitive decline and dementia.
This article originally appeared here and was republished with permission.