Too much sleep and time in bed (TIB) may raise the risk of developing dementia and Alzheimer’s disease (AD), new research suggests.

Results from a population-based study of almost 2000 older adults, all of whom were free of dementia at baseline, showed that risk for dementia was 69% higher in those who slept more than 8 hours daily vs 7 to 8 hours daily; and it was twice as high for those who went to bed before 9 PM vs at 10 PM or later.

These associations were most pronounced in people aged 60-74 years and in men, the researchers report.

“Our study shows that self-reported sleep problems, such as long TIB and early sleep timing, are independently associated with incident dementia and AD in Chinese rural older adults, and their associations with greater global cognitive decline vary by age and sex,” Rui Liu, MD, Department of Neurology, Shandong Provincial Hospital, Shandong University, Jinan, China, and colleagues write.

“This suggests that cognitive function should be monitored in older adults who report prolonged TIB and advanced sleep timing,” they add.

The findings were published online September 21 in the Journal of the American Geriatric Society.

“Poorly Understood” Association

The researchers note that sleep quantity, quality, and circadian rhythm change as individuals age.

Previous studies have suggested that extreme sleep duration, sleep fragmentation, and excessive daytime sleepiness are associated with cognitive decline or dementia. However, the association of TIB — a composite indicator of sleep duration, latency, and fragmentation — with dementia is “poorly understood,” the investigators write.

The longitudinal association of sleep timing with dementia and cognitive decline in older adults “has rarely been explored,” and most studies have investigated White populations in North America and Europe rather than rural adults in China, the investigators note.

It is additionally unclear whether sleep problems are associated with cognitive phenotypes that vary by demographics and APOE genotype.

To investigate these questions, the current researchers assessed a cohort of 1982 rural-dwelling Chinese older adults (mean age, 70 years; 59.6% women; 38.2% with no formal schooling).

Participants were drawn from the Shandong Yanggu Study of Aging and Dementia. Included were older residents in the western Shandong Province who underwent interviews, clinical examinations, and laboratory tests in late 2014.

Survivors of that cohort participated in a follow-up examination between March and September 2018 as part of the baseline assessments of the MIND-China study.

Sleep characteristics were assessed using the Chinese version of the Pittsburgh Sleep Quality Index and the Epworth Sleepiness Scale. Global cognitive function was assessed using the Mini-Mental State Examination (MMSE).

Participants were divided into the following tertiles, based on their sleep parameters:

All were followed for an average of 3.7 years. During this time, 97 developed dementia, including 68 with AD.

Selective Survival?

After adjusting for the confounders of age, sex, education, body mass index, alcohol consumption, smoking, hypertension, diabetes, dyslipidemia, coronary heart disease, stroke, and APOE genotype, results showed that long TIB and mid-sleep time were significantly associated with increased dementia risk, with a linear association between bedtime and dementia risk.

Every 1-hour advance in bedtime was associated with a 25% increased risk for dementia (95% CI, 1.03 – 1.53). Every 1-hour advance in bedtime and mid-sleep time was associated with an adjusted hazard ratio (HR) of 1.27 (95% CI, 1.01 – 1.59) and 1.49 (95% CI, 1.05 – 2.12), respectively.

The following table shows the fully adjusted HRs when sleep parameters were categorized into tertiles:

“Among individuals who were free of dementia at follow-up, baseline long TIB, early bedtime and mid-sleep time, early and late rise time, and prolonged TIB and advanced bedtime and mid-sleep time from baseline to follow-up were associated with a greater decline in MMSE score (P < 0.05),” the investigators report.

“Long sleep duration has been associated with global brain atrophy, more white matter hyperintensities, and proinflammatory biomarkers (eg, interleukin-6 and C-reactive protein), which may be the pathways linking long sleep duration to dementia,” write the researchers.

The age-varying associations of long TIB, early and late rise time, and early mid-sleep time with greater cognitive decline are “unknown,” they add. However, “selective survival may play a part in age-dependent associations because older age and sleep problems are both associated with mortality.”

Study limitations cited include the use of self-report, which may be subject to recall bias, and confounders not accounted for, such as sleep apnea.

Too Much, Too Little Sleep

Commenting for Medscape Medical News, Percy Griffin, PhD, director of scientific engagement at the Alzheimer’s Association, noted that although previous studies have shown associations between shorter sleep duration and cognitive decline, the current study showed an association with longer sleep time.

It “adds to our understanding of the importance of sleep and how it may impact the risk of developing cognitive decline as we age. The study is telling us that too little or too much sleep might be bad for cognitive health,” said Griffin, who was not involved with the research.

However, he cautioned that more work is needed, adding that “although interesting, there are some limitations to this study, including small sample size, self-reporting on sleep parameters, and the diversity of participants.”

Future research should “also ask questions about the quality of sleep and use more objective measures,” Griffin said.

The SYS-AD Study was funded by the Science and Technology Program for Public Wellbeing of Shandong Province, China. MIND-China was funded in part by grants from the National Key Research and Development Program of China, the National Natural Science Foundation of China, the Academic Promotion Program of Shandong First Medical University, the Taishan Scholar Program of Shandong Province, the Integrated Traditional Chinese and Western Medicine Program in Shandong Province, and the Brain Science and Brain-like Intelligence Technology Research Projects of China. Grants to individual investigators are listed in the original article. The investigators and Griffin have reported no relevant financial relationships.

J Am Geriatr Soc. Published online September 21, 2022. Full text

Batya Swift Yasgur, MA, LSW is a freelance writer with a counseling practice in Teaneck, NJ. She is a regular contributor to numerous medical publications, including Medscape and WebMD, and is the author of several consumer-oriented health books as well as Behind the Burqa: Our Lives in Afghanistan and How We Escaped to Freedom (the memoir of two brave Afghan sisters who told her their story).

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