By Salynn Boyles

Sleeping for extended amounts of time may be an early indicator of cognitive decline in older people, especially among those with lower education levels, researchers report.

Elderly participants who consistently slept more than 9 hours a night had double the dementia risk over a decade of follow-up in an analysis of data from the Framingham Heart Study.

Longer sleepers also had smaller brain volumes in the study of close to 2,500 older men and women.

Study participants without high school degrees who slept more than 9 hours a night had six times the risk of developing dementia over the decade-long follow-up, compared with participants who slept less than 9 hours a night.

The findings suggest that longer sleep duration may be a marker of early neurodegeneration, wrote Sudha Seshadri, MD, of the Boston University School of Medicine Alzheimer's Disease Center, and colleagues.

Another co-author, Matthew Pase, PhD, also of Boston University, stated that several previously reported studies examining self-reported sleep duration and dementia risk in older populations have had mixed findings: some suggested that it is actually sleeping for shorter periods that has the association with increased dementia risk, while others found long-duration (>9 hours) sleep to be associated with the greater risk.

The newly reported analysis of Framingham Heart Study data is the first to span two generations of participants asked about their sleep patterns twice during their participation in the study, Pace explained.

"Participants were asked about their sleep duration 13 years apart. We did not see an increased risk in people who said they were excessive sleepers at both time points. People who reported excessive sleep duration on the second inquiry, but not the first, showed the higher risk."

The analysis included 2,457 Framingham Heart Study participants (mean age 72±6; 57% women) who self-reported total hours of sleep as a three-level variable: 6 hours was categorized as short, 6 to 9 hours was considered the reference point, and more than 9 hours was categorized as long.

During the 10 years of follow-up, 234 cases of all-cause dementia were observed, and 181 cases (8% of total cohort) were clinically consistent with Alzheimer's disease. The analysis revealed that:

On multivariable analyses, prolonged sleep duration was associated with an increased risk of incident dementia (HR 2.01; 95% CI 1.24-3.26)

The findings were driven by individuals with baseline mild cognitive impairment (HR 2.83; 95% CI 1.06-7.55) and those without a high school degree (HR 6.05; 95% CI 3.00-12.18)

Transitioning to sleeping more than 9 hours over a mean period of 13 years before baseline was associated with an increased risk of all-cause dementia (HR 2.43; 95% CI 1.44–4.11) and clinical Alzheimer disease (HR 2.20; 95% CI 1.17-4.13)

Long sleep duration was also associated on cross-sectional analysis with smaller total cerebral brain volume (TCBV) (β±SE, -1.08±0.41 mean units of TCBV difference) and poorer executive function.

"Collectively, these results suggest that long sleep duration serves as an early biological marker of neurodegeneration, especially in those with low educational attainment," the researchers wrote.

Study limitations cited by the authors included possible confounding due to unrecognized mild cognitive impairment at baseline and the assessment of sleep duration through self-reporting, which may not be reliable. Still, self-reported sleep duration "is easy to collect, thus increasing the applicability of our results to general practice," the researchers said.

"Patients reporting long sleep duration and cognitive complaints to their primary care provider could be triaged for further dementia screening, without the need for overnight sleep studies. As our findings were driven by those with low educational attainment, we identified a rather select subgroup that may warrant further screening for dementia. Thus, self-reported sleep duration may be a useful clinical tool to help predict persons at risk of progressing to clinical dementia within 10 years."

Reviewed by F. Perry Wilson, MD, MSCE Assistant Professor, Section of Nephrology, Yale School of Medicine and Dorothy Caputo, MA, BSN, RN, Nurse Planner