Author: Heidi Splete
MedCentral
Older adults categorized as short sleepers with higher variability in sleep patterns were significantly more likely to show impaired cognitive performance, based on data from more than 800 individuals.
While previous research has associated short sleep duration (less than 7 hours) with cognitive impairment in healthy aging adults, most studies have relied on simple measures of sleep, wrote Samantha A. Keil, PhD, of the VA Puget Sound Health Care System, Seattle, Washington, and colleagues.
Her team’s new findings on sleep and cognition, published in JAMA Network Open last month, included data from 826 adults in the Seattle Longitudinal Study (SLS), a community-based cohort in Seattle. Self-reported sleep duration was based on data from 1993 and 2012 while cognitive performance data was pulled from 1997 to 2020. Data analysis was performed from September 2020 to May 2023. The mean age of the participants was 76.3 years; 56.7% were women.
Sleep Duration, Sleep Variability May Impair Cognition
The primary outcome was cognitive impairment, defined by below-threshold performance on the Mini-Mental State Examination (MMSE) and the Mattis Dementia Rating Scale.
In general, being a short sleeper and having higher sleep variability were significantly associated with later cognitive impairment (hazard ratios 3.67 and 3.06, respectively).
Sleep duration was based on self-reports and classified as short sleep, medium sleep, or long sleep, with median durations of 7 hours or less, 7 hours, and 7 hours or more, respectively, per night. Variability in sleep duration was defined as the standard deviation of median sleep duration.
The findings were limited by the researchers’ reliance on self-reports and lack of assessment of clinical sleep disorders, such as obstructive sleep apnea, circadian dysregulation, and insomnia.
However, the results suggest that longitudinal variability in sleep duration, as well as average sleep duration, may contribute to cognitive decline in older adults, wrote Keil et al.
Sleep Variability Means More than Sleep Duration
Overall data from the sleep and Alzheimer’s disease research field have shown a connection between short sleep and cognitive impairment, noted corresponding author Jeffrey J. Iliff, PhD, of the University of Washington, Seattle.
“As you get older, your sleep can change in many different ways, and we wanted to get a sense of how sleep changes over time relate to cognitive impairment,” he said.
The association between short sleep and later cognitive impairment was not surprising, and aligns with data from previous studies, he explained.
However, “we expected that when we looked at sleep over time, people with declining sleep duration would show more cognitive impairment, but we did not see that.”. Instead, those whose sleep variability shifted over time were more likely to experience cognitive impairment than more consistent sleepers, regardless of duration.
The mechanism for the impact of sleep variability on cognitive impairment remains unclear, Dr. Iliff told MedCentral. Changes in life experiences that come with age, including chronic pain, nighttime bladder issues, even loss of a spouse, can alter sleep habits, he said.
Next steps for research include examining what behaviors or other factors determine sleep variability, and exploring the durations and time scales that affect sleep variability, said Dr. Iliff. “This study measured sleep duration in the long term; we don’t really know what time domain of variability is actually the problem,” he explained.
In the meantime, the team’s findings do suggest “that sleep may be a modifiable risk factor for cognitive impairment later in life,” Dr. Iliff said. Sleep needs to be on the list of factors that physicians consider in routine care of older patients; this could mean addressing issues such as sleep apnea and insomnia with an eye toward maintaining cognitive stability, he said.
Previous Research Links Long and Short Sleep to Cognition
A previous cross-sectional study of 4,417 individuals aged 65 to 85 years showed a significant association between short sleep duration (7 hours or less) and reduced cognition. However, individuals who were long sleepers (defined as 9 hours or more) also scored significantly lower on measures of cognition compared to those with normal sleep duration, according to Joseph R. Winer, PhD, of Stanford University, Palo Alto, CA, who led the study.
Winer and coauthors measured cognition using the MMSE and the Logical Memory Delayed Recall (LMDR) test, and measured early signs of Alzheimer’s disease based on pathology (amyloid β).
Short sleep duration was linearly associated with elevated amyloid beta; long sleep was not, but both short and long sleep groups were associated with worse performance on cognitive measures. Short sleepers scored worse on the MMSE and LMDR than those with normal sleep duration, while long sleepers scored worse on the Digit Symbol Substitution Test (DSST).
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