New research provides evidence that heightened variability in sleep patterns, rather than just average sleep duration, is significantly associated with cognitive impairment in older adults. The findings, which have been published in JAMA Network Open, suggest that a more comprehensive evaluation of longitudinal sleep behavior might be necessary to understand the relationships among age, sleep disruption, and cognitive impairment.
There is a well-established association between sleep disruption and dementia. Up to 90% of patients with dementia experience disrupted sleep before the emergence of cardinal disease symptoms. Recent studies suggest that sleep disruption might not just be a consequence of dementia but could also contribute to the pathological processes underlying dementia.
However, previous studies on the link between sleep disruption and dementia often relied on simple, usually cross-sectional, measures of sleep. The new study aimed to provide a more comprehensive understanding by evaluating longitudinal sleep patterns. In particular, the researchers were interested in how changes in sleep patterns over time might influence cognitive health.
“Our lab studies a new function of sleep that has been characterized over the last ten years – that during sleep the brain shifts into a kind of ‘cleaning mode,’ clearing away wastes that accumulate through the course of the waking day,” explained study author Jeffrey Iliff, a professor of psychiatry and behavioral sciences, and of neurology at the University of Washington School of Medicine and the associate director for research at the VISN20 Northwest Mental Illness Research, Education and Clinical Center at the VA Puget Sound Healthcare system.
“Our work has shown that an impairment of this process appears to be one of the factors that may make the aging and injured brain vulnerable to the processes leading to conditions like Alzheimer’s disease. So when our group relocated to the University of Washington in Seattle about 5 years ago, we wanted to start working with local clinical studies to evaluate possible linkages between poor sleep earlier in life, and the development of cognitive impairment in the decades to follow. That is what led us to our initial collaboration with the longstanding Seattle Longitudinal Study.”
For their study, the researchers analyzed data from 826 participants, all part of the the Seattle Longitudinal Study. These participants, whose average age was 76.3 years, had been regularly completing a Health Behavior Questionnaire and undergoing neuropsychological testing for several years.
To gauge the sleep patterns of participants, the study used a questionnaire assessing their average nightly sleep duration over the past week. This questionnaire was repeated at intervals of 3 to 5 years over a period from 1993 to 2012. The researchers then categorized the participants’ sleep into three categories: short (less than 7 hours), medium (7 hours), and long (more than 7 hours). By having multiple years of data, the researchers could also observe changes in each participant’s sleep duration over time.
Additionally, the participants’ cognitive health was monitored through a series of neuropsychological tests conducted every 5 to 7 years between 1997 and 2019. These tests were designed to identify signs of cognitive impairment, focusing on general cognitive function and specific areas like memory, attention, and language skills. Participants who scored below certain thresholds on these tests were classified as cognitively impaired.
The researchers found that older participants generally reported longer sleep durations. Interestingly, the distribution of sleep duration categories varied significantly with age, with a higher percentage of older adults being long sleepers.
However, the key discovery was the relationship between sleep variability and cognitive health. The researchers found that variability in self-reported sleep duration – the changes and fluctuations in sleep patterns over time – was a more noticeable feature than consistent increases or decreases in sleep duration.
When examining the likelihood of cognitive impairment, being a short sleeper and experiencing high sleep variability were both significantly associated with increased risk. This suggests that it’s not just the amount of sleep one gets that matters, but also the consistency of sleep patterns over the years.
“We were not surprised to see that short sleep durations were associated with cognitive impairment – this is something that has been reported by several prior studies,” Iliff told PsyPost. “We expected though that people whose sleep got shorter over time would be at an increased risk of cognitive decline. But we didn’t see this.”
“It seems to be variability, rather than steady changes in sleep, that are connected with cognitive impairment. It may be gradual changes may matter less than the kinds of things that might go along with big changes in sleep duration – injury, illness, changes in marital status, depression – things that might be reading out in the lives of people as dramatic changes in sleep behavior.”
These findings align with some previous studies that have linked short sleep duration with a higher risk of cognitive impairment. They also bring to light the importance of sleep variability as a factor in cognitive health, an aspect that has not been extensively explored before.
“Your sleep matters, which is something we all understand intuitively,” Iliff explained. “But these findings, and others like them that have begun to emerge the last several years, suggest that the sleep that we get earlier in life – like in midlife – may be having an important impact our cognitive trajectories over the ensuing decades. So just like exercise and diet, paying attention to sleep may be important for maintaining cognitive function in the years to come.”
While the study offers significant insights, it’s important to understand its limitations. One key point is that the study relied on self-reported data for sleep duration. This means that the information about sleep patterns was based on participants’ own assessments, which can be inaccurate.
Looking ahead, future studies might benefit from including objective measures of sleep, such as sleep tracking devices, to complement self-reported data. Additionally, exploring the impact of clinical sleep disorders on sleep variability and cognitive health could provide a more comprehensive understanding of these complex relationships.
“The idea of ‘variability’ in sleep isn’t well understood,” Iliff said. “In this study, sleep was assessed every few years for a couple of decades. We don’t know if variability over the course of days, weeks, months, years, or decades matter the most for cognitive function long-term. That is something that we and others will need to try to understand better in the future.”
The study, “Longitudinal Sleep Patterns and Cognitive Impairment in Older Adults“, was authored by Samantha A. Keil, Abigail G. Schindler, Marie X.Wang, Juan Piantino, Lisa C. Silbert, Jonathan E. Elliott, Madeleine L.Werhane, Ronald G. Thomas, Sherry Willis, Miranda M. Lim, and Jeffrey J. Iliff.