A group of studies, all conducted independently, have reached a similar sobering conclusion: sleep problems—including several common sleep difficulties faced by millions of people on a regular basis—may, over time, lead to cognitive impairment and even dementia.
Four studies presented at the Alzheimer’s Association International Conference investigated the possible relationship between sleep disorders and cognitive function among older adults. Each study looked at the question from a different angle, and used different methods to obtain results. The collective result? A watershed of information that some of our most common sleep troubles—including sleep deprivation, oversleeping, daytime tiredness, and sleep-disordered breathing—may play a significant role in cognitive decline as we age.
Sleeping too little—or too much—linked to cognitive decline
Researchers at Brigham & Women’s Hospital, Boston used data from the Nurses’ Health Study to examine risk factors for cognitive problems in older adults. The Nurses’ Health Study is a landmark, long-running, and ongoing study of women’s health, using information collected from thousands of nurse-participants.
To investigate the relationship between sleep and cognitive decline, researchers examined data from 15,263 women, all of whom were 70 years or older at the time of the first cognitive assessment. The women received follow-up cognitive tests every other year for the following six years. Participants were also asked to report their daily sleep patterns and habits. Researchers found that sleeping too little and sleeping too much were both associated with cognitive decline over time:
* Women who slept five hours a day or less had lower average cognitive scores than women who slept seven hours per day.
* Women who slept more than nine hours had lower average scores than women who slept seven hours per day.
* Women whose sleep duration changed by more than two hours—up or down—had lower cognitive assessment scores than women whose daily sleep patterns did not change significantly.
* In a small subgroup of women, researchers analyzed blood samples for changes in protein levels that are considered markers for changes to the brain that are associated with Alzheimer’s disease. Women who slept more or less than seven hours were more likely to show evidence of these Alzheimer-indicative changes to protein levels.
Sleep-disordered breathing may increase risk of dementia
A study led by scientists at University of California San Francisco, also examined the relationship between cognitive function and sleep disorders among older women. In this study researchers did not use self-reported information about sleep. Instead, they measured sleep quality and quantity among 1309 women ages 75 and older, over a period of several days. Researchers used polysomnography and wrist sensors to collect detailed data about physical changes that occurred during sleep—including brain activity and breathing—as well as information about rest and activity levels. Five years later, the women were given tests to measure their cognitive functioning. The results of this study showed a connection between disrupted sleep—especially sleep-disordered breathing—and diminished cognitive function:
* Women with sleep-disordered breathing were more than two times as likely to show evidence of dementia than women without sleep-disordered breathing.
* Women who showed signs of disrupted sleep cycles were also more likely to demonstrate cognitive problems and dementia.
* Total sleep time was not associated with cognitive decline among these women. However, women who experienced greater periods of nighttime wakefulness scored lower on their cognitive function tests.
A preliminary version of these results was published last year in the Journal of the American Medical Association.
Daytime sleepiness may be a risk for cognitive impairment
An international group of researchers, from INSERM and Stanford University, used data from the Three-City Study to examine the effect of sleep on cognitive function among men and women age 65 and older. The study used self-reported sleep information from 4,894 adults, including details on trouble falling asleep and staying asleep, poor sleep quality, waking early, and daytime sleepiness. Participants were given cognitive assessments every two years over a 10-year period.
In contrast with some of the other findings on sleep and cognition presented at AAIC, researchers in this study did not find that sleep duration, or difficulty falling asleep and staying asleep, were associated with cognitive decline. They did find, however, that excessive daytime sleepiness was associated with diminishing cognitive function among these older adults.
Do disruptions to circadian rhythms lead to dementia?
A fourth study examined a hypothesis related to Alzheimer’s disease: that abnormal levels of a particular protein known as amyloid-beta contributes to dementia. Changes in the body’s production and clearance of this protein are considered a bio-marker for Alzheimer’s disease. Levels of amyloid-beta protein naturally change in the body over time, and also fluctuate within a 24-hour period, and we don’t currently know a lot about why or how these natural cycles work. This limited understanding makes accurately measuring and tracking abnormalities more difficult.
Scientists at St. Louis’ Washington University School of Medicine examined changes in amyloid-beta protein in relation to circadian rhythms, and found that the protein’s daily fluctuations conform to circadian patterns, suggesting that disruptions to circadian rhythms—which happen with age as well as with disordered sleep—may affect the production and regulation of this protein linked to dementia.
This is a lot to digest. In the case of each study, there is more work to be done to establish a causal link between disrupted sleep and cognitive decline. We’re still a long way from a real understanding about just how sleep affects cognitive function over the course of a lifetime, and how lack of sleep may contribute to the onset of dementia or other conditions involving cognitive impairment and decline. But such unanimity among the results of no fewer than four separate studies certainly feels like the sounding of an alarm.
Michael J. Breus, PhD
The Sleep Doctor™