CHICAGO - A multi-year study of older adults found that both short and long sleepers experienced greater cognitive decline than people who slept a moderate amount, even when the effects of early Alzheimer's disease were taken into account.
To tease apart the separate effects of sleep and Alzheimer's disease on cognition, researchers at Washington University School of Medicine in St. Louis turned to volunteers who participate in Alzheimer's studies.
Such volunteers undergo annual clinical and cognitive assessments, and provide a blood sample to be tested for the high-risk Alzheimer's genetic variant APOE4.
For this study, the participants also provided samples of cerebrospinal fluid to measure levels of Alzheimer's proteins, and each slept with a tiny electroencephalogram (EEG) monitor strapped to their foreheads for four to six nights to measure brain activity during sleep.
In total, the researchers obtained sleep and Alzheimer's data on 100 participants whose cognitive function had been monitored for an average of four and half years. Of them, 88 had no cognitive impairments, 11 were very mildly impaired, and one had mild cognitive impairment. Their average age was 75 at the time of the sleep study.
The researchers found a U-shaped relationship between sleep and cognitive decline. Overall, cognitive scores declined for the groups that slept less than 4.5 or more than 6.5 hours per night, as measured by EEG, while scores stayed stable for those in the middle of the range.
The U-shaped relationship held true for measures of specific sleep phases, including rapid-eye movement (REM), or dreaming, sleep; and non-REM sleep. Moreover, the relationship held even after adjusting for factors that can affect both sleep and cognition, such as age, sex, levels of Alzheimer's proteins, and the presence of APOE4.
Each person's sleep needs are unique, and people who wake up feeling rested on short or long sleep schedules should not feel compelled to change their habits, said first author Brendan Lucey, an associate professor of neurology and director of the Washington University Sleep Medicine Centre.
But those who are not sleeping well should be aware that sleep problems often can be treated, he said.
Co-senior author Beau M. Ances, a professor of Neurology, treats patients with dementia and other neurodegenerative conditions. "Often patients report that they're not sleeping well. Often once their sleep issues are treated, they may have improvements in cognition," he said. "Physicians who are seeing patients with cognitive complaints should ask them about their quality of sleep. This is potentially a modifiable factor."
The findings were published Wednesday in the journal Brain.
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