Sept. 21 (UPI) -- Getting too much or too little sleep may increase the risk for cognitive decline, or dementia, in older adults, according to a study published Monday by JAMA Network Open.
In an analysis of the sleep habits of more than 20,000 English and Chinese adults age 48 to 75, people who slept for fewer than four hours or more than 10 hours per day showed evidence of declines in cognitive function, including memory and language comprehension, researchers said.
"This study is an observational study and cannot demonstrate a causal relationship," study co-author Yanjun Ma told UPI, so the findings don't necessarily prove that lack of sleep or excessive sleep causes a decline in cognitive function.
Observational studies are intended to assess only the effect of an intervention -- in this case, sleep -- on study participants, without trying to modify it to compare differences.
It's possible that diminished or excessive sleep is an early sign of cognitive decline or dementia, as opposed to a risk factor, researchers said.
"Future mechanism studies, as well as intervention studies examining the association between sleep duration and cognitive decline are required," said Ma, of the Peking University Clinical Research Institute in China.
As many as 6 million Americans have some form of dementia, and changes in sleep patterns are common, according to the Alzheimer's Association.
To date, research has shown that sleep disturbances can result from cognitive impairment, while animal studies have found links between lack of sleep and increased levels of brain proteins that are thought to be signs for Alzheimer's disease, said Dr. Yue Leng, who authored a commentary on the study findings.
Leng is an assistant professor of psychiatry at the University of California-San Francisco.
For their research, Ma and colleagues analyzed data on sleep behaviors and cognitive function in 20,065 adults from the English Longitudinal Study of Aging and the China Health and Retirement Longitudinal Study, and tracked them for about eight years, on average.
In addition to finding higher levels of cognitive decline among those who slept fewer than four or more than 10 hours per day, the researchers also observed that people with these sleep habits had "faster cognitive decline" than those who slept seven to nine hours per day, Ma said.
"It's usually believed that sleep deprivation might lead to cognitive decline, but it's unclear why too much sleep might be bad for cognitive health," Leng added. "Older adults should pay more attention to their sleep habits, as these might have implications for their cognitive health."