It's not clear yet whether treating the sleep apnea can help prevent that memory decline, but researchers say future studies should address that question.
"It makes sense that good sleep is going to be protective to the brain," said Dr. Robert Thomas, who studies sleep at Harvard Medical School in Boston and was not involved in the new study.
But, he added, clear answers have been lacking on the link between problem sleeping and memory. "We simply don't have data to answer many of the simple questions people may have in the sleep clinic," he told Reuters Health.
To try to shrink that gap, Dr. Kristine Yaffe of the University of California, San Francisco and colleagues gave an overnight sleep apnea test to 298 women without dementia, who were an average of 82 years old. The test looks for changes in breathing and oxygen flow during the night, as well as for the short, frequent breaks in sleep that are signs of sleep apnea.
Just over a third of the women had sleep apnea—which is especially common in older, overweight people.
About five years later, the researchers brought those same women in for a set of thinking and memory tests, and doctors evaluated any of the women who showed signs of memory decline.
In total, a little over a third (36 percent) of the women were diagnosed with mild cognitive impairment or dementia.
Among women who had shown signs of sleep apnea on their overnight tests five years earlier, 45 percent had developed thinking and memory problems, compared to 31 percent of women who didn't have sleep problems.
When Yaffe and her colleagues took factors such as race, weight, and other diseases and medications into consideration, women with sleep apnea were almost twice as likely to test positive for cognitive impairment or dementia, according to findings published in the Journal of the American Medical Association.
Sleep apnea has also been linked to a host of other health problems, including high blood pressure and cholesterol. Researchers pointed to lower blood flow to the brain during sleep as a possible culprit in cognitive problems down the line.
Indeed, when the authors looked at the specific factors that went into a diagnosis of sleep apnea, they found that the lack of steady oxygen overnight was related to thinking and memory problems, not how much total sleep women got or how many times they woke up during the night.
Thomas said that not everyone with sleep apnea has symptoms, which include fatigue and snoring, and that people who are overweight or have heart and blood pressure diseases should also consider getting tested.
But researchers still don't know to what extent treatment, which involves wearing a mask that delivers pressurized air from a "CPAP" machine to ease breathing at night, can prevent the complications of sleep apnea, including cognitive decline.
"The single biggest hole in sleep apnea (research) is: what are the outcomes of treatment?" Thomas said.
Yaffe agreed. "That's obviously a next step and important question," she told Reuters Health.
Seva Polotsky, a sleep apnea researcher from Johns Hopkins University School of Medicine in Baltimore, said it's important nonetheless that people with sleep apnea follow whatever treatment regimen their doctor recommends. Still, "there's only one cure for apnea so far we've found, and this is weight loss," he told Reuters Health.
People with sleep apnea should also try to be tested occasionally for signs of cognitive decline, Yaffe concluded.