Older men who have breathing difficulties or spend less time in deep sleep may be at greater risk of brain changes that can precede dementia, a new study suggests.
Experts said the findings don't prove that breathing disorders, including sleep apnea, lead to dementia. But they add to evidence that poor sleep may play a role in some older adults' mental decline.
Past studies have suggested that people with certain sleep disturbances may face a greater dementia risk, the researchers note. But the reasons remain unclear.
The new findings "help to explain how sleep disturbances may actually contribute to the development of cognitive impairment and dementia," said study leader Dr. Rebecca Gelber, of the VA Pacific Islands Health Care System in Honolulu.
Specifically, the researchers found that elderly men who had less oxygen circulating in their blood during sleep tended to show more "microinfarcts" in the brain. Microinfarcts are tiny abnormalities in brain tissue that can precede dementia.
Meanwhile, men who spent less time in slow-wave sleep -- the deep, restorative stage of sleep -- tended to show more atrophy in their brain tissue.
"Microinfarcts and atrophy are known to be much more common, and more severe, in people with dementia, than in people without memory problems," Gelber explained.
But this is the first study, she said, to show that certain sleep features are related to those brain changes.
However, an expert who reviewed the study urged caution in interpreting the results.
"All this shows is a linkage, not cause-and-effect," said Keith Fargo, director of scientific programs and outreach for the Alzheimer's Association in Chicago.
Fargo stressed that research into the role of sleep in dementia risk is "very important," but there are still many questions.
"What we can say is, people with sleep disturbances appear more likely to have brain changes associated with dementia," Fargo said. "There is some relationship, but researchers don't understand it yet."
The findings -- published online Dec. 10 in Neurology -- are based on brain autopsies from 167 elderly Japanese-American men who'd taken part in a long-term health study before their deaths. As part of that study, the men had their oxygen levels and brain activity monitored as they slept.
Overall, Gelber's team found, the one-quarter of men with the lowest oxygen levels during sleep were almost four times more likely to show microinfarcts in the brain, versus men with the highest oxygen levels.
Lower oxygen levels can be caused by conditions such as emphysema or sleep apnea -- where breathing stops and starts repeatedly during sleep. This study did not actually show that sleep apnea, per se, is related to brain changes, Fargo pointed out.
Gelber's team also found that study participants who'd spent less time in slow-wave sleep tended to have more brain atrophy.
"Slow-wave sleep has been considered the deep, restorative stage of sleep and is important in processing new memories," Gelber said. With age, people tend to spend less time in slow-wave sleep, the researchers say.
She added, though, that further studies are needed to see whether slow-wave sleep has a "restorative role in brain function" and whether there are ways to boost the amount of time an older person spends in that stage of sleep.
There is some evidence that treating sleep apnea can improve people's mental performance. But no one knows yet whether boosting oxygen levels during sleep actually wards off or slows down dementia, Gelber said.
Fargo agreed. He noted that people with poor sleep can have problems with memory, thinking and concentration that have nothing to do with dementia.
"If your sleep is disturbed and you're having daytime symptoms, you should see your doctor," Fargo said. "One reason is that you may find out your cognitive problems are not related to dementia at all."