Sleep apnea is a sleeping disorder whereby breathing stops temporarily during sleep. It is characterized by snoring and leads to reduced levels of oxygen in the blood.
The study, published in Neurology, discovered that people with lower levels of oxygen in their blood during sleep - found in patients with sleep apnea - were more likely to have brain tissue lesions called microinfarcts, an abnormality linked to the development of dementia.
The researchers also found that spending less time in deep sleep - referred to as slow wave sleep - increased the likelihood of losing brain cells. Loss of brain cells has also previously been associated with dementia.
Slow wave sleep is key to the processing of newly acquired memories and remembering facts. People typically spend less time in slow wave sleep as they get older.
The National Heart, Lung, and Blood Institute (NHLBI) report that sleep plays a vital role in keeping physically healthy, with sleep deficiency associated with increased risks of heart disease, kidney disease, hypertension, diabetes, stroke and obesity. Sleep is involved in healing, growth, hormonal balance and maintaining a strong immune system.
Sleep tests and brain autopsies conducted
Using the Honolulu-Asia Aging Study (HAAS) - a prospective cohort study of Japanese American men in Honolulu, HI - the researchers conducted sleep tests on 167 men with an average age of 84.
All participants were followed-up until they died. The researchers then conducted autopsies on their brains, assessing potential brain cell loss and searching for evidence of microinfarcts and brain abnormalities associated with the development of Alzheimer's disease and Lewy body dementia.
The researchers divided the participants into four different groups, each containing 41 or 42 men, based on the amount of time they had spent with lower-than-normal oxygen levels during sleep. The lowest group spent up to 13% of their time sleeping with low oxygen levels, while the highest group spent between 72-99% of their time sleeping with low oxygen levels.
In the lowest group, only four of the 41 men were found to have microinfarcts in their brains, compared with 14 of the 42 men in the highest group. As a result, men in the highest group were four times more likely to have brain damage than those in the lowest group.
The participants were also divided by the researchers into four groups, depending on the amount of time they had spent in slow wave sleep. The researchers found that 17 of the 37 men who spent the least time in slow wave sleep had brain cell loss. In comparison, only seven of the 38 men who spent the most time in slow wave sleep demonstrated brain cell loss.
"These findings suggest that low blood oxygen levels and reduced slow wave sleep may contribute to the processes that lead to cognitive decline and dementia," states study author Dr. Rebecca Gelber.
Additional investigations required
After adjusting for potentially confounding factors, such as smoking, and excluding the participants that had died relatively early into the study's follow-up period, the results remained the same.
The authors acknowledge that within their study, the causes of low oxygen levels during sleep were unclear. These may have had a bearing on their findings, along with the daytime oxygen saturation measurements for the participants, which were similarly unavailable.
They also note that the lesions found in the brains of the participants may have developed prior to the commencement of the sleep tests.
"How [short wave sleep] may have a restorative role in brain function, and whether prevention of nocturnal hypoxia may reduce the risk of cognitive decline, require additional investigation," conclude the authors.
Continuous positive airway pressure machine (CPAP) use - a form of treatment for sleep apnea - has been found to improve cognition, even in patients who already have dementia, Dr. Gelber notes.
Previously, Medical News Today reported on a study suggesting that an approach to preventing Alzheimer's disease could lie in targeting a protein that rouses the brain from sleep.