What Happens When Sleep Apnea Occurs During REM Sleep, and Why Women Are Affected More

A new study published in the journal Alzheimer’s Research & Therapy found that sleep apnea may be linked to impaired cognition. (1) The study, led by a research team from the University of California, Irvine, showed that the frequency of sleep apnea events during REM sleep may be linked to the severity of verbal memory impairment in older adults at risk for Alzheimer’s disease. And as the study unfolded, researchers also discovered an interesting difference between the sexes.  

The Study

The study featured 81 middle-aged and older adults, 62 percent of whom were female, from the Wisconsin Alzheimer’s Disease Research Center. (1) During the study, participants underwent polysomnography (sleep studies that record brain waves, eye movements, muscle activity, blood oxygen levels, heart rate, and breathing during sleep) and verbal memory assessments, which measured their cognitive ability to retain and recall information presented through voice or text. (1) (Impairments to the latter are symptoms commonly associated with Alzheimer’s.)

Ultimately, researchers found that higher ratios of sleep apnea events during REM sleep were associated with worsened memory performance  — especially in those with a predisposition to Alzheimer’s and those with a parental history of the disease. (1)

Another Battle of the Sexes

This study found a notable difference between the sexes. More specifically, they observed that women were more likely to have a greater proportion of their hypopneas during REM sleep in comparison to men, a potential contributor to their higher risk of Alzheimer’s disease — women are almost twice as likely to be diagnosed with Alzheimer’s Disease and tend to have a faster memory decline and a more severe disease progression. (1) We’ll add here that while the researchers wrote that there’s “some evidence that suggests women with obstructive sleep apnea (OSA) may develop stronger OSA-related memory impairments than men with OSA,” they make it a point to highlight the fact that “women typically remain underdiagnosed.” (1)

Women Are More Likely to Have Apneic Events in REM Sleep in Comparison to Men

Bryce Mander, University of California, Irvine associate professor of psychiatry and human behavior and co-author of the study, said that while it wasn’t fully explored in the literature, “there is a sex difference in how common sleep apnea events occur in NREM and REM sleep, and that difference “has been known clinically for years.” Leaning on a study from 2020 that examined these disparities, Mander tells us, “it has to do with the sex difference in the physical and functional composition of male and female bodies. This includes how the body stores fat and how breathing muscles function with increasing age.” (2)

Mander also clarifies, “It isn’t that women have more events in REM sleep than men; it’s that they are more likely to have their events in REM, while men have events in NREM and REM sleep. That said, because the physiology is different in NREM and REM sleep, apnea events in REM tend to last longer and be associated with greater drops in blood oxygenation.” (2)

What Does This Mean for the Diagnosis and Treatment of Sleep Apnea and Alzheimer’s?

In terms of what this means for the diagnosis and treatment of sleep apnea, Ruth Benca, M.D., professor and chair of psychiatry and behavioral medicine at Wake Forest University School of Medicine and co-author of the study, says, “This finding suggests that basing treatment solely on an apnea-hypopnea index (AHI — the number of apneic events per hour of sleep) may lead to undertreatment of individuals who only have apnea during REM sleep.”

And while the team’s research certainly connects some dots, both researchers say that it may not necessarily change outcomes because effective treatment and management of OSA still lies with the individual. Historically, adherence is the rub for CPAP therapy. (3)

“One of the problems is that optimal outcomes from treating sleep apnea will likely be related to good adherence to therapy,” says Benca. “For example, if an individual with sleep apnea only uses their CPAP (continuous positive airway pressure) machine for part of the night, they could still be subjected to the deleterious effects of sleep apnea.” Ultimately, Benca adds, “Insufficient adherence could be more problematic for those with REM-related apnea. Since more REM sleep occurs later in the night,  if the person removes their mask partway through the night, they will not be administering treatment during the time when their apnea is most prevalent.” 

Regarding Alzheimer’s, the researchers are hesitant to say what the implications for its diagnosis and treatment may be. Mander does say, however, that “Much of the sleep and Alzheimer’s literature focuses on NREM sleep and slow wave sleep in particular, so findings like this remind us that there are multiple aspects of sleep that are relevant for AD risk.” 

Can Treatment for Sleep Apnea Potentially Mitigate the Risk of Alzheimer’s?

While both researchers indicate that further research is necessary, they seem cautious but optimistic. 

“Our work identifies an important relationship between sleep apnea severity and memory impairment in older adults at risk for Alzheimer’s disease,” says Mander. “These findings support the hypothesis that sleep apnea treatment may reduce Alzheimer’s risk in older adults with sleep apnea, but this may depend on whether that treatment reduces events during REM sleep.”  

Noting that “the links between sleep apnea and Alzheimer’s disease and related dementias are strong, Benca says, “longitudinal clinical trials are needed to answer that question.” However, she adds, “Certainly, there is evidence that treatment of sleep apnea can lead to better cardiovascular outcomes, so it would not be surprising if it also improved cognitive outcomes.” 

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