New research suggests that obstructive sleep apnea (OSA) may impair a person's ability to form meaningful memories about their personal life. Such dysfunction may, in turn, be a sign of depression, caution the researchers.

OSA is a sleep disorder that affects more than 18 million adults in the US and over 100 million people worldwide. Those with the condition often have fragmented sleep because OSA interrupts their breathing briefly but repeatedly. OSA also lowers a person's oxygen levels, and the combination of poor sleep and oxygen deprivation raises the risk of cardiovascular problems, mood disorders, and memory problems.

New research zooms in on a potential consequence of OSA—depression. Previous studies have found that the rates of depression are higher among people with OSA, but the mechanisms behind this association were unclear. The new study, which was led by Melinda Jackson, a senior research fellow at the Royal Melbourne Institute of Technology (RMIT) University in Melbourne, Australia, investigates the relationship between OSA and autobiographical memory.

What is autobiographical memory?

Autobiographical memory refers to a person's ability to memorize specific episodes and retain information about their personal lives. Research has previously linked impaired autobiographical memory with depression.

"We know that overly general autobiographical memories—where people don't remember many specific details of life events—are associated with the development of persistent depression," Dr. Jackson explains.

She goes on to lay out the motivation for her research. "Sleep apnea is also a significant risk factor for depression, so if we can better understand the neurobiological mechanisms at work, we have a chance to improve the mental health of millions of people."

In the new study, Dr. Jackson and colleagues examined the link between OSA and autobiographical memory; the researchers published their findings in the Journal of the International Neuropsychological Society.

'Overgeneral' memories and sleep apnea

Dr. Jackson and team examined 44 adults who had OSA but were not actively treating it and 44 healthy adults without OSA. The researchers looked at the individuals' abilities to remember various kinds of memories from their childhoods, early adult lives, and recent events. The study revealed that people with OSA had considerably more "overgeneral memories" than people without OSA. Overgeneral memories describe memories that people cannot recall in much specific detail.

In the current study, more than 52% of those participants with OSA had overgeneral memories, whereas less than 19% of participants in the control group had overgeneral memories. Moreover, the study compared semantic memory with episodic memory. The former describes detailed facts and information about someone's personal history, whereas the latter describes the ability to remember broader events or "episodes." The researchers found that while the episodic memory of people with OSA was intact, their semantic memory was impaired. Also, they established a correlation between a higher number of autobiographical memories and worse semantic memory across both groups.

"Our study suggests sleep apnea may impair the brain's capacity to either encode or consolidate certain types of life memories, which makes it hard for people to recall details from the past," explains Dr. Jackson. "Brain scans of people with sleep apnea show they have a significant loss of grey matter from regions that overlap with the autobiographical memory network," the scientist continues.

Dr. Jackson, a senior research fellow at RMIT's School of Biomedical and Health Sciences, goes on to outline some directions for future research.

"We need to look at whether there's a shared neurobiological mechanism at work—that is, does the dysfunction of that network lead to both depression and memory problems in people with sleep apnea?" In the future, Dr. Jackson and her team plan "to determine whether successful treatment of sleep apnea can also help counter some of these memory issues or even restore the memories that have been lost."