When Shary Smith noticed that she was forgetting details she should’ve easily remembered, she never suspected that her cognitive impairment had anything to do with her snoring. But when she looked up “short-term memory loss” on the Internet after her neurologist’s diagnosis, sleep apnea popped up.

“I never thought of putting two and two together,” said the retired dog groomer from Weston. “I didn’t think they were related.”

A sleep study confirmed she had sleep apnea, a disorder that causes abnormal pauses in breathing or very shallow breathing during the night, so she was fitted with a continuous positive airway pressure device, more commonly known as a CPAP. That was eight years ago and now, per her doctor’s orders, she wears a facial mask that is connected to the CPAP machine by a flexible tube every night. By generating air pressure, the machine keeps Smith’s airways open while sleeping.

Her sleep is better now, and her memory hasn’t gotten any worse.

Sleep apnea is not uncommon — about 12 million Americans suffer from it. Yet, the America Sleep Association estimates that as many as 80 percent of the people with the disorder are left undiagnosed, mostly because they are unaware of their condition.

Untreated sleep apnea, however, can be dangerous.

“It affects your brain health and your heart health,” says Cleveland Clinic neurologist Dr. Po-Heng Tsai, adding it can be easily diagnosed and treated. “It must be addressed.”

Sleep apnea has been linked to diabetes, hypertension and cardiovascular disease. And now more recently it’s been associated with cognitive impairment and dementia.

Though it’s not the first of its kind, a 2011 study published in the Journal of the American Medical Association shows that sleep apnea contributes to cognitive decline and dementia, probably because of hypoxemia or a drop in the level of oxygen in the blood.

The study, performed on 298 healthy women with an average age of 82, was the first to count the number of disordered breathing events during a night of sleep using EEGS and other equipment. It was also the first to follow up to see if healthy people without dementia but with disordered sleeping patterns were more likely than regular sleepers to decline mentally over time.

After five years, 45 percent of healthy women with sleep-disordered breathing had developed mild cognitive impairment or dementia. In the normal sleepers, the number was 31 percent. The scientists, led by a professor of psychiatry at the University of California San Francisco, then controlled for age, body mass index, race, education level and other risk factors, including the use of certain medications. The resulting figure of prevalence was even higher. In fact, women with sleep apnea were almost twice as likely to develop cognitive impairment.

Sleep experts caution that the study has certain limitations. Sleep data were gathered only one night and the subjects were mostly white women. It’s not known if the results would apply to men as well, or other races.

Sleep apnea is more common in men and in African-Americans, Hispanics and Pacific Islanders. However, at any age and in every ethnic group, “we know and have known that sleep apnea definitely affects cognitive function,” says Dr. Jeremy Tabak, medical director of the Sleep Diagnostic Center at South Miami Hospital. “That doesn’t mean we are saying sleep apnea may be a cause of Alzheimer’s.”

In sleep apnea, each pause in breathing — or apnea — can last from 10 to 20 seconds or more. This can occur 20 or 30 times an hour. and leads to brief drops in the oxygen levels in blood. It also results in poor sleep quality as a person moves out of deep sleep and into light sleep several times.

Without deep sleep, people cannot retain information. Their reflexes slow. They can’t concentrate, experience mood swings, suffer from morning headaches and feel irritable.

“Sleep is important for memory consolidation,” Tsai says. “It’s vital for the processing of information.”

There are two kinds of sleep apnea. The most common, obstructive sleep apnea, is a result of the tongue and soft palate blocking the airway. Central sleep apnea is caused by weak signals from the brain to the diaphragm. The snorts and choking sounds a person suffering from sleep apnea emits are a result of the restarts of normal breaths.

Sleep apnea is more common in older people and as many as 60 percent may suffer from it, says Dr. Alberto Ramos, co-director of UHealth Sleep Center and assistant professor of neurology at the University of Miami Miller School of Medicine. “We lose muscle tone and we gain weight,” he adds. Both are risk factors for the condition.

But aging also brings a host of other problems that interrupt an older person’s sleep, even if they don’t suffer from sleep apnea. “Slow wave sleep, or what we call delta sleep, decreases as we age,” Ramos says. “And that’s what we need to feel refreshed the next day.”

Older adults take more medications, which may affect sleep. They also suffer more from pain because of chronic conditions, and may have trouble falling asleep and staying asleep. Aging also changes our circadian rhythms.

The good news is that seniors can be screened for sleep problems, including sleep apnea. If diagnosed and treated early, a person can enjoy deep, restful slumber. For those suffering from cognitive impairment, it may even slow the rate of mental decline.

“I don’t think I’ve recovered what I lost,” Smith says, “but I’m staying level. I’m not getting worse.”