Fisher also had cocaine, methadone, ethanol and opiates in her system.

Sleep apnea is when a person stops breathing while sleeping, for 10 seconds to a minute or longer. This can happen repeatedly during the night, causing blood-oxygen levels to dip and putting a strain on the heart.

Obstructive sleep apnea, the most common type, is caused by the upper throat muscles relaxing, which makes the airway narrow and close. Another type is central sleep apnea, where the brain doesn't send the right signals to the muscles that control breathing. Complex sleep apnea, the third type, is when a person has both obstructive and central sleep apnea.

Fisher's autopsy report said she had a history of sleep apnea. According to the coroner, she had episodes while sleeping on a plane ride from London to Los Angeles in December and then went into cardiac arrest. Four days later, she died at Ronald Reagan UCLA Medical Center at the age of 60.

Fisher's cause of death was listed as "sleep apnea and other undetermined factors," with "atherosclerotic heart disease, drug use" as other conditions. However, the office ultimately declared her "manner of death undetermined."

The Los Angeles County Department of Medical Examiner-Coroner's Office declined to comment further on Fisher's case.

More than 18 million Americans are affected by obstructive sleep apnea, according to the National Sleep Foundation . The condition is very common, and levels of severity vary by individual.

"Everybody has these events where they may stop breathing for a brief period of time while sleeping," said Dr. Ilene Rosen, president of the American Academy for Sleep Medicine and associate professor of clinical medicine at the University of Pennsylvania. "But when the events start happening more than five times an hour, that's when we start to diagnose the person as having sleep apnea."

Alcohol or drugs such as sedatives, narcotics or opioids can worsen sleep apnea. Those substances further relax the upper airway muscles, causing the airway to become even more narrow.

Rosen said alcohol and drugs can also make you less likely to partly wake and breathe in air, which she calls "brain arousal."

"The individual may not remember they had an arousal during sleep, but that's what opened up their airway," she said. "So those same drugs that make the airway likely to collapse also change the brain arousal threshold, such that an individual is less likely to wake up when they're having an event."

Sleep apnea is closely associated with heart problems including high blood pressure, irregular heart rhythms or arrhythmias, and strokes. That's in part due to what happens when we experience brain arousal and resume breathing, said Dr. Lee Surkin, a cardiologist and sleep medicine physician in Greenville, North Carolina.

"We basically go through a reflex maneuver to force open the airway. That maneuver causes the chest and abdomen muscles to move in opposite directions of each other, and that reopens the airway," said Surkin, who is also founder of the American Academy of Cardiovascular Sleep Medicine. "When the chest and abdomen contracts, the increased pressure is transmitted directly to the heart muscle. So when you couple that with a repetitive drop in oxygen level, that is a tremendous strain on the heart."

Even with this increased risk, another expert says, the chance of a person dying from sleep apnea is very low.

"There's data on increased mortality, but again, it's about 15 years after you are diagnosed with sleep apnea. And you usually die from the consequences because you develop hypertension, coronary heart disease or congestive heart failure," said Dr. Richard Schwab, an expert in obstructive sleep apnea also at the University of Pennsylvania. "But of all the years that I've taken care of patients, it's very uncommon. Most patients don't get in trouble with their sleep apnea unless it's very severe."

Rosen believes the risk is not clear. "Does sleep apnea kill you? We don't know," she said.

Anyone who has certain symptoms -- such as loud snoring, sudden awakening and being short of breath, waking with a dry mouth or throat, waking with a headache, not being able to stay asleep or being very sleepy during the day -- may have sleep apnea and should see their primary care doctor.

The doctor may refer the patient to a sleep specialist, who will probably advise an overnight sleeping test, called a polysomnography, at a sleep disorder center or a take-home sleep test.

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The most effective type of treatment for sleep apnea is use of a continuous positive airway pressure, or CPAP, machine. These deliver air directly through the airway, ensuring that it remains open. Other treatment options include oral appliances or, in very severe cases, tissue removal or jaw surgery.

Rosen, Schwab and Surkin all emphasized that diagnosis and treatment of sleep apnea is crucial to ensuring heart health.

"Patients with sleep apnea may have this disease for many, many years, and no one event, or no one night, puts them at risk," Rosen said. "It is the accumulation of sleep disorder breathing, and the consequences that go with it, night after night, after a long period of time, that puts people at risk. That's why treatment is so important."