Sudden cardiac arrest affects an estimated 350,000 Americans each year, with between 17% and 41% of these cases occurring at night. Now, new findings published in the journal Heart Rhythm suggest that a woman’s heart is more likely to stop beating after dark than a man’s, reports Cedars-Sinai.

For the investigation, researchers from the Center for Cardiac Arrest Prevention in the Smidt Heart Institute at Cedars-Sinai examined the health records of 4,126 patients. Of these patients, 3,208 sudden cardiac arrest events took place during the day, and 918 occurred at night.

Findings showed that more women experienced nighttime cardiac arrest than men (25.4% versus 20.6%). “We were surprised to discover that being female is an independent predictor of these events,” said Sumeet Chugh, MD, director of the Center for Cardiac Arrest Prevention and senior author of the study.

Scientists theorized that respiratory problems might be responsible for women’s increased risk for cardiac arrest at night.

In addition, setting aside the sex of patients, researchers noted that those stricken by sudden cardiac arrest at night were more likely to suffer from lung disease—specifically, chronic obstructive lung disease and asthma. (Former or current smokers experienced a higher incidence of nighttime cardiac arrest too.)

“Brain-affecting medications, some of which have the potential to suppress breathing, were also found to have a significantly greater usage in nighttime compared to daytime cardiac arrest,” Chugh added.

This finding suggests that doctors ought to be more careful when prescribing sedatives or drugs for pain and depression to high-risk patients, particularly women.

Additionally, these results help to continue sex-based research in cardiology that’s key to making needed advances in the field.

For related coverage, read “For People With Sudden Cardiac Arrest, Where They Live May Matter” and “African-American Infants More Likely to Suffer Cardiac Arrest.”