If you’re going to have a heart attack, don’t have it during evenings, weekends, or holidays. Why? Because going to the hospital during those times increases your risk of dying 13% compared to patients arriving during workday hours, according to research in Circulation.

To prevent death, it’s critical to restore blood flow as quickly as possible by surgically opening the blocked vessel with balloons and stents or administering IV clot-busting drugs.

“Slower door-to-balloon times for people who arrived at the hospital during off hours is likely due to staffing. In the middle of the night, the hospital catheterization lab where angioplasty and other artery-opening procedures are performed is closed,” said Dr. Jorge Saucedo, lead author of the study and chief of cardiology and co-director of the Cardiovascular Institute at Northshore University Health System in Evanston, Ill.

Patients who arrived at the emergency room during regular workday hours had an average door-to-balloon time of 56 minutes compared with 72 minutes for patients who arrived during evenings, weekends, and holidays. The American Heart Association’s guidelines for the treatment of such patients recommends angioplasty in 90 minutes or less.

During angioplasty, a catheter is threaded into the heart with a deflated balloon at the tip. The balloon is then inflated in the artery where blood flow has been reduced or blocked. Door-to-balloon time is the period from the patient’s arrival to the hospital until the blockage is opened by angioplasty. Following angioplasty, doctors may also implant a mesh tube called a stent to help keep the artery open.

Heart blockage is a 911 emergency—no matter what time of day or night. Don’t guess and waste precious minutes.