A trip to the emergency room is bad, and a heart attack is worse, but combining the two on at night or on a weekend lowers one’s chances of surviving the episode, according to a new report.

More than 250,000 people each year have the worst type of heart attack possible, caused by complete blockage of blood flow in the heart. Time is of the essence because blood flow must be restored in order to keep the heart muscle alive.

Researchers analyzed the treatment and survival records for more than 27,000 patients who suffered the most severe kind of heart attack and sought medical attention at hospital emergency rooms after normal business hours. These records were compared to those of 16,000 patients with the same kind of heart attack who were treated during regular daytime hours. All patients had their heart attacks between January 2007 and September 2010 and were treated at one of 447 hospitals across the U.S.

Time of day and day of week were not factors in several recorded variables, including when aspirin was given, when imaging tests were done, and when clot-dissolving medications were given. However, those treated during normal business hours received angioplasty an average of 16 minutes sooner than those who were treated outside of these hours.

Angioplasty involves the insertion of a catheter and deflated balloon on the end into a blood vessel. The apparatus is then threaded into the heart, and the balloon is gently inflated to open up the blood vessel, allow the clot to dislodge, and blood flow to resume. In some cases, a stent, or mesh tube, may be left in place to keep the blood vessel open.

The report was published on Tuesday in the journal Circulation: Cardiovascular Quality and Outcomes.