Researchers said that heart-attack patients admitted to hospital at night or on weekends run a five-percent higher risk of dying than those treated during regular hours.
In the United States alone, this translates to an extra 6,000 deaths per year, they reported in the British Medical Journal (BMJ).
The study was an overview of 48 published investigations into the outcome of nearly 1.9 million people hospitalised with an acute myocardial infarction, or heart attack.
Twenty of the investigations were carried out in the United States or Canada, 16 in European countries, and the others were either multinational or conducted in Australia, Brazil, Israel, Japan, South Korea, Thailand or Turkey.
One of the probes found that the risk of dying in hospital or within 30 days of admission after hours was nearly fivefold, while another found a fourfold risk.
Overall, though, the increased risk was far smaller but still significant at five percent, the researchers said.
The study found that after-hours patients experienced a delay of nearly 15 minutes on average before getting a "balloon," a common procedure to inflate the coronary artery after infarction.
Patients admitted at night or on weekends "experience delays in urgent care and worse outcomes, and the gap seems to be increasing over time," doctors at the University of Toronto said in an accompanying BMJ editorial.
They urged health watchdog to pay close attention to what happens outside of regular hospital hours, "with the goal of providing high-quality care 24 hours a day and seven days a week."