A growing body of evidence suggests that time of day plays an important role in how well various medical treatments work.

Researchers looked at the way circadian rhythm — the body’s internal clock — affects  the outcomes of a patient recovering from a complex heart procedure. Patients who underwent open-heart surgery in the afternoon experienced better health outcomes compared to those who got operated on in the morning, study authors found after six years of observing nearly 600 patients who underwent heart valve replacement.

In the subsequent 500 days after surgery, researchers found, those patients who had surgery after noon had half the risk of a major cardiac event — for instance myocardial infarction, acute heart failure, or death — as those who had surgeries before then.

Researcher said that, while it's impossible to abandon morning heart surgeries completely, the highest-risk patients should be moved to another time of day.