Having heart surgery later in the day may be safer than having it in the morning.

Researchers studied 596 patients, half of whom had undergone open-heart surgery for aortic valve replacement in the morning and half in the afternoon. They followed their recoveries for 17 months.

On almost every measure — heart attack during the operation, major adverse cardiac event, acute heart failure, cardiovascular death — afternoon surgery produced significantly fewer complications.

They followed this observational study with a randomized trial of 88 patients, 44 assigned to morning surgery, 44 to the afternoon. During 12 days of follow-up they found that levels of troponin, a measure of heart muscle damage, were significantly lower in the afternoon group. The study is in Lancet.

The genetic mechanisms that protect tissue under stress function differently in the morning and the afternoon — in all organs, not just the heart — and the authors believe that these circadian variations account for their findings.