Open heart surgery appears to be safer in the afternoon because of the body's internal clock, scientists have said.
The research, published in the Lancet, suggests the heart is stronger and better able to withstand surgery in the afternoon than the morning.
Doctors need to stop the heart to perform operations including heart valve replacements.
This puts the organ under stress as the flow of oxygen to the heart tissue is reduced.
The doctors and researchers looked for complications including heart attacks, heart failure or death after surgery.
Dr Mike Knapton, from the British Heart Foundation, said: "Thousands of people now have open heart surgery in the UK.
If this finding can be replicated in other hospitals this could be helpful to surgeons planning their operating list, for non-urgent heart surgery. »
