The body clock - or circadian rhythm - is the reason we want to sleep at night, but it also drives huge changes in the way our bodies work.
The research, published in the Lancet, suggests the heart is stronger and better able to withstand surgery in the afternoon than the morning.
And it says the difference is not down to surgeons being tired in 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.