Mornings are the best time to have an operation, according to research which shows more complications occur when surgery is performed in the afternoon.
Morning operations came out best
A US team at Duke University analysed the outcomes of over 90,000 operations carried out at their hospital.
Adverse events were most common when surgery started between 3pm and 4pm and least common with 9am-noon operations.
The authors say natural changes in the body's pain levels during the day and staff fatigue could be involved.
Lead researcher Dr Melanie Wright said: "Healthcare is a 24-hour-a-day business, and it is not unexpected that factors such as fatigue, circadian rhythms, personnel shift changes and scheduling may affect patient care over the course of a day."
Very few of the adverse events Dr Wright's team identified caused lasting harm to the patients.
Most were related to problems with pain management and postoperative nausea and vomiting.
Other problems included prolonged sedation, wound infection, dangerous changes in blood pressure and operating room equipment problems.
The analysis also highlighted 9,497 administrative delays which, although not categorised as adverse events, may have influenced them, say the researchers.
When they matched the adverse events with the time that the patient's surgery began they found a striking pattern.
"We found that adverse events were most common for operations starting between 3pm and 4pm," said Dr Wright, from the university's Human Simulation and Patient Safety Center.
She said many factors could explain why this might be.
Stress and fatigue
For example, patients may be more susceptible to pain or postoperative nausea and vomiting in the late afternoon.
Issues such as not having eaten all day or spending a stressful day waiting to have the surgery might also be important.
Late afternoon is also a time when the anaesthetic teams in the hospital changed shifts and when the body's has a dip in its natural circadian rhythm, which regulates sleep, brain wave activity and other bodily functions.
The researchers are planning another study to compare each step in the delivery of care for patients having surgery during two time periods - 9am to noon and 3pm to 6pm.
Their current work is published in the journal Quality and Safety in Healthcare.
Professor Alistair Chambers, honorary secretary of the Association of Anaesthetists of Great Britain and Northern Ireland, said the findings would need to be replicated, pointing out that working hours and procedures differed between hospitals and countries.
If other studies corroborate the findings, the factors causing the difference would also need to be identified, he said.
"But it is interesting. It perhaps does mean that we should caution against suggesting that we go to round-the-clock operating to get better use of operating theatres."