Patients who have surgery during the night are twice as likely to die compared to patients who are operated on during the day, a study has found.

The aim of this study was to investigate the relationship between postoperative mortality and the time of the day of surgery.

The researchers evaluated all surgical procedures for five years and a retrospective review of 30 days postoperative in hospital mortality was carried out at the hospital.

A database was constructed collecting variables about surgical interventions and the working day was divided into three time blocks — daytime, evening and night.

There were 41,716 elective and emergency surgeries performed on 33,942 patients in 40,044 hospitals.

The researchers found that those operated on at night were 2.17 times more likely to die than those during regular daytime working hours, while patients operated on in the afternoon were 1.43 times more likely to die.

"This study demonstrates that late day and night emergency surgeries are associated with higher mortality when factoring in American Society of Anesthesiologists (ASA) score and patient age," said Michael Tessler, Associate Professor at McGill University Health Centre.

"Postoperative 30-day in-hospital mortality rate should include start time of anaesthesia, along with other known variables, as a risk factor," said Tessler.

The study was presented at the World Congress of Anaesthesiologists (WCA) and published in the journal World Federation of Societies of Anaesthesiologists.