A new study found that certain times for surgery could mean a higher risk of death for the patient. According to the preliminary results reported in the European Journal of Anesthesiology, there are daily, weekly, and seasonal variations in hospital mortality. In particular, there was a significantly higher risk associated with afternoons, weekends, and the month of February.

CBS News reports that afternoons were correlated with a 21 percent higher risk of death, weekends a 22 percent higher risk, and February a 16 percent higher risk. Also, another study similarly found a higher risk on weekends as compared to weekdays. While the results are statistically significant, what is going on is still unclear. The researchers do not know why there is a change in the risk.

One hypothesis is that the standard of care may change throughout the day and between weekdays and weekends. This means that health professionals are treating differently, leading to poorer outcomes for patients at those times. Another theory is that more severely ill patients might be coming in at certain times, leading to higher risk for the same surgeries.

The preliminary results were presented at the European Society of Anesthesiology (ESA) Annual Meeting. The ESA is an organization committed to highest standards of practice and safety in anesthesia, perioperative medicine, intensive care, emergency medicine, and pain treatment through education, research, and professional development throughout Europe.