The current study was designed as a retrospective cohort study and was carried out between December 2015 and August 2022, at Södersjukhuset (Stockholm South General Hospital), Stockholm, Sweden. It is one of the seven emergency hospitals in Stockholm County and has a referral area of roughly 700,000 adult inhabitants. In 2021, the total number of patients presenting in the emergency department was 112,826 (of which 21,600 were children aged < 18 years) and 49,781 patients were admitted to the hospital. As of November 2022, the Department of Surgery had 84 inpatient beds and 345 employees, of which 66 were doctors.

Previous to September 2016, only acute appendicitis patients aged > 14 years were treated at the Department of Surgery at Stockholm South General Hospital. Younger Stockholm patients with surgical conditions were treated at Karolinska Universitestsjukhuset (Karolinska University Hospital), by pediatric surgeons. However, since September 2016, all pediatric acute appendicitis patients in the range of 10–14 years are treated Södersjukhuset. These patients are staying in a pediatric ward, but medical decisions and daily rounds are done by general surgeons.

At the Department of Surgery, office hours are between 7:30 and 16:00 Monday–Friday, non-Holidays. Night shifts are staffed by a doctor who is off during the day before as well as the day after the night shift. The night shift starts at 19:00 Mondays–Thursdays, at 16:00 on Fridays, and at 18:00 during weekends. Night shifts end at 07:30 during the week and at 8:00 on weekends. A night shift week is made up by three shifts, every other night, starting either on Saturday or Sunday. The Friday night is a separate shift, which is staffed by a doctor who has been working daytime Monday–Thursday. Monday–Thursday, the evening hours 16:00–19:00 are covered by a colleague working during the day, who, thus, prolongs the workday by 3 h. The doctors covering the on-call shifts are either specialists in surgery or residents from year two or later. A senior back-up consultant is available per telephone around the clock, and needs to be physically present within 30 min if needed. Every day, a junior resident is supporting the on-call doctor between 16:00 and 23:59, by covering all phone calls from the nurses of the Surgical Clinic’s inpatient wards. On-call work duties primarily imply medical responsibility for the inpatients of the Department of Surgery, while the emergency department is run by specialists and residents in emergency medicine, employed the emergency department.

During office hours, there are two operating rooms (ORs) designated for acute and semi-acute cases in general- and vascular surgery. From 16:00 until 21:00, one OR is available for acute general surgery, vascular surgery, and urology. From 21:00 until 7:30, one OR is distributed between acute general surgery, vascular surgery, gynecology/obstetrics, urology, orthopedics, and hand surgery.

At the Department of Surgery at Södersjukhuset, there is a local quality register including all patients who have undergone an appendectomy [15]. The patients are continuously identified through the local operation logistic software (Orbit5, TietoEvry, Kristianstad, Sweden), using codes from NOMESCO Classification of Surgical Procedures version 1.15: JEA 00 (Appendectomy), JEA01 (Laparoscopic appendectomy) and JEA10 (Appendectomy with drainage) [16]. Procedure-related data are extracted into the register from Orbit5 and other parameters are retrieved from electronic medical records (TakeCare, CompuGroup Medical, Helsinki, Finland).

Patients undergoing acute appendectomy between December 2015 and August 2022 were included in the current study. Method of surgical approach (laparoscopic, open, converted from laparoscopic to open in complicated cases) and eventual perforation was registered manually, using the operating surgeon´s medical chart entry. Postoperative data, including complications, readmission and death, was registered manually 30 days after surgery, using medical records from the hospital. Medical records from other hospitals and/or primary health care centers were also registered, as most healthcare providers in the region are using the same software for medical records (TakeCare). Surgical complications were defined as wound infection, intrabdominal abscess, paralytic ileus, mechanical small bowel obstruction, bleeding/hematoma, and others (e.g., urinary tract infection, wound dehiscence).

Data on hospital stay, such as length of stay, date, and time of arrival in the emergency department, were retrieved automatically from TakeCare.

Operation-related parameters were obtained automatically from Orbit5. Physical status was determined by the responsible anaesthesiologist, using American Society of Anaesthesiologists (ASA) assessment [17]. Time to surgery was defined as the elapsed time from the surgeon´s decision of appendectomy, until start of surgery. Length of surgery was defined as time from skin incision to skin closure.

Start of surgery was defined as the whole hour of the day when surgery was initiated (0–23). Start of surgery was further divided into 8-h blocks defined as day (8–15), evening (16–23), and night (0–7). For example, a procedure initiated at 7:59 would be classfied as night, while surgery inititated at 8:00 would be classfied as day.

For the statistical analyses, IBM SPSS Statistics Version: 28.0.0.0(190) was used. The exposures of interest were surgery at day, evening, and night, and the cohort was, thus, divided into three groups accordingly. Differences in patient characteristics of the three groups were compared using a two-tailed chi-square test for categorical variables and an ANOVA test for continuous variables. Categorical variables were presented as numbers and percentages, and continuous variables were presented as means and 95% confidence intervals (CIs).

Main outcome was postoperative complication registered within 30 days after surgery and secondary outcomes were readmission within 30 days, death within 30 days and length of hospital stay. Risk estimates of complication, readmission, and death were computed using uni- and multivariable logistic regression models with daytime surgery as reference, and presented as odds ratios (ORs) and 95% CIs. The multivariable model was adjusted for age, sex, surgical approach, ASA classification, perforation, time to surgery and length of surgery.

Timing of surgery and the effect on length of hospital stay were estimated in a multiple linear regression model, including the same covariates as in the logistic regression model, with daytime surgery as the reference.

A P-value of ≤ 0.05 was considered significant, for all statistical analyses.

Ethical approval of the study was obtained from the Swedish Ethical Review Authority (Dnr 2019-05976).