Number of studies using a specific time point, first 24-hour postoperative pain recordings. Data from 32 studies are included with a total of 162 measurements, equaling a mean of 5.1 pain recordings per study on POD1. *Erden et al. measured pain within time intervals 0–3, 4–7, 8–11, 12–15, 16–19, 20–24 hours. To include the data, the middle range was chosen as time of measurement. **Jo et al. recorded pain 5 hours postoperatively and then three times daily (at 0:00 am, 9:00 am, and 5:00 pm). Since we do not know what time of day the patients were operated, only the first recording after 5 hours is included in the graph. ***Ryu et al. measured pain hourly until request of rescue analgesic which was requested at a mean of 11 hours and 45 minutes postoperatively on which basis we added these 11 pain recordings to the graph.