This cohort study examined the relationship between surgery start time and anesthetic adverse events (AEs) using a large database of anesthesia procedures at an academic medical center. The incidence of AEs was increased for surgical procedures starting in the late afternoon compared with those starting in the morning. The authors hypothesize that this finding could reflect fatigue (as demonstrated in a prior simulation study) or problems with care transitions; however, they were not able to directly measure case load or composition of the care team. Moreover, for most AEs, the authors could not determine whether patients were harmed or whether the error was preventable.