Background: Preoperative anxiety is a common issue in surgery. Preoperative anxiety may lead to increased mortality, pain and dissatisfaction with surgery. Melatonin is a drug with only mild adverse effects and has previously been shown to reduce anxiety and pain in the perioperative setting. The aim of study was to investigate the anxiolytic effect of melatonin in the preoperative setting. Methods: This study was a randomised, double-blinded, placebo-controlled clinical trial. Included patients were allocated in the ratio 1:1 to either the placebo group or the melatonin group. Patients received 10 mg melatonin or placebo the evening before surgery, 2 h before surgery, immediately after surgery and the evening after surgery. Preoperative anxiety was examined 1 h before surgery, using the State Trait Anxiety Inventory (STAI) questionnaire, along with a Visual Analogue Scale (VAS) of anxiety. Furthermore, participants filled out a sleep log each morning, where we examined sleep quality, comfort and drowsiness. Results: Sixteen patients received placebo treatment and 17 patients received melatonin. Median state anxiety on the STAI scale at baseline was 27 (interquartile range [IQR] 22–34) for the melatonin group and 27 (IQR 23–31) for the placebo group. In the primary outcome of preoperative anxiety 1 h before surgery, the median state anxiety on the STAI scale was 27 (IQR 24–34) for the melatonin group and 28 (24–33) for the placebo group, p =.814. VAS anxiety was median 7 (0–28) for the melatonin group and median 7 (1–13) for the placebo group, p =.813. Furthermore, there were no statistically significant differences between the two groups in the secondary outcomes of sleep quality and consumption of analgesics. Conclusion: Melatonin did not reduce preoperative anxiety in patients undergoing hernia repair. Preoperative anxiety levels were low, which limits the generalisability of our findings.