STUDY OBJECTIVES: To investigate whether administration of an oral dose of 6 mg melatonin before bedtime perioperatively in breast cancer surgery could change sleep outcomes measured by actigraphy. METHODS: This paper reports secondary outcomes from a double-blind, placebo-controlled, randomized clinical trial where patients received 6 mg melatonin (n = 27) or placebo (n = 21) approximately 60 minutes before bedtime 3 nights preoperatively until at least one week postoperatively. Participants were monitored in the entire period with actigraphy, and were instructed to complete visual analogue scale (VAS) for sleep, and the Karolinska Sleepiness Scale (KSS) each morning. RESULTS: Administration of 6 mg oral melatonin approximately 1 hour before bedtime resulted in significantly increased sleep efficiency and reduced wake after sleep onset for the entire 2-week postoperative period. No other significant differences for actigraphy determined sleep outcomes or subjective outcome parameters in the perioperative period were found between the groups. Overall, the patients sleep outcomes were within normal ranges and no participants had pathological sleep disturbances. CONCLUSIONS: Melatonin significantly changed sleep efficiency and wake after sleep onset after surgery, but had no effects on other objective sleep outcomes or on subjective sleep quality (VAS and KSS).