Background. Sleep disturbances and cognitive dysfunction are common in patients with breast cancer. Disturbed sleep leads to poor cognitive performance and exogenous melatonin may improve sleep and attenuate cognitive dysfunction. We hypothesized that melatonin would improve sleep and cognitive function after surgery. Methods. This study reports secondary endpoints from a randomized, double-blind, placebo-controlled trial. Women, 30-75 years, were randomized to 6mg oral melatonin/placebo for 3 months. We assessed postoperative cognitive dysfunction (POCD) with a neuropsychological test battery, sleep with a diary, and sleep quality with VAS. Results. 54 patients were randomized to melatonin (n = 28) or placebo (n = 26); 11 withdrew (10 placebo, 1 melatonin, P = 0.002). The incidence of POCD was 0% (0/20) [95% CI 0.0%; 16.8%] in the placebo group and 0% (0/26) [95% CI 0.0%; 13.2%] in the melatonin group 2 weeks postoperatively (P = 1.00) and 6.3% (1/16) [95% CI 0.0%; 30.2%] in the placebo group and 0% (0/26) [95% CI 0.0%; 13.2%] in the melatonin group 12 weeks postoperatively (P = 0.38). Sleep efficiency was significantly greater in the melatonin group; mean difference was 4.28% [95% CI 0.57; 7.82] (P = 0.02). The total sleep period was significantly longer in the melatonin group; mean difference was 37.0 min [95% CI 3.6; 69.7] (P = 0.03). Conclusion. Melatonin increased sleep efficiency and total sleep time but did not affect cognitive function. The dropout rate was significantly lower in the melatonin group. This trial is registered with Clinicaltrials.gov NCT01355523.