The effect of melatonin and lighting schedule on energy metabolism in broiler chickens was studied. Eight groups of six female broiler chickens each were assigned to a continuous lighting schedule [23 h light (L):1 h darkness (D)] or an intermittent lighting schedule (1L:3D), and were fed a diet with or without melatonin (40 ppm). At 21 d of age, the chickens were placed in respiration chambers for 20 d. Energy and nitrogen balances, heat production and physical activity were measured per group. The only effect of melatonin on energy metabolism, was a decreasing effect on activity-related heat production. The intermittent lighting schedule induced improved feed conversion, higher metabolizability of the diet, and lower physical activity compared to continuous lighting. No interactions between melatonin and lighting schedule were found on energy metabolism traits. Lighting schedule strongly affected daily heat production pattern (total, activity-related, and nonactivity-related heat production). Melatonin had a reducing effect on activity-related heat production during the day, especially during light periods. The present study demonstrated that reduced energy expenditure for physical activity, caused by the supplementation of melatonin to the diet, might be a reason for the often observed improvement of feed conversion. Furthermore, this study showed that feed conversion was improved with an intermittent lighting schedule, which was related to higher metabolizability and lower energy expenditure on physical activity, compared to continuous lighting.