||Light pollution is one of the most rapidly increasing types of environmental degradation, especially in the hospital ward and ICUs. Exposure to constant light (LL) condition disrupts circadian rhythms in behavioral, physiological and endocrinal processes. Whether melatonin can reverse those disruptions remains unclear. We used male Sprague – Dawley rats (7 weeks of age at the time of purchase)to explore 1) the effects of LL condition on social memory, anxiety–like behavior and circadian system, 2) whether exogenous melatonin can reverse the changes. Compared with LD + Veh (LD: standard 12h:12hlight-dark cycles; Veh: receive vehicle intraperitoneal injection) group, rats exposed to LL for two weeks had less anxiety-like behavior, impaired the social memory, elongated the rhythm cycle of free running wheel and disrupted secretion of corticosterone (Cort) and melatonin (Mel). Application of exogenous melatonin couldn't rescue the impairment of the social memory and the disrupted circadian system. These findings suggest that LL condition can disorganize the circadian system and exogenous melatonin cannot reverse these changes.