: This brief review summarizes some of the biological effects of light exposure at an inappropriate time (during the normal dark period) and the potential negative physiological consequences of this light exposure. Two major systems are significantly influenced by light at night. Thus, the circadian system and melatonin synthesis are altered when light is extended into the normal dark period or when the dark period is interrupted by light. This summary reviews the potential sequelae of chronic inappropriate light exposure and the suppression of endogenous melatonin levels. Given that melatonin is a free radical scavenger and antioxidant, conditions that involve free radical damage may be aggravated by light suppression of melatonin levels. The conditions of particular interest for this review are excessive DNA damage (which potentially leads to cancer), cellular destruction in neurodegenerative diseases and aging itself. Further research should be conducted to more accurately define the potential negative impact of light at abnormal times on animal and human pathophysiology.