In industrialized countries, certain types of cancer, most notably, breast and prostate, are more frequent than in poorly developed nations. This high cancer frequency is not explained by any of the conventional causes. Within the past decade, numerous reports have appeared that link light at night with an elevated cancer risk. The three major consequences of light at night are sleep deprivation, chronodisruption, and melatonin suppression. Each of these individually or in combination may contribute to the reported rise in certain types of cancer. In this article, the potential mechanisms underlying the basis of the elevated cancer risk are briefly discussed. Finally, if cancer is a consequence of excessive nighttime light, it is likely that other diseases/conditions may also be exaggerated by the widespread use of light after darkness onset.