Circadian rhythm refers to the approximately 24-hour cycles in human and animal physiology and behavior, regulated by an internal biological clock. The master circadian clock is located in the hypothalamus, specifically the suprachiasmatic nucleus, which receives light input from the retina. These circadian rhythms evolved to protect DNA from UV radiation and help entrain organisms to the light-dark cycle. Core body temperature, melatonin secretion, and cortisol levels are classic markers used to measure circadian rhythms. Light exposure can advance or delay circadian rhythms depending on timing.