“Biological night” is defined as the period between the onset and cessation of melatonin secretion. During this period, melatonin is secreted, blood cortisol levels rise, core body temperature goes down, and we become sleepy. Melatonin is produced only during darkness and stops upon optic exposure to bright light, with light in the blue portion of the visible spectrum proving the most potent at suppressing production [for more information about circadian rhythm and blues, see “What’s in a Color? The Unique Human Health Effects of Blue Light,” p. A22 this issue].