The definition of light by the Illuminating Engineering Society (IES) is limited to how it stimulates the visual system. In addition to stimulating the visual system, light incident on the retina stimulates other biological functions, most notably circadian rhythms. Circadian rhythms are bodily rhythms that, in humans, oscillate with a period close to, but slightly longer than 24 hours (h). They are generated and regulated by a biological clock located in the suprachiasmatic nuclei in the hypothalamus in the brain. Light/dark patterns incident on the retina reset the biological clock daily and are the major sychronizer of circadian rhythms to the 24 h solar day. Lighting characteristics (quantity, spectrum, timing, duration and distribution) effective to the circadian system are different than those effective to the visual system. Since circadian rhythms researchers are mainly biologists and physicians, not lighting scientists, it is common that the specification of the light stimulus used in experiments is not well characterized or correctly reported.