Circadian rhythms are seen at every level of biology, from single cells to complex behaviors. The timing of every biological function in mammals is governed by the master clock in the suprachiasmatic nuclei (SCN), which has an intrinsic period of slightly longer than 24 hours. The light/dark pattern incident on the retina synchronizes the SCN to the 24-hour local time, coordinating and enabling diverse biological functions to occur at the correct time of day and night for optimum species survival. Without exposure to a regular, daily pattern of light and dark, circadian rhythms become disrupted. A wide range of modern maladies, from sleep disorders to cancer, has been linked to light-induced circadian disruption. Light has, however, been defined only in terms of the human visual system, not the circadian system. Light source and systems development should consider the needs of both the visual and non-visual systems. Discussed are the lighting characteristics impacting these two systems and the implications for designing light for various healthcare and medical applications.