Implications of the interplay between light and dark for human health in the built environment
24-hour cycles of light and dark are one of the most significant factors in the built environment that affect human health and wellbeing. In addition to stimulating visual responses, light induces a range of circadian, physiological and behavioral responses in humans, including sleep quality, hormone production, alertness and cognitive performance. These nonvisual responses are primarily mediated via a novel type of photoreceptors that contain the photopigment melanopsin. These photoreceptors can function independently of the classical photoreceptors, rods and cones used for seeing, thus the human eye plays a dual role in detecting light. Since the overall purpose of lighting is to serve the needs of humans, the discovery of the melanopsin-containing photoreceptors should be addressed in architectural lighting design and engineering in buildings. The optimal daily doses of light and dark needed for synchronizing circadian rhythms or promoting other physiological and behavioral nonvisual responses are currently unknown, however there are evidences that light can have beneficial or harmful effects depending on the timing of the light exposure. Daytime light exposure can reduce sleepiness and improve performance but light exposure should be avoided at night, where it can suppress melatonin production resulting in circadian disruption. Human-centered approaches are rarely applied to evaluate building performance, where conventional recommendations for lighting are mainly based on the amount of brightness at a horizontal work plane. One of the challenges is to assess the amount of light received by occupants throughout the 24-hour day, whether the light source is daylight or electric. This paper summaries major implications of the interplay between light and dark for human nonvisual health in the built environment and discusses recent concepts that aim to integrate these findings into practice for supporting healthy lighting design.