This presentation will discuss how recent findings in photobiology about the correlation between lighting and the human circadian system can prospectively be applied to architectural design, with a focus on healthcare and housing environments. Outcomes of photobiology research were used to define threshold values for illumination in terms of spectrum, intensity, and timing of light at the human eye, and were translated into goals for simulation – and ultimately for building design. While no actual recommendations can - or should - yet be made because of our limited understanding of the effects of exposure to light on human health and circadian organization, especially during daytime, we can still discuss the relevance of some critical design parameters to electric lighting or – to a greater extent – to daylighting.