Delivering effective lighting in buildings to support circadian entrainment of the occupants is of growing interest and importance to lighting specifiers. Circadian-effective lighting is not simply about how much light to provide; it also depends upon how long that light is provided. Recent research suggests that the amount of light and the duration of light exposure are reciprocally related for entrainment in the morning hours (i.e. 08:00–12:00). This opens up a number of design options heretofore unaddressed by lighting specifiers.