Figueiro et al. have proposed a 24-hour lighting scheme that maximizes circadian stimulation during the day and minimizes it at night, while maintaining good visibility at any time.36 The evidence suggests that high circadian stimulation during daytime waking hours can probably be achieved by receiving about 400 lux at the cornea of a 6500 K (cool-white) light source. Relatively dim ambient evening light (less than 30 lux at the eye) can be provided by light sources such as a 2700 K (warm-white) light source.
High circadian stimulation during the day and low circadian stimulation during the evening can be achieved using a combination of daylight and electric light sources. Daylight can certainly be an effective light source for regulating the circadian system. Outdoor lighting has the ideal quantity, spectrum, distribution, timing, and duration, but it should not be assumed that daylight in buildings will always provide people with a suitable circadian light pattern. Again, a quantitative analysis of daylight in the space should be carefully made because the amount of daylight varies considerably throughout a building. Daylight levels in a room drop very quickly as the distance from the window increases; daylight levels are quite low 3-4 meters away from a window, even on a sunny day. It should also be noted that if sunlight from the window penetrates the room, discomfort glare may cause occupants to draw blinds or shades, eliminating daylight entirely from the space. Since it cannot be assumed that daylight in buildings is always going to be a solution for circadian entrainment, electric light must always be considered.
The relatively dim evening lighting scheme is entirely consistent with current lighting recommendations and practice, and although the light levels recommended for daytime applications are relatively high compared to current practice, they are not difficult to achieve, particularly if controls are incorporated into the design.
For older adults in long-term care facilities, the designer should consider using the 24-hour lighting scheme in common areas where residents are more likely to spend their waking hours. This may vary with the application. For example, in a single-room assisted living setting, the dual lighting scheme needs to be implemented in the room and in a common area, such as the dining room. In nursing homes, the dual lighting scheme can be implemented in day rooms and dining rooms. If power density requirements are still an issue, however, local supplementary lights from blue LEDs can be integrated into the design. Compliance to light treatment via supplementary lighting system may be an issue. In cases where compliance is not an issue, such as when the treatment is applied to healthy older adults living at home, ambient light levels can be reduced to 80-100 lux at the cornea (which is about 200-300 lux on the horizontal surfaces) of a neutral light source (3000-4100 K lamp) and the use of supplemental task light delivering 30 lux at the cornea of a blue light (λmax = 470 nm) from LEDs can be used for at least 2 hours in the morning.
Lighting design has focused largely on providing sufficient light for visual performance, minimal glare, good color rendering, and energy conservation. Little attention has been given to understanding how light affects the non-visual systems, including circadian regulation that affects sleep and mood. Strategies to increase circadian light exposures in buildings should be a consideration in architectural design. The use of new technologies such as LED lighting can enable greater control over both the amount of light and its spectral characteristics, both of which are known to influence circadian processes and health outcomes.