Table 3.

Unanswered questions related to properties of daylight and or electric light indoor lighting conditions.

Spectral range	Does the fact that the spectral power distribution of daylight goes beyond the visible range make a difference for non-visual functions, when compared to electric light? For example, the infrared (IR) portion that is always present in daylight is non-existent in electric light (except for banned incandescent light sources). It might have an important role for the retina, since a large body of literature suggests connections between long-wavelength radiation and (beneficial) physiological functions in the retina (see e.g., [118,119]). In addition, there is probably a specific role of the spectral power distribution and colour of twilight, as shown in rodents [103,104].
Duration and thresholds	-What are daylight or mixed electric/daylight exposure durations [52,120] and thresholds for non-visual functions, under real life circumstances? -What is the threshold of daylight/electric light conditions in the evening/night that does not interfere with sleep onset/propensity?
Dose-response relationships	What are the dose-response relationships for daylight and mixed daylight/electric light exposures for non-visual functions? In particular, with regard to the interaction of circadian and homeostatic processes with environmental conditions – that include electric light.
Irradiance and spectral composition	Daylight is spatially and temporally variable. These changes take place over very wide frequency bands: -What are these frequency bands, irradiances and are these variations required for the circadian system and other non-visual functions? -What is the influence of different contrasts of daylight or mixed electric light/daylight for photoreceptors (i.e., high vs. low melanopic irradiance) as was shown for electric light on visual perception [121], pupil light reflex and alertness [84,86]?
24 h-Dynamics	How do the 24-h dynamics of daylight impact on non-visual functions?
Light history	How can prior light history (see Table 4) of any light exposure be incorporated into the above-mentioned outcomes?
Therapeutic use of light	Light treatment has been established for winter and other depressive disorders and circadian sleep disturbances [122,123,124,125]. What should the daylight exposure recommendations be for these different therapeutic interventions (regarding intensity, spectrum, timing and prior light exposure)?