Table 1.

Some open questions in mixed electric light/daylight research related to metabolic functions, sleep, alertness and cognition as well as physical activity.

(i) Metabolic functions
How does electric light and/or daylight affect metabolic functions?	This question is closely related to meal timing, caloric intake and meal composition, or weight loss. A few studies have investigated these aspects under laboratory conditions [72,73,74], but not yet under daylight conditions. The question is whether daylight exposure specifically affects the timing of meals, post-prandial responses, temperature regulation, metabolism, body composition and the gut microbiome?
(ii) Sleep
Does daylight and/or mixed daylight/electric light conditions during the day mediate better sleep quality at night, and if yes, how?	There is some evidence that indoor and/or outdoor bright light exposure during the day leads to longer sleep duration and increases sleep quality as was shown in laboratory and field studies [63,64,75]. Electric light exposure (with a spectral power distribution closer to daylight than standard LED-light) during daytime enhances slow wave sleep (=‘deep sleep’) [76] and brighter ambient light (compared to dim light) increased homeostatic sleep pressure during wakefulness [77]. The following questions relate to daylight and sleep: Does insufficient sleep (partial and/or chronic sleep deficit) counteract beneficial daylight exposure effects? Can sufficient daytime light exposure offset negative consequences of electric light exposure at night—with respect to entrainment, sleep, performance and health outcomes [78,79,80]?
(iii) Alertness and cognition
How do daylight-specific properties affect alertness and cognitive functions?	Many laboratory studies with steady state electric lighting showed light-dependent alertness [45,46,49,81,82,83] and cognitive repercussions [46,53,55]. A few studies have looked at spectrally tuned electrical light conditions and alertness and cognition [76,84,85,86].
(iv) Physical activity
Does physical activity and daylight interact to induce larger phase shifts of the circadian clock?	From studies in the laboratory there is a phase-response curve of physical activity with both phase delays and advances at specific times of day [87]. Activity facilitates phase delays in very dim light [88]. The question relates to the interaction of exercise with (day-) light exposure as shown with cycling performance [89].