## 4 Discussion
### Reflection on the Results  
The intervention lighting that was placed in the participants' bedrooms, kitchens, and living rooms theoretically delivered up to 850 lx extra (580 lx EDI\({}_{\text{mel}}\); see Table 2, depending on the phase of the dynamic scenario) at eye level. These estimates were based on measurements in a small white room without daylight. The actual light exposure at eye level of the participants if seated correctly under the lamp, of course, depends on the exact placement of the lamp, furniture, and finishing of the room, as well as the available daylight. The measurements performed with the spectrometer in the rooms of the participants, under the lamps, indeed established an increase of 670-1250 lx (350-650 lx EDI\({}_{\text{mel}}\), depending on the room and measured in the morning, so at the peak of the scenario; see Table 3) between the intervention and baseline conditions. However, in daily life, the actual personal light exposure largely depends on the dynamic scenario and the person's physical activity and location (e.g., spending time outdoors, opening and closing curtains, and screen usage). For instance, the added effect of the lighting system is only modest in comparison with the effect of direct exposure to daylight when people are outside. The amount of light on a sunny day is orders of magnitude higher than the amount provided by any indoor lighting system. The effectiveness of the placement of our intervention lighting furthermore depends on the time they would actually spend in the designated seating areas under the lamps. We therefore considered it important to establish the effectiveness of a light intervention in everyday situations and over a prolonged period of time. Indeed, the personal light exposure data showed statistically significant differences between the two intervention periods and the second baseline week for most measurements. The difference in exposure in the first baseline week did not reach significance, potentially due to substantially better weather conditions in those weeks than in the remaining weeks. However, to put these findings into perspective, we should also note that the amount of extra light that was actually received according to the worn sensors was less than 20 lx (on average) during the participants' waking episodes. This is the reality of field interventions, as also observed in a recent office-based field study [28]: what you see is not necessarily what you get. On the other hand, the 20 lx may be an underestimation of the actual circadian effect, as our loggers could not collect full spectra and hence potentially did not pick up spectral shifts towards the blue spectrum specifically. In our sample, there was actually quite some variation in the effects of the lighting system. Although most participants showed effects in a positive direction, not all individual comparisons were statistically significant, and one participant (pp3) even had lower average light exposure when the lighting system was first presented in phase B1 and higher average light exposure when the lighting system was removed in phase A2. A possible explanation why this individual showed this deviant pattern is that he or she was not feeling well during this period and spent a lot of time in bed, where he or she only received the 30 min of wake-up light, not the full daylight curve of the free-floor standing lights. An advantage of the SCED design is that the data can be directly linked to this participant. An additional explanation for the differences in the results on the worn sensors compared with the results on the spectrometer is that the quality of devices to achieve an accurate quantification of light exposure can differ. Furthermore, the location where the sensors were worn can also have an impact on the results. [PERSON] et al. [46] previously studied several commonly used wearable light measurement devices.