## 4 Discussion
To decrease \(f_{1}\)\({}^{\prime}\), it will be necessary to reduce the error resulting from the difference between the simulated and fabricated optical filter, and to continuously provide refractive index feedback for the materials used (SiO\({}_{2}\) and TiO\({}_{2}\)). In addition, a separate diffuse lens system can be introduced above the optical filter to reduce \(f_{2}\)[24]. In terms of reproducibility, the repeatedly measured values of the circadian light meter are slightly different from those of commercially available products, but not at a level that is highly recognizable. Therefore, with small measurement deviations, the melanopic EDI and CIL values can be determined immediately without complicated calculations. Nevertheless, the CIL values were not included in the intra-daily analysis using the fabricated circadian light meter. As shown in Figure 2, the filters for melanopic EDI and CIL were prepared to mimic their action spectrum, but it was not clear whether the spectral resolution of the filters could properly discriminate the difference between the melanopic EDI and CIL. Similarly, as summarized in Table 1, there were small differences between the melanopic EDI and CIL under the identical lighting environment, but their effects on the human body were not verified. Meanwhile, the intra-daily illuminance according to the time of exposure of the participants tended to differ from that of natural light. This difference occurs because the sleep onset time of the participants was 00:48 \(\pm\) 01:08, and they mostly spent their personal time indoors, typically in their homes, until they fell asleep. In addition, their wake-up time was 08:51 \(\pm\) 00:51, but the VIL increased before that time as sunlight affected the room after sunrise. This trend is also confirmed in the melanopic EDI value because that illuminance is proportional to VIL under a light environment with an equivalent SPD. Furthermore, the melanopic irradiance expressed as a radiometric quantity show the identical trend as the melanopic EDI. Participants were free to move as they wished outdoors; however, they were exposed to very low levels of light compared to the level of sunlight. In other words, during the daytime, the participants spent most of their time indoors, and so their melatonin levels may therefore be significantly upregulated, which can lead to increased subjective sleepiness or decreased attention, even when their rooms were sufficiently bright [27; 28]. Here, it is important to note that after sunset, the participants were exposed to artificial lighting that was hundreds of times brighter than natural light. Specifically, the participants were exposed to artificial lighting thousands of times brighter than the natural light illuminance at midnight. In fact, participants were exposed to light at midnight with average VII. and melanopic EDI values of 70 k and 61 k, respectively, which may have suppressed their melatonin levels, resulting in poor sleep or affecting their circadian rhythms [29; 30]. This trend is natural because the average activity time of a person has increased due to artificial lighting, even at night and without sunlight, ever since artificial lighting was invented. Furthermore, we mostly have at least one display device, such as a personal computer, a laptop, or a smartphone, and we can use them whenever desired, even at night. A survey from the National Sleep Foundation found that 90% of Americans report using their devices an hour before falling asleep [31]. General white artificial lighting sources, such as fluorescent lamps that do not allow changes in the CCT or brightness, can disturb human circadian rhythms by the blue light contained in them. In particular, blue light is known to suppress melatonin at nighttime, and thus night workers may see an increased incidence of melatonin-related diseases, such as insomnia, depression, breast cancer, and even heart disease [6; 32; 33]. We checked the VIL values and non-visual effects of light exposed to participants using a designed circadian light meter.