# The Calculated Circadian Effects of Light Exposure from Commuting
## 4 Discussion
### Limitations  
Although the results suggest a positive circadian impact of daylight on commuters, it should be noted that the measurements were conducted in a metropolitan environment in Sydney, Australia and this conclusion may not apply to all circumstances or age groups. For example, travelling in suburban areas may result in a higher circadian impact than travelling in a central business district, as suburban areas have fewer tall buildings that can block the light [10]. Travelling toward the light source (sun) will result in more circadian effects than travelling away from the light source. Older individuals may be impacted differently than younger people traveling on the same commute, as lens transmitances varies with age [46]. While people are commuting, many factors affect how much daylight people are exposed to, and not all these factors were reflected in these measurements. For example, wearing sunglasses will reduce the amount of light entering the eyes and, consequently, reduce the circadian impacts. Wearing a pair of prescription glasses with blue light filters may also reduce the circadian impacts compared with wearing glasses without filters, as the human circadian systems are most sensitive to blue light [2, 36, 37, 38]. These measurements were taken in the summer and autumn and the effects of commuting on circadian entrainment may differ in the winter and spring. The Sydney peak morning commuting hours occur after the sun has risen [47] year-round and the sun angle is lower in winter [48], so it's possible that commuters' corneas would receive more light in winter than in summer, which would lead to a greater circadian impact. In this research, no physiological measurements were made. The conclusions are based on the calculated results of the three metrics. When the \(CS\) value is over 0.3 and EML value is over 200 melanopic lux, the lighting conditions can be beneficial to people's circadian synchronization to the local day-night cycle, and are likely to improve one's daytime alertness and sleep quality [12; 19; 32; 34; 35; 45]. However, the improvement of daytime alertness and sleep quality from daytime light exposure can be varied between different individuals. A recent literature review conducted by [PERSON] et al. shows that the non-visual effects of light on subjective alertness and sleepiness are inconclusive [49]. Several studies also show inconclusive results [50; 51; 52]. There are also some inconsistencies between the results of the three metrics reported in this paper. The results of this study cannot conclude which metric is more accurate. Future research could investigate this inconsistency by measuring a circadian rhythm marker, such as melatonin suppression [7], pupil constriction [53], phase shifting [41], and/or changes in core body temperature [54], to identify the actual circadian effects of commuting on people. Several studies have used the melanopic equivalent daylight (D65) illuminance (melanopic EDI) to quantify light's circadian impact [55; 56; 57]. In addition to the five \(\alpha\)-opic irradiances, the melanopic EDI is defined as one of the five \(\alpha\)-optic equivalent daylight (D65) illuminances (\(\alpha\)-optic EDIs) by the CIE in CIE S 026/E:2018 [16]. It can be simply calculated as the melanopic irradiance divided by the melanopic efficacy of the luminous radiation for daylight (D65), which is 1.3262 (mW/lm) [16]. Compared with other types of cells, the melanopsin-based ipRGCs predominantly contribute to the non-visual effects, including circadian impacts [57; 58; 59; 25]. However, the actual non-visual effects from light exposure rely on the combined responses of all photoreceptors [16; 22]. It is necessary to report the response of each photoreceptor type [16]. As the five \(\alpha\)-optic irradiances and five \(\alpha\)-optic EDIs can be easily converted between each other, to avoid repetition, only irradiances are reported here. Readers who are interested in melanopic EDI values can calculate them from the \(\alpha\)-optic irradiances.