Comparative analysis of circadian lighting models: melanopic illuminance vs. circadian stimulus

The influence of light exposure on human circadian rhythms has been widely recognized. This effect is mediated by a phototransduction process projected by the intrinsically photosensitive retinal ganglion cells (ipRGCs). The process also involves signal inputs from visual photoreceptors. However, the relative contributions of each photoreceptor to this process remain unclear; accordingly, two different types of circadian lighting models have been proposed: (i) the melanopic illuminance model based solely on ipRGC activation, including melanopic equivalent daylight D65 illuminance (m-EDI) and equivalent melanopic illuminance (EML), and (ii) the circadian stimulus (CS) model, which considers the participation of both ipRGC and visual photoreceptors. However, the two models can yield conflicting predictions. In this study, we assessed and compared the accuracies of these circadian lighting models by fitting a substantial amount of experimental data extracted from multiple laboratory studies. Upon evaluating the results across all exposure durations, data-fitting accuracy of the intricate CS model did not surpass that of the much simpler melanopic illuminance model. Consequently, the latter appears to be the more suitable model for lighting applications. Moreover, a recurring limitation of prior research was revealed: the lighting spectra were not tailored to effectively reflect the fundamental distinctions between the two types of models. Therefore, drawing clear conclusions regarding the accuracies of the models is challenging. To address this problem, we introduced a method for designing contrast-spectra pairs. This method can provide lighting spectra to highlight the difference in circadian illuminance based on one model while keeping the circadian illuminance of others constant.