OPN4 and obesity due to melanocortin 4 receptor deficiency: Compared to other opsins primarily involved in image formation, OPN4 containing cells function more as photoreceptors for non-visual tasks, including the previously mentioned ones: photoentrainment, PLR, and potential effects from light sensitivity, such as SAD, insomnia, and even associations with metabolism, obesity, and type 2 diabetes (Fleury et al., 2020).