Reply to Bracke et al. Comment on “Prayag et al. Light Modulation of Human Clocks, Wake, and Sleep. Clocks&Sleep 2019, 1, 193–208”

Abstract

We thank Bracke and colleagues [1] for their commentary on our recent article ‘Light Modulation of Human Clocks, Wake, and Sleep’ by Prayag et al. (2019a) [1]. This gives us an opportunity to expand on our reported findings and interpretations and also to remind readers that mathematical models are aimed at simplifying complex biological mechanisms, and hence should be used parsimoniously and never be considered as absolute truths. We also underscore that results should always be interpreted cautiously, taking into account the experimental conditions the data were collected, and the non-visual responses investigated. We appreciate Bracke and colleagues’ [1] concern that our findings may be inappropriately interpreted by the non-specialist, in particular, Figures 4 and 5 of our review paper [2]. They raise two important points: (1) “ . . . Figure 5 could be misinterpreted by this audience as if monochromatic light sources need less melanopic-weighed irradiation than white light sources for the same response”, and (2) “ . . . since the data for the monochromatic light sources are for pharmacologically dilated pupils, with normal pupil constriction (as for the white light irradiance response curves in Zeitzer et al., 2000) a much higher melanopic irradiance will be required for the same monochromatic response”. In our review paper [2], Figure 4 is the illuminance-response curve for melatonin suppression (in response to 6.5 h polychromatic white light exposures) that we redrew from Zeitzer and colleagues (2000) to extend the illuminance axis. Figure 5 is the melanopic illuminance-response curve we modelled in one of our recent articles [3], derived from data obtained in response to 1.5 h of monochromatic light exposures [4]. In the interest of clarity, we would like to provide some elements of context and then address these aspects point- by-point.