Melanopic illuminance defines the magnitude of human circadian light responses under a wide range conditions.

Ocular light drives a range of non-visual responses in humans including suppression of melatonin secretion and circadian phase-resetting. These responses are driven by intrinsically photosensitive retinal ganglion cells (ipRGCs) which combine intrinsic, melanopsin-based, phototransduction with extrinsic rod/cone mediated signals. As a result of this arrangement it has remained unclear how best to quantify light to predict its non-visual effects. To address this, we analysed data from nineteen different laboratory studies that measured melatonin suppression, circadian phase resetting and/or alerting responses in humans to a wide array of stimulus types, intensities and durations with or without pupil dilation. Using newly established SI-compliant metrics to quantify ipRGC-influenced responses to light we show that melanopic illuminance consistently provides the best available predictor for responses of the human circadian system. In almost all cases melanopic illuminance is able to fully account for differences in sensitivity to stimuli of varying spectral composition, acting to drive responses that track variations in illumination characteristic of those encountered over civil twilight (~1-1000 lux melanopic equivalent daylight illuminance). Collectively our data demonstrate widespread utility of melanopic illuminance as a metric for predicting the circadian impact of environmental illumination. These data therefore provide strong support for the use of melanopic illuminance as the basis for guidelines that seek to regulate light exposure to benefit human health and to inform future lighting design. This article is protected by copyright. All rights reserved.