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

Apr 6, 2020Journal of pineal research

Melanopic light levels predict the strength of human circadian responses in many situations

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Abstract

Melanopic illuminance is identified as the best predictor for nonvisual responses to light in humans, including melatonin suppression and circadian phase resetting.

Nineteen laboratory studies were analyzed to measure human responses to various light stimuli.
Melanopic illuminance accounted for differences in sensitivity to light stimuli with varying spectral compositions.
Responses tracked variations in illumination typical of civil twilight, ranging from approximately 1 to 1000 lux.
The findings suggest widespread applicability of melanopic illuminance in predicting circadian effects of environmental light.

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Ocular light drives a range of nonvisual 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 nonvisual 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.

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

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