This white paper discusses the latest scientific insights on how light affects people’s sleep, health and well-being. It also provides some initial guidance and recommendations for the application of melanopic lighting in indoor environments. With the recent insight that light plays a much bigger role than vision only, two new quantities have been defined to specify the lighting environment with respect to its ability to support health and well-being aspects [39]: The effectiveness of a lighting condition to activate melanopsin-based photoreception and drive ipRGC responses can be expressed in terms of the absolute quantity melanopic equivalent daylight illuminance (melanopic EDI). During daytime the guideline for melanopic EDI is: the more, the better, although this should not supersede existing guidelines [12]. Based on a comprehensive analysis of the sensitivity of human non-visual responses to retinal light exposure Brown et al. [11] recommend a minimum melanopic EDI of 250 lx at the eye for daytime indoor environments (measured in the vertical plane at ~1.2 m height) as to promote optimal physical and mental health and performance. The melanopic daylight efficacy ratio (melanopic DER) is a dimensionless relative quantity that describes a spectral characteristic of a light source and expresses the melanopic activation of a (test) light source as compared to a reference light source that emits a daylight spectrum (D65) and produces the same photopic illuminance as the test light. For instance, a test light with a melanopic DER of 1.25 or 0.75, has a melanopic activation (per lumen) that is 25% more or less as compared to daylight D65, respectively Note: the melanopic equivalent daylight illluminance (lx) (Melanopic EDI) equals the product of the amount of light (photopic illuminance in lx) that falls on the eye and the melanopic DER: melanopic EDI = illuminance * melanopic DER