Opinion: How do we define ‘healthy’ lighting?

Abstract

Increased awareness about how the light to which we are exposed throughout the day influences our health, performance and well-being has sparked interest in practical solutions to optimise lighting design and practice to promote and maintain human health. What has been lacking until now, however, is an agreed definition as to what an appropriate daily pattern of light exposure should entail to inform these goals. It has long been known that light is a key regulator of our internal circadian clock, hormone levels, sleep and other aspects of physiology and cognitive function (so-called non-visual responses to light). These adaptations evolved to allow us to tune cycles of rest, activity, energy intake and associated body functions to the availability of natural light. However, our patterns of light exposure are now increasingly at odds with the natural solar cycle. Hence our dimmer days (due to reduced exposure to daylight) and brighter nights (due to ready access to electric light sources) can disrupt the normal functioning of these biological control mechanisms and contribute to acute and longer-term impacts on health and performance. The obvious solution is to make days brighter and evenings/nights dimmer, but how bright or dim? This has proved a particularly challenging problem because it does not just require agreeing a set of numbers but also identifying an appropriate way of measuring ‘brightness’ in the context of the light-sensing machinery that regulates such responses. Significant progress here became possible due to a new international standard for metrology (CIE S 026/E:2018) based around the properties of intrinsically photosensitive retinal ganglion cells (ipRGCs) known to mediate non-visual responses. The ipRGCs possess their own photopigment, melanopsin, which is maximally sensitive in a part of the spectrum (~490 nm in vivo) distinct from the cone photoreceptors whose properties underlie established photometric units. Since ipRGCs can also convey signals from rods and cones, however, there has been uncertainty as to the relative contribution of melanopsin versus these other signals. Accordingly, the new standard has formalised approaches for quantifying light separately for each of the photoreceptors that may contribute to ipRGC responses, and thereby has facilitated numerous retrospective evaluations of historical data and informed new hypothesis-driven investigations designed to address the existing uncertainty. These advances have now enabled the first expert scientific consensus, quantitative, recommendations for daytime, evening and night-time light exposure to best support physiology, sleep and wakefulness.1 The recommendations derive from an international group of leading specialists in the non-visual effects of light and associated metrology, informed by detailed evaluation of laboratory data and corroborating field studies which reveal melanopsin-weighted light exposure provides a reliable model of the sensitivity of practically relevant non-visual responses to light in healthy adults. These recommendations now provide a clear framework to inform how we light indoor spaces spanning homes, workplaces, educational and healthcare establishments, expressed in an easily measured, SI-compliant, quantity: melanopic equivalent daylight illuminance.