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A new study published in PLOS Biology looked at recommendations for daytime, evening, and nighttime indoor light exposure to best support physiology, sleep, and wakefulness in healthy adults.

“It has been known for some time that our daily patterns of light exposure exert important influences on health and well-being by regulating our internal body clock, cycles of hormone release, night-time sleep and daytime alertness,” study author Timothy Brown told us. “Accordingly, there have been concerns that our modern lifestyles might be detrimental to health, as a result of reduced exposure to bright natural outdoor light during the day and/or increased exposure to light during the evening and night through electric lighting and visual displays.”

While the obvious solution suggests Brown, would be to increase light exposure during the day and decrease it in the evening and nighttime the question has always been how bright is sufficient for daytime, how dim/dark at night? Importantly, answering those questions has been more challenging than simply agreeing a set of numbers but requires also working out how to properly measure the ability of light to influence the body clock and associated so-called ‘non-visual’ effect of light.

“Hence we have known for some time that our established methods of measuring ‘brightness’, which approximate perceptual brightness sensitivity to light of different wavelengths (e.g. lux), do not adequately capture the propensity of different types of light and lighting to trigger these non-visual responses,” Brown told us. “The aim of our study was therefore to draw of the many years of research into these topics, to establish concrete recommendations for daytime, evening and nighttime light exposure based around a methods of measuring light relevant to the effects of light on these non-visual responses.”

Researchers drew on over 20 years of scientific research into the biological mechanisms regulating effects of light on circadian rhythms, hormone secretion and sleep alertness. Such studies revealed that a specialized cell type in the eye, distinct from the rod and cone cells responsible for vision, played a major role in controlling these non-visual effects of light. These cells express a specific light –sensitive protein called melanopsin which is most effective at sensing light in the sortwavelength (‘blue’) part of the spectrum.

“We expected light measurement approaches based around the unique properties of melanopsin would likely be important but there were also suggestions that signals from the cone cells in the retina might also be important,” Brown told us. “These non-visual effects of light exert important influences on health, well-being and performance. There was therefore a great need for research-based guidance on appropriate daily patterns of light exposure to inform lighting and building design and associated regulatory guidance.”

Researchers gathered an international group of experts on non-visual effects of light on circadian rhythms and sleep as well as representatives of the international lighting regulatory body the CIE (Commission Internationale de l’Eclairage). They reviewed the scientific literature, drawing on a new international light measurement standard relevant to the melanopsin-expressing cells in the retina (CIE S026), and formed an expert consensus during an in person meeting (held in Manchester UK in 2019).

“The consensus emerging from the meeting was a set of recommendations for daytime, evening and nighttime light exposure expressed in a newly defined unit of measurement that accurately predicts the propensity of different types of light and lighting to impact health and physiology (melanopic equivalent daylight illuminance),” Brown told us. “We very pleased that we were able to establish a unanimous agreement across scientific experts from all over the world to address this important issue.”

Brown and his team believes the outcomes provide a key milestone in the goal towards designing healthier lighting environments. They expect the recommendations to directly inform the design of lighting and buildings (ranging from increasing availability of daylight to bulbs and lighting systems that better approximate the effects of natural daylight and/or minimise unwanted effects of evening light without compromising vision) and to inform changes to official lighting regulations that take account of non-visual effects of light, in addition to the visual considerations already covered.

“We further expect the recommendations should help inform future clinical guidelines relating to light-therapy applications,” Brown told us. “Finally, we expect that the future availability of personal light monitoring devices tailored to the new measurement system employed by our recommendations will increase the accessibility with which general members of the public can best make use of the recommendations to optimize their own daily patterns of light exposure, as part of strategies to promote health and well-being.”