Welcome to the lighting revolution
In lighting applications, the adoption of light-emitting diodes (LEDs) and organic LEDs promises to reduce lighting energy use dramatically over the next few decades. There is an equally marvellous scientific revolution in biology and psychology. In 2002, we learned conclusively that there is a class of photoreceptive cells in the retina, the intrinsically photoreceptive retinal ganglion cells (ipRGCs), that is separate from the rod and cone cells that transduce visual signals (1). Thus, the eye-brain connection is far more complex than previously thought, and the more we learn the more complex we find it to be (2). Ever since this discovery, debate has raged concerning how to apply this knowledge, and how quickly to do so (3).
One reason for caution is that lighting installations serve many functions, and our recommendations reflect this complexity. As I wrote in a previous blog entry, lighting quality exists at the nexus of the needs of individuals, the environmental and economic context, and architectural considerations. Strong evidence is needed to intelligently blend new discoveries into coherent guidance in balance with the other considerations.
What we know now
Light provides the strongest signal for daily rhythms of waking and sleeping (circadian rhythms), mediated by the hormone melatonin. Darkness signals melatonin production; light exposure suppresses melatonin. In a healthy person living a regular schedule of daytime activity and night-time sleep, the circulating melatonin level begins to rise in the evening, reaching a peak in the middle of the night before falling abruptly around dawn. Melatonin production remains very low throughout the daytime hours before rising again the following evening.
Our understanding of how light exposure regulates circadian rhythms, although not complete, far exceeds our understanding of light’s influences on other behavioral and physiological processes. This fact is reflected in the detailed pathway from the retina to melatonin in the figure above, as contrasted with the more general path to other brain structures.
One way to think about how light might influence human health is in the expression of principles of healthy lighting, as seen in CIE Publication 158:2004/2009 (4). Some of these are extracted here as bullet points, with commentary on their current status.
- The daily light dose received by people in Western [industrialized] countries might be too low.
Investigations continue to show that people who experience increases in light exposure during daytime show beneficial effects (5). Time-use studies consistently show that people spend ~90% of the day indoors, which raises the possibility that interior light level recommendations might need to be higher than is currently the case. This could be controversial because of the need to reduce lighting energy use. Even with smart lighting systems using solid-state lighting and advanced controls, providing higher light exposures without increasing lighting energy use will demand careful design and planning.
- Healthy light is inextricably linked to healthy darkness.
Although circadian regulation is not the only function influenced by ipRGC stimulation, it is an important one. There need to be signals for both light and dark. Without a period in darkness each day, night-time melatonin is suppressed. Growing evidence links this to serious health consequences, from cancer to metabolic disorders (6).
- Light for biological action should be rich in the regions of the spectrum to which the nonvisual system is most sensitive.
Part of the evidence for the existence of ipRGCs was the observation that night-time melatonin suppression by light followed a different spectral response function than any of the then-known retinal photoreceptors. This fuelled excitement about the prospect of improving well-being by changing the light source spectrum. Extensive research since then, and an international expert workshop, has established consensus concerning the action spectra for the five known photopigments (Figure below) (2, 7). The consensus placed the peak of the action spectrum for ipRGCs at 490 nm, in the blue region of the spectrum – but also concluded that no single photoreceptor type explains physiological responses to light. The various biological and psychological processes appear to respond differently to changes in light source spectrum. Thus, although the original principle remains true, it is too imprecise to guide lighting practice.
- The timing of light exposure influences the effects of the dose.
In addition to the light source spectrum, the intensity, duration, timing, and pattern of exposure all influence our physiological and behavioral responses. Understanding the effect of timing on circadian rhythms has led to practical advice for shift work and jet lag adaptation (8) by timing light exposure in relation to the lowest point of the circadian cycle. We have much to learn about the influence of the other parameters and how they might influence lighting practice in places where people only spend parts of their time each day.
What the future holds
The importance of a regular rhythm of bright light and darkness for circadian regulation, and the moderating effects of the timing and pattern of exposure, lead to the conclusion that healthy lighting is not only an architectural issue: It is a public health matter, and individuals will need to take responsibility for their own light hygiene. Most people (with some obvious exceptions) do not spend all of their time in one place lit with one set of lights. For most of us, our personal behaviors will largely determine the daily light-dark pattern to which our bodies respond. Among the most important science to be done is to determine what that pattern ought to be, with enough detail to support integrated lighting recommendations both for architectural spaces and for personal light hygiene.
CIE will soon issue a new technical report, “Research Roadmap for Healthful Interior Lighting Recommendations”, outlining expert consensus on specific topics and setting priorities. Among the challenges ahead is sustained interest (and research funding) to develop that knowledge, so that future lighting systems fulfill the promises of delivering “light for life”.
A longer version of this article will appear in Information Display, December 2015.
- Berson DM, Dunn FA, Takao M. Phototransduction by retinal ganglion cells that set the circadian clock. Science. 2002 Feb 8;295(5557):1070-3.
- Lucas RJ, Peirson SN, Berson DM, Brown T, Cooper HM, Czeisler CA, et al. Measuring and using light in the melanopsin age. Trends in Neurosciences. 2014;37(1):1-9.
- Commission Internationale de l’Eclairage (CIE). CIE statement on non-visual effects of light: Recommending proper light at the proper time. Vienna, Austria: CIE; 2015.
- CIE. Ocular lighting effects on human physiology and behaviour (CIE 158: 2004/2009). Vienna, Austria: CIE..
- Smolders KCHJ, Kort YAWd, van den Berg SM. Daytime light exposure and feelings of vitality: Results of a field study during regular weekdays. Journal of Environmental Psychology. 2013;36(0):270-9.
- Stevens RG, Zhu Y. Electric light, particularly at night, disrupts human circadian rhythmicity: is that a problem? Philosophical Transactions of the Royal Society of London B: Biological Sciences. 2015;370(1667):20140120.
- CIE. Report on the First International Workshop on Circadian and Neurophysiological Photometry, 2013 (CIE TN 003:2015). Vienna, Austria: CIE.
- Revell VL, Eastman CI. How to trick mother nature into letting you fly around or stay up all night. Journal of Biological Rhythms. 2005;20(4):353-65.
Jennifer Veitch (@JenniferVeitch1) is a Principal Research Officer in NRC Construction, where she has led research into the effects of indoor environment effects on health and behaviour since 1992. She chaired the CIE technical committee that wrote CIE Publication 158:2004/2009, and currently chairs CIE TC 3-46, which is writing the “Research Roadmap for Healthful Interior Lighting Recommendations”. She is a Fellow of several associations in lighting and psychology. In 2011 she received the Waldram Gold Pin for Applied Illuminating Engineering from the International Commission on Illumination (CIE). She currently serves CIE as Director of its Division 3, Interior Environment and Lighting Design.