National Research Council of Canada
Welcome to the lighting revolution
It’s difficult to overstate the changes that have occurred in recent years to the lighting industry, with the development of both new light sources — light-emitting diodes (LEDs) and organic LEDs — and advanced control systems. We are beginning to see reductions in energy use associated with these technologies, as well as new product developments that marry sensors, controls and light sources to produce colour-tunable LEDs and new applications.
Scientists are equally excited about what we are learning about how light affects biology. We have known for ~15 years about the intrinsically photoreceptive retinal ganglion cells (ipRGCs), the photoreceptor class that is separate from the rod and cone cells that transduce visual signals (1). 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). The pressure to move from investigation to application has never been greater – but the need for caution is, in my view, unchanged.
In all the excitement, it is important to recognize that the fundamentals have not changed. Lighting installations should strive to provide the best lighting quality consistent with the context (Figure 1). Well-being has many components.
Light: Not just for vision
Light provides the strongest signal for daily rhythms of waking and sleeping (circadian rhythms). Melatonin is the signalling hormone. Darkness triggers 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. Physiological processes including immune function, digestion, cellular repair and regeneration, start and stop in synchrony with the rise and fall of melatonin.
Interestingly, it now appears that light exposure during the day has additional effects. We have long known that the ipRGCs connect to many brain structures. It now appears that there are five subtypes of ipRGC, although we do not yet know the functions of all of them, nor are we certain what their spectral sensitivities might be. These other ipRGCs might be involved in the processes that underlie the observations that bright light exposure during the day can improve the quality of social interactions and increase alertness (5).
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 (6). 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 (7). 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 (8).
- 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. Extensive research since then, and an international expert workshop, has established consensus concerning the action spectra for the five known photopigments (Figure 2) (2, 9). 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.
This finding underlies much of the current excitement about colour-tunable lighting. Surely we can boost exposure to short-wavelength light for some of the day to boost the circadian regulation signal, and then reduce it at other times to add to the amplitude? Surely we can use relatively more short-wavelength radiation to increase the circadian signal strength while avoiding the need to increase overall light levels (and therefore energy use)?
We know more about when to avoid light exposure to short-wavelength light than we do about the right times to increase it. Late in the evening, as we ready ourselves to go to sleep, it makes sense to avoid light exposure of all kinds, and particularly in the spectral range that most strongly suppresses melatonin. However, it does not follow that it is necessary or desirable to increase short wavelength exposure at other times of day, as the next principle makes clear.
- The timing of light exposure influences the effects of the dose.
Light exposure at night suppresses melatonin immediately, but light exposure during daylight cannot, because there is little or no circulating melatonin to suppress. There is evidence that the same brain structures that are very sensitive to short-wavelength radiation in the middle of the night are much less sensitive during the day – but current research is telling us how those daytime exposures influence our behaviour and well-being later that same day or evening. There are guidelines available today that can aid shift work adjustments and jet lag adaptation (10) by timing light exposure in relation to the lowest point of the circadian cycle, but we don’t know enough yet about the necessary intensity, spectrum, timing, or pattern of light exposure to make good recommendations for specific people in places where they spend only parts of their day.
What the future holds
Healthful 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 – meeting the full range of lighting quality goals — and for personal light hygiene. Until we know this, the foundations of recommendations for using advanced controls for colour-tuning, intensity, and timing of lighting remain strictly “under construction”.
This is an updated version of a longer article that appeared in Information Display, December 2015.
Dr. Jennifer Veitch (e-mail: firstname.lastname@example.org; Twitter: @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 committees that wrote CIE Publications 158:2004/2009, and CIE 218:2016. 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.
- 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. http://files.cie.co.at/783_CIE%20Statement%20-%20Proper%20Light%20at%20the%20Proper%20Time.pdf
- Veitch JA. Commentary: On unanswered questions. Proceedings of the First CIE Symposium on Lighting Quality. Vienna, Austria: CIE; 1998. CIE x015:1998: 88-91.
- CIE. Research roadmap for healthful interior lighting applications (CIE 218:2016). Vienna, Austria: CIE.
- 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. http://www.cie.co.at/index.php?i_ca_id=978
- 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.