Light for Life: Emerging Opportunities and Challenges for Using Light to Influence Well-Being
Both lighting technology and our understanding of the relationship between light and human health have advanced rapidly in recent years. The latter needs to be carefully evaluated, particularly as longer-lasting light sources enter the market.
by Jennifer A. Veitch
ADVANCES in science and technology, occurring separately but in parallel, have brought new life to all aspects of lighting research and application. With this has also come tension between those who argue in favor of rapid adoption of practical applications based on the scientific advances and those who favor a slower approach that waits for deeper understanding of related human physiological issues.
Readers of Information Display need hardly be reminded of the solid-state-lighting (SSL) revolution. It has rendered the old cathode-ray-tube monitor obsolete and is partly responsible for the portable computing we now enjoy. Most industrialized countries have now enacted energy-efficiency regulation for light sources that have resulted in common incandescent lamps being withdrawn from the market. Although compact fluorescent lamps generally fulfill the new light-source efficacy requirements, it is light-emitting-diode (LED) sources that are projected to dominate the market in the coming years, with organic LEDs (OLEDs) lagging a little behind.1 Whereas most household lighting formerly delivered, at best, 20 lm/W, and commercial fluorescent systems delivered 70–80 lm/W, inexpensive LED replacement lamps now deliver ~100 lm/W. The U.S. Department of Energy predicts that 200 lm/W will be achieved before 2020.1 If that goal is met, that would be a 10× increase in energy performance for common light sources used in homes (LEDs vs. incandescent lamps) and a ~3× increase for common commercial light sources (LEDs vs. electronically ballasted fluorescent systems) in under two decades.
The scientific revolution is no less marvelous. In 2002, in the culmination of several decades of investigation and debate, we learned conclusively that there is a class of photo-receptive cells in the retina that is separate from the rod and cone cells that transduce visual signals.2,3 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.4 These findings have captured the attention of many labs in the intervening years: a search of the Scopus database using the terms (light AND (health OR circadian OR melatonin)) for the periods 1996–2001, 2002–2007, and 2008–2013, returned, respectively, 8172, 12,886, and 20,692 records.
Not long after the identification of what are now known as intrinsically photoreceptive retinal ganglion cells (ipRGCs), some authors began to advocate for changes to lighting practice to take advantage of the burgeoning research in the field.5 Others argued in favor of a slower pace of adoption, citing the risks of applying only partial knowledge and possibly causing unintentional effects.6 Ten years later, this debate continues. In late June 2015, the International Commission on Illumination (known as CIE for its French name, Commission Internationale d’Eclairage) issued a statement on “Recommending the proper light at the proper time,”7 outlining its cautious step-by-step approach to applying the findings of this burgeoning area of investigation.
One reason for caution is that lighting installations serve many functions, and recommendations need to reflect this complexity. Lighting quality exists at the nexus of the needs of individuals, the environmental and economic context, and architectural considerations (Fig. 1). New technologies offer new opportunities to use light to influence well-being, and research is providing new (but still emerging) understanding of how light can influence well-being – but there exist many challenges as we seek to blend these opportunities into coherent guidance or practice in balance with the other considerations.
The remainder of this article will expand upon the state of knowledge and the possible guidance for its application.
Fig. 1: Good lighting quality requires balancing the individual needs of people who will experience the installation, its economic and environmental context, and the architectural setting. From Veitch.8
Eye–Brain Pathways
Extensive investigations by photobiologists have revealed to us that the connections between the eye and brain are far more extensive than are needed for vision alone. Figure 2 provides a schematic guide to some of these connections. The connections through the retino-hypothalamic tract leading to the control of melatonin secretion are the best understood. Melatonin is an important marker of circadian rhythms. Melatonin is a hormone that is secreted in high levels at night, but not by day, in all vertebrate species that have been studied.9 In some species (e.g., rats), melatonin signals waking and activity, whereas in others (including humans) it signals sleep and rest. The secretion profile of this hormone helps the organism to match activity levels and internal processes to the external cues of day and night, a process called entrainment.
Fig. 2: This simplified schematic diagram of two eye–brain pathways is taken from CIE 158:2009. The light received by the eye is converted to neural signals that pass via the optic nerve to these. © CIE, 2009. Used by permission.
In a healthy person with a regular schedule of daytime activity and nighttime sleep, melatonin secretion begins to rise in the evening, reaching its peak in the middle of the night before falling abruptly around dawn. Its level remains very low throughout the daytime hours before rising again the following evening. Other processes rise and fall in response to this signal. As melatonin rises, we become less alert and more sleepy, and digestive and immune processes alter, for example, by increasing infection-fighting processes while the organism is at rest.9 As melatonin levels fall in the morning, we become more alert; the hormone cortisol increases in level, signaling a period of activity.
The observation that nighttime melatonin secretion by humans is acutely suppressed by nighttime light exposure10 was an important step in understanding how light entrains our daily rhythms of waking and sleeping. After early missteps that led to the development of tight experimental controls, this paradigm has been the foundation of much that is currently known colloquially as “light and health.” Melatonin is relatively easy to measure (in either blood or saliva samples), making it a practical starting point for understanding these effects. Consequently, our understanding of how light exposure regulates circadian rhythm, although not complete, far exceeds our understanding of light’s influences on other behavioral and physiological processes.
In the 1980s and 1990s, researchers focused attention on the physiology of circadian regulation and on identifying the photoreceptive mechanism. Extensive discussion (and some heated debate) concerned whether or not the signals to the suprachiasmatic nucleus began with the classic photoreceptors – the rods and cones well known from vision research – or with a different cell type. Evidence that the spectral sensitivity of the melatonin suppression response to nighttime light differs from any of the then-known photoreceptors11,12 was part of the process leading to the identification of ipRGCs.2,3 The curves reported by various researchers all differed slightly, but were consistent in showing peak sensitivity for radiation between 460 and 490 nm in the blue range.
Among the important consequences of this finding is the need for new quantities to characterize the intensity of light exposures that are intended to trigger effects other than vision. Light is unique in the International System of Weights and Measures (Système Internationale, SI) in that its definition is tied to a human biological response, the photopic spectral luminous efficacy function,13 commonly known as V(λ). This function is a combination of the spectral responses of the medium- and long-wavelength cones at the fovea; the response is strongest for radiation of 555 nm (green). Once it was established that ipRGCs are responsible for light detection that leads to parts of the brain not responsible for visual processing, it became clear that if investigations report light exposures in illuminance or luminance units, they would be a poor indicator of the stimulus strength for these other processes.
Although the need for a new quantity has been known for many years, only with the publication of CIE Technical Note TN 003:2015 have we taken the first step toward the goal of appropriate SI-compliant units to characterize the stimulus strength for processes other than vision, including circadian regulation. TN 003 is an extensive report of a consensus workshop held in 2013 to establish the action spectrum for ipRGC stimulation, first reported by Lucas et al.4 Figure 3 displays the spectral efficiency functions for the five photoreceptors. TN 003:2015 is a free document, which also comes with an Excel toolkit for calculating light exposures using the weighting functions for the five photoreceptors. Note that the functions published in TC 003:2015 and in Lucas4 do not have the status of international standards. CIE has formed a technical committee to translate the consensus expressed in TN 003 into the first international standard on quantifying irradiance with respect to stimulation of all ocular photoreceptors.
Fig. 3: There are five known photoreceptive cells in the human retina, each with a different action spectrum, shown here as relative sensitivity normalized to their peaks. The three cone types are responsible for color vision and fine-detail detection and are present primarily in the fovea. Rods are present across the retina and are primarily responsible for vision at low light levels; their activity is suppressed at daytime and indoor light levels. The ipRGCs are irradiance detectors, sending signals to the brain through the retino-hypothalamic tract (see Fig. 2). Data used to prepare this figure are from CIE.14
Since the identification of ipRGCs, and the awareness that their action spectrum shows greater sensitivity to short-wavelength radiation, there has been a desire among many in the lighting industry to use this knowledge in practical applications. This is challenging, however, because of growing awareness that the pathways shown schematically in Fig. 2 are very complex. The photoreceptors interact. For example, it now appears that pupil size is regulated by different photoreceptors at different times. The immediate pupillary light reflex response to light exposure occurs because of rod and cone responses, but the sustained response occurs because of ipRGC stimulation.4,14
Moving from science to application is further complicated by the fact that in addition to the light-source spectrum, four other parameters also influence our physiological and behavioral responses to light exposure: light intensity, duration, timing, and pattern. Some of these are better understood than others. For example, we know how to influence circadian rhythm by changing the timing of light exposure in relation to the nadir of the cycle. Light exposure before this point (e.g., late in the evening, after melatonin secretion has begun) tends to delay the cycle – it slows down melatonin secretion. Light exposure after this point (e.g., around dawn) tends to advance the cycle – melatonin secretion will begin again at an earlier clock time than on the previous day. This knowledge is the basis for recommendations for shift-work adaptation and the avoidance of jet lag.15
Principles of Healthy Lighting Redux
One way to think about how light might influence human health is in the expression of principles of healthy lighting. The first consensus report in this field was CIE publication 158, first published in 2004 (re-issued in 2009 with errata corrected). Although based on the knowledge available at that time, subsequent evidence has not displaced these principles; rather, it has emphasized their importance, as will be shown briefly here. Each bullet point below is a principle of healthy lighting as articulated in CIE 158:2004/2009.
• The daily light dose received by people in Western [industrialized] countries might be too low.
Since 2004, evidence for this has mounted. Several investigations show that people who experience increases in light exposure during daytime show beneficial effects.16,17 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, nighttime melatonin is suppressed. Growing evidence links this to serious health consequences from cancer to metabolic disorders.18
The importance of a regular rhythm of bright light and darkness (the first two principles) leads to a 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 do not spend all of their time in one place lit with one set of lights. There are notable exceptions, which demand special attention, such as care homes, hospitals, and prisons, where a single authority is largely responsible for establishing and maintaining the light pattern. For most of us, however, our personal behaviors will largely determine the amplitude of the daily light–dark pattern to which our bodies respond. Even relatively low-intensity ambient illumination (200 lx) late in the evening can influence melatonin levels at night.19
• Light for biological action should be rich in the regions of the spectrum to which the nonvisual system is most sensitive.
Even in 2004, when there was no consensus concerning the action spectrum for ipRGCs, it was clear that short-wavelength radiation had a greater effect on circadian regulation. This evidence has led to the introduction of many products designed to increase short-wavelength light exposure by day, such as fluorescent lamps with high correlated color temperatures and dynamic color-changing lighting systems that can vary the spectral content of the light source and its intensity over time. As we learn more about the complexity of these processes, the limitation of this principle becomes clear: We need to think carefully about what biological action we wish to influence in order to choose the correct spectrum (and intensity, duration, timing, and pattern) of light exposure.
• The important consideration in determining light dose is the light received at the eye, both directly from the light source and reflected off surrounding surfaces.
This principle remains true for circadian regulation and processes mediated by eye– brain pathways. Anyone developing a lighting system needs to be aware that the effects are not determined by the technology alone but by how it is used. For architectural lighting, this means that the room surfaces are part of the lighting system. For display-screen devices, determining the light exposure will mean thinking about how the viewer holds the device, as well as the ambient environment in which it is used.
• The timing of light exposure influences the effects of the dose.
System sensitivity has long been known to be time-dependent, but we are beginning to learn how complex the system can be. Evidence coming in now shows us that prior experiences influence subsequent responses: thus, when one experiences a day of lower light exposure, one shows a lower nighttime melatonin suppression response.20 As we learn more about how pattern affects various physiological responses, we might become better able to deliver the greater peak light exposure referred to in the first principle but without increasing lighting energy use by employing information about duration, timing, and pattern to better effect.
Recent evidence has shown that illuminated-display-screen use in the evening, before bed, can influence subsequent sleep quality and disrupt circadian rhythms.21 This has been attributed in part to the relatively high level of short-wavelength radiation emitted from many display screens22 to which the ipRGCs are most sensitive, particularly at that time of day. Personal light hygiene and technology together can play a role in preventing problems. Altering the display properties in relation to the time of day is one possibility (e.g., using software such as f.lux, https://justget flux.com/). (That is, avoid using a light source with emissions in the most-sensitive region of the spectrum if one wishes to avoid biological effects.) Other options are to reduce the screen intensity or to reverse the text polarity (supported by many eReaders). A simple solution requiring no technology is to stop using a display screen late into the evening, just before bedtime.
Other Healthful Lighting Issues
The principles of healthy lighting set out above focus primarily on stimulating ipRGCs, but to use light to benefit well-being in all aspects of life – as would be required to achieve good-quality lighting – is more than this. There is more to learn about the non-visual processes because we do not yet have a clear understanding of the purposes of the links from the RHT to brain structures other than the hypothalamus. Some evidence suggests that there are acute effects on alertness separate from circadian regulation;23,24 Whitehead25 identified issues related to light-source properties as they can affect color vision. Well-being also includes environmental perception, such as evaluations of spaciousness and feelings of visual comfort.26 Lighting for life demands an integration of many considerations.
One issue that many had thought was resolved with the advent of electronic ballasts for fluorescent lamps is light-source flicker. Electronic ballasts operate between 20 and 40 kHz, whereas the magnetic ballasts they replaced operated between 100 and 120 Hz (depending on the supply frequency). The change occurred in order to achieve energy efficiency but had the demonstrated benefit of reducing the incidence of headaches and eye strain.27 LED drivers, however, show great diversity in their operating characteristics, which means that there is a vast range of diversity in the flicker properties of LED devices on the market.28 Some of these operate in the ranges previously associated with adverse health effects. In an attempt to educate the industry about these issues and to provide guidance concerning the safe range of operating conditions, IEEE published a recommended practice for LED operation to mitigate health risks from flicker.29 This is the first guidance of its kind and is likely to be revised as new information becomes available, particularly concerning parameters that have not previously been investigated (e.g., duty cycle and waveform). It is a new document and not yet referenced in any legislated requirements (to my knowledge), but we can expect the issue to remain on the agenda for healthful lighting in the months and years to come.
What the Future Holds
Recent revolutions in both lighting technology and lighting research make this a very exciting time to be affiliated with these fields. With 2015 having been designated by UNESCO as the International Year of Light and Light-Based Technologies (www.light2015.org), this excitement is reflected in the popular press to a far greater extent than in previous years. People are hungry to know how to use light for living, and manufacturers are eager to provide them with the tools to do so.
Nonetheless, we are only at the beginning of understanding how light can affect us, and there may be surprises ahead as our understanding expands and strengthens. Choices made today about technology that might have a long operating lifetime need to be carefully weighed, lest we find ourselves unintentionally conducting a natural experiment with unwanted consequences. These important decisions about light sources, their operating properties, their use in devices, the installation of those devices in the environment, and our personal patterns of light and dark exposure, all require bodies of knowledge. One investigation ought not to be the basis of national, regional, or international standards or regulations; look for replications and extensions that provide consistent results that follow predictions based on a deep understanding of underlying anatomy, physiology, and psychology. When reading about exciting new lighting products that are said to deliver healthful lighting, we need to consider whether they provide a complete lighting quality solution.
In the coming months, CIE will issue a new technical report, “Research Roadmap for Healthful Interior Lighting Recommendations,” outlining requirements for that body of knowledge. Among the challenges ahead is sustained interest (with research funding) to develop that knowledge so that future lighting systems fulfill the promises of delivering “light for life.” Fortunately, the advances in technology, including controls and imaging, offer many new tools to build systems that take advantage of these research findings. Partnerships between industry and researchers will sustain progress for both.
References
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