The lighting industry continues to explore the potential for lighting solutions to benefit and optimize human health. Studies indicate light has physiological effects that go beyond vision. In recent years, the industry has focused on the effect of light on the human circadian system and how electric lighting choices can affect circadian health.
Light and circadian rhythms
The circadian system produces and regulates bodily functions based on 24-hour cycles, or circadian rhythms. A big example is the sleep-wake cycle. Disrupting circadian rhythms can contribute to poor nighttime sleep, increased daytime napping, and greater risk of depression, obesity, diabetes and seasonal affective disorder.
The daily change from light to dark is the primary stimulus for synchronizing circadian rhythms to our location. The human eye has cells that are receptive to light and connect directly to the brain’s master clock, converting light into neural signals that regulate circadian rhythms. For millions of years, the sunrise-sunset cycle performed this job, but in the modern era, we rely on electric lighting systems.
Traditionally, these lighting systems are designed for vision, disregarding light’s nonvisual impact. Mariana G. Figueiro, Light and Health program director at the Lighting Research Center (LRC) at Rensselaer Polytechnic Institute, Troy, N.Y., said there are four major factors in designing a lighting solution that optimizes circadian health.
First and foremost is intensity, the quantity of light falling on the eye’s photoreceptors during the day. Of importance here is light at the plane of the cornea or eye, not light falling on the horizontal workplane. Next is spectral power distribution (SPD)—the combination of light wavelengths the light source emits. Circadian regulation is most responsive to short-wavelength light (460 nm, or “blue”). (Note SPD and correlated color temperature, or CCT, roughly correlate but not exactly; request SPD data for a light source, not just CCT.) Meanwhile, longer-wavelength light (“red”) also can produce an alerting effect. Finally, timing, duration and photic history also are important—when the light is received, how much light cumulatively falls on the eye throughout the day, and previous light exposure.
Ideally, a circadian lighting solution exposes occupants to high-intensity light (at least 20–40 foot-candles, or fc, at eye level) in the morning, which can be enhanced with short-wavelength light. Daylight is ideal. Otherwise, electric direct-indirect general lighting, task lighting, luminous workstation partitions and wall lighting can increase vertical light levels. Controls can play an important role in adjusting light levels and spectrum throughout the day. (Meanwhile, as an aside, steps should be taken to minimize glare and objectionable flicker.)
Due to differences in human physiology, one’s mileage may vary. Studies tend to suggest outcomes for average populations under certain conditions. Lifestyle is a major determinant, along with evening and nighttime lighting. Overall, circadian health is a puzzle. For commercial buildings, designers may choose to ensure their piece is optimized.
“Architectural lighting isn’t just for vision anymore,” Figueiro said. “Clients are increasingly requesting and expecting lighting systems and applications that can support human health and wellbeing.”
She said, if a company wants to attract and retain top talent in today’s labor market, the office environment must be designed with employee needs and desires at the forefront. Beyond that, companies that care about the health and wellbeing of their workforce should consider healthy lighting.
“People are the most important asset of an organization,” Figueiro said. “Why not provide them with the best lighting? Providing occupants with proper circadian lighting is similar to providing them with ergonomic chairs or flat-screen computer monitors.”
For lighting professionals, this emerging trend poses several challenges. For starters, achieving circadian response with a vertical light level of 20–40 fc translates to roughly 80–120 horizontal fc, which prevailing commercial building energy codes do not support.
Tunable-white LED lighting can play an important role here. It can adjust spectral emission to emphasize short-wavelength light, which can increase circadian response by a factor of two to three. Figueiro found, when targeting 30 fc on the horizontal workplane, circadian stimulus required an SPD emitting more short-wavelength light (6,000 K correlated color temperature, or CCT). At 40 fc, it required an SPD emitting less short-wavelength light (4,500 K CCT).
“In situations where renovations may be impossible due to budgetary or architectural constraints, low-cost and low-impact light oases can be established,” Figueiro said. “Such oases can be quite effective when occupants are provided with information on light therapy and the health value of circadian stimulus, and can be tailored for limited spaces ranging from small offices to submarines.”
Lighting professionals also are seeking further research confirming positive outcomes for circadian lighting, along with tools and metrics they can use to evaluate, compare and implement solutions. The LRC recently published the results of an office circadian lighting research project and a circadian stimulus metric and predictive tool.
“Much has been learned over the past decade about the impact of light on circadian rhythms, and interest in the topic of light and health is certainly on the rise,” Figueiro said. “New metrics are now being proposed. But rather than wait until standard-setting bodies agree on new metrics or guidelines, lighting professionals can begin to apply current research to help people live better right now.”
CS metric and calculator
As a design objective, circadian light stimulation differs from achieving sufficient functional illumination for visual acuity and safety, requiring a new metric. The LRC developed the circadian stimulus (CS) metric as a proposed basis for evaluation, comparison and application of circadian lighting solutions. CS is based on an LRC model of how the retina converts light stimulation into neural signals, focusing on the quantity of circadian-effective light falling on the eye’s cornea.
As manufacturer claims about circadian benefits have begun to proliferate, proposed metrics such as CS can be very helpful. The CS metric has been successfully used in field applications, including people with Alzheimer’s disease and U.S. Navy submariners.
Exposure to a CS of 0.3+ at the eye for at least one hour in the early part of the day is considered effective for circadian stimulation. Circadian response activates at a CS of 0.1 and caps at a saturation point of 0.7.
“Although responses to circadian-effective light vary … , a lighting system that delivers a circadian stimulus greater than 0.3 during the day—particularly in the morning—and less than 0.1 in the evening is a great starting point,” Figueiro said.
Subsequently, the LRC released the CS calculator, a free tool designed to help lighting professionals choose light sources and levels ideal for circadian stimulus. The lighting professional establishes the base condition using the CS calculator and software such as AGi32. Various lighting strategies are then considered, which can be fine-tuned to achieve the right balance between circadian stimulus, IES recommendations, energy codes and owner requirements. Download the CS tool at http://bit.ly/2lD5X8G.
“The CS calculator enables a lighting professional to quickly and easily convert the photopic illuminance provided by any light source at any light level into the effectiveness of that light for stimulating the human circadian system,” Figueiro said. “It helps one compare effectiveness of various light sources for the circadian system.”
While the CS tool is helpful for designers, additional interest is coming from manufacturers. Some may publish CS values in their cut sheet. Others develop lighting control schemes around CS.
The LRC put its ideas to the test in a field study conducted at five office buildings managed by the U.S. General Services Administration (GSA). It found office workers receiving a substantial dose of circadian-effective light in the morning have better sleep and lower levels of depression and stress than workers who spend their mornings in low light levels. GSA intends to use this data to support its efforts in developing new healthy lighting practices at federal buildings.
The study used the Daysimeter, a device the LRC developed to measure the amount of circadian stimulus received throughout the day. The 109 study participants wore the device for seven consecutive days in summer and winter months, from 2014 to 2016. The LRC collected data during these periods covering sleep and mood, using five standard questionnaires. The participants also logged bedtimes and wake-up times. Sleep latency, quality of sleep and naps were calculated using Daysimeter data.
The LRC determined that participants receiving a morning electric or daylight CS of at least 0.3 displayed greater circadian entrainment than participants receiving a morning CS of 0.15 or lower. They were able to fall asleep faster at bedtime, particularly in winter. They also experienced higher-quality sleep. With no seasonal variation, they reported lower stress levels. Participants receiving lower CS reported taking about 45 minutes to fall asleep at bedtime.
“The results are a first step toward promoting the adoption of new, more meaningful metrics for field research, providing new ways to measure and quantify circadian-effective light,” Figueiro said.
Clearly, circadian lighting is a new field, and more work needs to be done in regard to daylight integration, surface characteristics, controls and understanding special populations. Standards and best practices need to be formulated. It’s important to recognize blue light is not the only answer and not to overstate results and benefits. Successful implementation requires an adequate budget and a high degree of design influence over general lighting, task lighting, controls and potentially daylight, furnishings and finishes.
That being said, Figueiro believes circadian lighting is actionable now and points to fresh research, metrics and tools as ways for lighting professionals to begin exploring opportunities with their projects. As interest in circadian lighting grows—and should that interest translate to owner demand and best practices—it may spark a revolution in lighting design. Lighting that not only provides visual and aesthetic benefits but also supports circadian regulation.
“We strongly encourage lighting professionals to seek opportunities that provide a deep understanding of the many ways light can affect health and wellbeing, and to become adept at addressing and designing lighting for special applications effectively,” she said.