Light doesn’t only serve as the basis of vision. It can influence behavior and how people feel. Further, light plays an important role in human health. As researchers gain insight into the relationship between light and health, the lighting industry is beginning to consider health effects in product and lighting design best practices.

Heart of the issue


At the core of one of light’s biggest effects on human health is its nonvisual effect on the body’s circadian system. The circadian system produces and regulates bodily functions based on 24-hour cycles, called circadian rhythms. Examples of circadian rhythms include sleep-wake cycles, core body temperature changes, and the release timing of hormones, such as melatonin. Disruption to circadian rhythms can lead to poor nighttime sleep and increased daytime napping as well as a 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 local position on Earth. The human eye contains various cells receptive to light. Together with rods and cones, recently discovered intrinsically photoreceptive retinal ganglion cells, which are connected directly to the brain’s master clock, convert light into neural signals for regulating the timing of the circadian system.


For millions of years, sunrise and sunset primarily regulated the human circadian system. Today, we use electrical light sources, which also stimulate our circadian system. Designed primarily for vision, electrical lighting can provide too much or too little illumination to properly drive the circadian system. This uncertainty in light exposure can cause circadian disruption.


As our understanding of the science increases, it also creates opportunities to enjoy the visual and aesthetic benefits of electrical lighting while designing lighting systems that are conducive to proper circadian regulation.


What we can do


Mariana G. Figueiro, professor and light and health program director at the Lighting Research Center, Rensselaer Polytechnic Institute, Troy, N.Y., has been studying the relationship between light and health for years. She said four primary characteristics influence light’s effect on the circadian system.


Intensity is the quantity of light the eye’s photoreceptors receive during the day. In application, we are talking about light levels falling on the eye.


Figueiro said this factor may be predominant in circadian response. She said that vertical light levels are important here, rather than the horizontal (workplane) light levels that are typically the focus of lighting design for visual performance. This would entail ensuring vertical surfaces, such as cubicle walls, are properly lighted. It is a shift in traditional practice.


Optimal light levels can be calculated, but generally, a light level of about 30–40 vertical foot-candles (which translates to roughly 80–120 horizontal foot-candles) is desirable in the morning. Providing these light levels is currently at odds with commercial building energy codes. In fact, light levels suitable for energy codes based on visibility are sometimes too low to activate the circadian system, which is a big problem, Figueiro said. Ideally, light level will be variable throughout the day through supplemental task lighting and controls.


In the evening, starting about two hours before bedtime, a much lower vertical light level of 1–2 foot-candles is desirable.


[SB]Spectrum is the wavelength (typically associated with related color perception) of the light. While visual acuity is most responsive to medium-wavelength light [around 555 nanometers (nm), or “green”], circadian regulation is most responsive to short-wavelength light (460 nm, or “blue”). As a result, there is greater interest in lamps with a very cool correlated color temperature (5,000K-plus) and color-tuning light-emitting diode (LED) luminaires.


However, while research suggests spectrum can increase circadian response by a factor of two, Figueiro said increasing light levels is a more effective way to entrain the circadian system during the day. Both spectrum and quantity of light must be considered.


Further, while short-wavelength light can trigger a strong circadian response, red light can increase daytime and nighttime alertness and affect production of hormones, such as cortisol. So, red wavelengths, in addition to blue, are important.


Timing is when the eye’s photoreceptors receive light. People who work during the day require different light-exposure timing than people who work at night. The same intensity and spectrum delivered in the morning will have a different effect on sleep time than if received in the evening. In short, morning light aids going to bed earlier, while evening light will delay the timing of sleep.


Duration is the amount of exposure time. Intensity is important, but the circadian system really responds to the cumulative quantity of light the eye’s photoreceptors receive during the day. In short, the circadian system doesn’t turn on a dime. It essentially sums up morning and evening light and uses the net result to either advance or delay the body’s internal clock.


“We need to know these characteristics before we can determine what constitutes healthy lighting for each of us,” Figueiro said.


What we know and what we don’t


We know that exposure to light and dark influences the timing of the circadian system with related health effects. The mechanics of optimizing light exposure for circadian health—intensity, spectrum, timing and duration—are coming into focus. As evidenced by studies, properly designed circadian lighting can produce positive outcomes, notably greater alertness and a reduction in feelings of sleepiness during the day and better sleep for people with Alzheimer’s disease.


However, there’s still a lot we don’t know. Common sense might suggest designing lighting to be supportive of circadian health may seem to be a positive goal, but current research does not conclusively link specific lighting design strategies to improved performance, sleep and overall health.


“I am comfortable saying that light during the day can increase alertness, but we still don’t know to what extent it can affect daytime performance,” Figueiro said, adding that this might be enough for some organizations.


Figueiro said that many studies were conducted under controlled laboratory conditions and produced average responses.


“We do not know much about individual responses,” she said. “And, we do not know the exact amount of light needed to affect circadian rhythms outside of laboratory conditions.”


In other words, not everyone who leads an active lifestyle with multiple kinds of light exposures may respond positively to a building’s circadian lighting solution. It depends on the person’s entire light exposure history, not just lighting in one building. Ideally, a building’s lighting design would take into account every photon to give each individual user access to their own optimal solution.


Meanwhile, nighttime light exposure is as important as daytime light exposure, though nighttime lighting is outside the control of a building’s workplace lighting designer. Exposure to a high intensity of short-wavelength light within a two-hour period before one’s normal bedtime—such as using a laptop or tablet in close proximity to one’s eyes—can delay sleep. Even if someone works and lives under ideal circadian lighting, lifestyle is a major determinant in how much sleep he or she gets.


As a result, lighting should not be regarded as a panacea for good sleep hygiene but, instead, as an important piece of the puzzle that facilitates rather than fixes.


“While we cannot state that everyone will sleep better, we think that a good portion of the population would be more entrained and, therefore, sleep better if lighting was better controlled during the day and at night,” Figueiro said. “Better sleep is closely associated with better performance and better health. Moreover, if anything, we will not have dingy, dark environments during the day.”


Therefore, getting from lighting to health remains a complex mechanism. Figueiro argues that circadian lighting is nonetheless actionable in building lighting applications.


“Don’t be afraid to try,” she said. “It may not help everyone, but if it helps only half of the people occupying the building, it is worth it.”


It also poses no risk of harm when delivered at the right time, Figueiro said.


“Taking walks outside is important,” she said. “How is implementing circadian lighting in a commercial building any different than exposing oneself to daylight during the day?”


Where we can start


While circadian lighting poses potential benefits for a variety of applications, ideal uses are controlled environments occupied 24/7 on predictable schedules, such as assisted living and nursing homes. This is because the lighting system can be used to adjust occupants’ light-dark pattern throughout the 24-hour cycle.


“Lighting for older adults, including Alzheimer’s disease patients, is the low-hanging fruit,” Figueiro said. “It is ready for prime time.”


Schools are another “killer app,” as she put it.


“But for it to be successful, we need to also inform the kids and their parents on what is the best lighting for the evening hours—what to do and what to avoid at home,” she said.


Figueiro emphasized that circadian lighting can be beneficial in many other applications.


“People are the most important assets of an organization,” she said. “Why not provide them with the best lighting pattern we can? Providing occupants with proper circadian lighting is similar to providing them with ergonomic chairs or flat-screen computer monitors. In addition, offices with daylight are more valuable and can be sold or rented for higher value. The same can be established for offices with good circadian lighting.”