Human-centric lighting (HCL), or circadian lighting, is all about capitalizing on the body’s response to natural light, saving energy and improving human health, productivity and well-being.

“Artificial light revolutionized our lives by allowing us to work and play at any time, increasing productivity—but for a healthy, efficient rhythm of life, we need the natural changes of daylight,” said Linda Conejo, business development manager for national accounts at lighting manufacturer LEDvance, Wilmington, Mass. “Daylight is one of the most important sources of energy for our biological system, and a lack of it can cause a serious disruption to our day/night rhythm and cause a degradation of our performance and overall well-being.”

Circadian rhythm is the body’s 24-hour cycle and is affected by light, Conejo said. In the evening, without any light stimulus, the body begins to produce melatonin, which helps induce sleep. As morning comes, melatonin levels drop and the body releases cortisol to program the body for daytime activities.

If a person receives too little light during the day, their melatonin levels fall too low and they will not sleep well at night, she said. As a result, they may be sluggish, tired, listless and less productive throughout the day. An HCL system compensates for this by adjusting lighting levels and colors indoors to mimic the outdoors more closely.

“This is a new way of thinking of lighting beyond just dimming fixtures,” Conejo said. “Human-centric lighting places the focus on the people using the space and their need for natural light. For example, a neutral or cool white color of light promotes activity, while a warm white color of light promotes relaxation.”

In an HCL system, there is a lighting control unit; a commissioning tool, such as an app; and luminaires that adjust to different color temperatures and illuminances. This simulates the natural course of daylight to positively support the circadian rhythm of humans, she said. In wireless systems, the contractor only needs to connect the control unit to a power supply and install the fixtures in the rooms so they can communicate with it.

In some cases, the HCL control unit can be preprogrammed with lighting profiles that the user can adjust depending on the time of day.

“With these types of controllers, the lighting profiles will even adjust automatically to your location and local time,” Conejo said. “For example, if you install an HCL system in a classroom and you have students writing or taking an exam, you can set the control unit to ‘boost.’”

Indeed, students in schools can achieve up to 14% higher scores under HCL lighting, according to the World Green Building Council. Also, employees in offices can perform up to 12% better, worker productivity can increase by up to 18% and retailers can achieve up to a 25% increase in sales.

“Human-centric solutions are for customers looking to improve spaces beyond just functional lighting to create a positive impact on the people using the spaces,” she said.

LED lighting has been refined to save energy over the past 10 years but, only recently, has the color rendering within lighting systems been optimized. The next stage emerging is increasing the efficiency of the spectrum for health, said Tom Jory, vice president of illumination marketing at Luminus Inc. The Sunnyvale, Calif., company makes the actual LED light source—a semiconductor chip packaged with phosphor blends—that is sold to lighting fixture manufacturers.

Behind the human eye’s rods and cones used for vision are intrinsically photosensitive retinal ganglion cells that detect cyan light and send signals to the brain to suppress melatonin during the day and release it at night when the sun has set, Jory said.

“If a person doesn’t see enough cyan light during the day, their body is going to be confused and may start sending the wrong signals,” he said. “So it’s really important during the day to have lighting with the right amount of cyan to keep awake and equally important at nighttime to make sure they are under light that is largely free of cyan content so their body can relax and go to sleep.”

Quantifying the effect

The melanopic/photopic ratio, also known as the M/P ratio or MR, is a metric to quantify lighting’s circadian effect, Jory said. Warm correlated color temperatures (CCTs) such as 2,200K tend to have low MR. They have less blue/cyan content and are ideal for nighttime relaxation and preparation for sleep. Cool CCTs such as 6,500K tend to have high MR. They have more blue/cyan content that suppresses melatonin and are ideal for daytime alertness and productivity in schools, factories and offices.

Lighting designers who follow the WELL Building Standard will specify melanopic lux in the vertical plane, not just the usual “500 lux on the desk” in the horizontal plane, he said. WELL suggests a minimum of 200 melanopic lux (lux multiplied by the MR of the LED spectrum) in the vertical plane during the day.

“The MR ratio in most office buildings is too low, which means that workers are not getting enough cyan content in the lights during the day,” Jory said. “Today’s 4,000K or 3,500K office lights don’t provide enough melanopic lux in the vertical plane to meet WELL, and adding more of those lights is not the solution.”

Lighting with Luminus’ Salud LEDs enables a lighting plan that meets the WELL Building Standard with high quality of light and without over-lighting, he said. In February 2022, Luminus introduced two-channel CCT tunable modules that are ideal for HCL applications—cool white 5,000K Salud with an MR of 0.95 and 90 CRI, and warm 2,200K with an MR of 0.33 and 90 CRI.

Terry McGowan, director of engineering at the American Lighting Association, was one of the earliest proponents of HCL lighting. McGowan is involved with the committee work of the Illuminating Engineering Society and the International Commission on Illumination. His lighting experience includes some 20 years with GE Lighting in Cleveland where he managed the GE Lighting Institute and then several years as manager of the Lighting Research Office of the Electric Power Research Institute, which included organizing the first international conference on light and human health in 2002.

“Now, almost two decades later, light and health has become an important factor in lighting designs and healthy building specifications such as the WELL Building Standard,” McGowan said. “Major medical institutions are also increasingly involved in lighting research.”

For example, Mount Sinai Hospital in New York City established the Light and Health Research Center last year as part of its Icahn School of Medicine.

“Healthy lighting can make people feel better, both physically and mentally,” he said. “It can also help people avoid problems that result from lack of quality sleep. Research now underway is looking at the role lighting can play for hospital patients recovering from surgery or diseases.”

The research center at Mount Sinai is currently working with members of the American Lighting Association to develop new luminaires, lamps and controls for residential and residential-like environments, such as home offices, retirement communities and nursing homes.

“It’s time to include light and health considerations in practical lighting applications,” McGowan said. “Lighting for circadian entrainment is a good place to start.”

The electrician’s role

Electrical contractors are an important part of applying this new technology, because they can control the intensity, position and direction of installed lighting, he said. Circadian lighting must vary according to the time of day, and luminaires must be located and directed so light reaches the eye with minimal glare, rather than just into a space or onto a surface where there is a visual task.

“It’s a change from suggesting and installing lighting products for lighting a desktop or kitchen counter,” he said.

Patrick Laidlaw, director of business development for integration at WAC Lighting in Port Washington, N.Y., agreed that lighting can improve health, citing studies conducted in long-term care facilities for seniors, in which the lighting was adjusted to be warmer at night and cooler during the day. Researchers found that dementia patients functioned better and experienced better sleep patterns.

“There is also the ability to have circadian lighting on demand, for when people have to be awake at 4 a.m. for work or asleep at noon,” Laidlaw said. “These lighting systems are typically tunable and allow warmer and cooler color temperatures to be adjusted to mimic sunset or sunrise, to help the body awaken more naturally.”

Within new home construction, tunable systems are currently installed in about 75% of the medium- to higher-end homes, but typically not in entry-level homes at this time, he said. Within commercial facilities, the adoption rate is about 25%, but it should be closer to 75% by 2030 or “maybe even sooner,” Laidlaw said.

Commercial buildings with circadian lighting have optional sensors built into the lighting fixtures, which can turn the color temperature up a bit after lunch to help workers feel more awake and productive, Laidlaw said. But when spaces are vacant, the sensors detect that and turn the lights down halfway or even off to save energy. Data from sensors can also communicate to building management systems to let managers know when and how often spaces are being used.

“We now know more about how lighting impacts the human body than we have ever known before in the history of mankind,” he said. “So the future of lighting is tunability—having the color temperature you want on demand.”

Most tunable circadian lighting systems operate with low-voltage control systems under DMX protocol, so some contractors are now considering low-voltage work or partnering with an integrator, Laidlaw said. “There’s no doubt the future growth of circadian lighting on demand is a great opportunity.”