Craig’s Lighting Articles, Light + Health

Circadian Lighting 101

Below is my contribution to the June 2018 issue of tED Magazine, the official publication of the NAED. Reprinted with permission.

Circadian lighting describes lighting systems designed to support human health by stimulating the body’s circadian system. While a young trend, it is potentially transformative in how we design and use lighting.

Circadian health

The body’s circadian system regulates digestion, hormone release, the timing of alertness and sleepiness, and other bodily functions. It is synchronized with the 24-hour day by exposure to light and darkness. These 24-hour biological cycles are called circadian rhythms. Disruptions to the circadian system can lead to poor sleep and health problems.

In the early 2000s, scientists discovered a third type of light-sensitive cell in the human eye, believed related to circadian response and not vision. This discovery eventually led to an understanding that how we design lighting systems can affect circadian health.

Circadian lighting

For millions of years, sunrise and sunset synchronized the human body clock. Today, Americans on average spend most of their lives under electric lighting. Traditionally, this lighting was designed with an emphasis on light levels delivered on a horizontal workplane, typically with no variation in light level or wavelength (associated with color perception).

The problem is this approach may not deliver sufficient light to the eye itself. The key factors in circadian-effective lighting are amount of light falling on the eye, spectrum (wavelength), and timing and duration of exposure. Vertical illumination is considered the most important, with spectrum (shifting from cool to warm over the day) acting as an entrainer.


As circadian stimulation involves a non-visual response to light, it required a dedicated metric. Several metrics have been proposed, such as melanopic lux, melanopic content, and circadian stimulus (CS).

CS was developed by the Lighting Research Center (LRC) at Rensselaer Polytechnic Institute. This metric indicates how well a one-hour exposure to a light source producing a certain light level and wavelength of light stimulates the circadian system, based on its ability to suppress the hormone melatonin. It ranges from 0.1, the threshold for circadian activation, to 0.7, which represents saturation. The LRC found exposure to a 0.3+ CS is effective for stimulation. The LRC offers an online calculator enabling designers to produce a CS value for their designs, available at

Design templates

To provide design guidance, the LRC developed Lighting Patterns for Healthy Buildings (, sponsored by the Light and Health Alliance. This site contains baselines and model alternative designs for typical spaces in various building types, including schools, healthcare buildings, senior facilities, and office buildings. Each pattern presents lighting plans, renderings, and generic luminaire information.

As an example, consider a small (20×22 ft.) windowless classroom with an occupancy of the teacher plus 16 students. Pendant 32W T8 luminaires are mounted on a 9.5-ft. ceiling with a 2×4 ceiling grid. While delivering satisfactory task light levels, the lighting system produces a CS of 0.16, too low for good circadian system activation.

An alternate, circadian-friendly design might replace the fluorescents with dedicated LED pendant luminaires with supplemental LED wallwashers along one wall. In the morning, all lighting is at full output with a very cool correlated color temperature (CCT) of 6500K, producing an active CS of 0.42 (see Figure 1). In the afternoon, the light dims to 40 percent of full output while warming to 3000K, reducing CS to 0.15 (Figure 2). The system operates efficiently and produces satisfactory light levels while featuring sufficient vertical illumination and flexibility to support circadian stimulus.

Putting it to the test

The LRC recently concluded a study at a series of Federal office buildings and discovered that people working under circadian-effective lighting had better sleep and were more alert during working hours.

In an earlier study led by LRC Director Dr. Mariana Figueiro, the researchers measured light levels in five Federal buildings, which were designed to maximize daylight availability for their indoor workspaces. Though some spaces featured large and numerous windows, the LRC discovered workers were not receiving enough light, due to a number of factors such as seasonal variation and window shading. As a result, the LRC theorized that supplemental task lighting could be effective at boosting CS.

In a subsequent study, the LRC tested this theory by installing circadian-effective task lighting (designed by LRC and shown in Figure 3) in two Federal office buildings and two embassies. The embassies are located in Iceland and Latvia, which experience far fewer daylight hours during the winter. Daysimeters measured the amount of CS received by each of the study’s 68 participants before and then during the two-day intervention.

The new lighting was found to significantly improve CS, resulting in participants self-reporting less sleepiness and greater energy and alertness. The findings were consistent across all four buildings. According to Figueiro, the study confirmed previous LRC studies by demonstrating that high CS during the day better aligns circadian rhythms and increases alertness during the workday.

Healthier lighting?

Built on the latest scientific understanding of the relationship between light and circadian health, circadian lighting is a new and emerging trend in the lighting field. With the recent introduction of metrics and model designs, and with equipment readily available, it is increasingly actionable in commercial buildings. Research suggests it can influence circadian health and alertness during working hours. Implementation need not be complex, though it requires education, extra effort, and expanded design practices to better incorporate vertical light levels, layered lighting, and flexibility.

In this rendering of a small classroom, morning lighting is at full output with a very cool CCT of 6500K, producing an effective level of CS. Image courtesy of the Lighting Research Center.

The lighting in the afternoon in this small classroom dims to 40 percent of full output while warming to 3000K, reducing CS. Image courtesy of the Lighting Research Center.

LRC-designed task lights delivered sufficient light to the eye at least one hour per morning during a two-day study at four Federal office buildings, resulting in self-reported scores reflected improved mood and vitality. Image courtesy of the Lighting Research Center.

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Craig DiLouie


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