The Light of Your Life: Building the Case for Circadian Lighting

By Craig DiLouie | Sep 15, 2018




Circadian lighting is a generic term used to describe lighting designs that support alignment of circadian rhythms to a day-active and night-sleeping pattern. The lighting approach offers the additional potential to increase alertness during working hours. As interest in circadian lighting grows, the lighting industry is developing metrics and products while validating approaches.

This article provides a refresher on circadian lighting and focuses on three recent studies pointing to potential benefits.

Circadian lighting

The circadian system produces and regulates bodily functions based on 24-hour cycles, or circadian rhythms. One critical function is the timed release of melatonin, which tells the body it’s time to sleep. Disruption of this internal clock can lead to poor nighttime sleep and health disorders.

Until the invention of electric lighting, the daylight cycle regulated circadian rhythms. Today, many people spend most of their time indoors, so electric lighting plays a very important role. However, traditional lighting design focuses on vision and visual comfort, not circadian response.

For circadian response, the key factors are quantity of light falling on the eye’s photoreceptors during the day, requiring sufficient light level on a vertical visual plane; spectrum or wavelength of light; when light is received by the eye; and the amount of time of light exposure.

An approach to circadian electric lighting might involve significant vertical illumination produced by light reflected on walls or emitted by task lighting. It might then include automatic dimming that produces a higher light level in the morning and entrains circadian response by reducing light levels and changing spectrum from cooler to warmer light as the day goes on. Access to daylight is ideal.

Healthcare project

In August 2017, the U.S. Department of Energy (DOE) published a Gateway evaluation of a circadian lighting project at a hospital. The report documents the strategy and resulting circadian stimulus (CS) measurements. While it did not capture data showing realized health or behavioral effects, it nonetheless provides a useful demonstration for how circadian lighting can be implemented in a healthcare application.

Following renovation of two floors of an existing hospital wing, the new Swedish Medical Behavioral Health Unit (BHU) opened to serve patients struggling with mental health conditions. Designed by ZGF Architects, the BHU comprises 15,000 square feet and houses 22 beds.

Hospital patients can lose track of time and sense of a normal routine. The designers required the lighting be tunable in intensity and color output to imitate the daylight cycle. Specifically, a warm sunrise, brightening and cooling in appearance until noon, and then tapering off to lower light levels and a warmer appearance toward the evening.

The design team kept an existing outdoor space and large skylight in the common area. In corridors and the dining/activity space, they specified downlights that provide programmable dimming and color tuning.

The downlights’ correlated color temperature (CCT) changes in alignment with a daylight cycle, ranging from 6,000K midday to 2,400K at night. (CCT is affiliated with spectral output, which was the real tool being used.) Intensity dims from high levels during the day to lower levels at night, producing energy savings. The design goal was to achieve a 0.3+ in the morning and a <0.1 CS in the evening and night hours. Getting to the right range of CCTs and light levels required two rounds of commissioning.

The DOE’s Gateway report uncovered some interesting lessons from the project. First, circadian lighting can be challenging in that this field is still emerging, particularly in commissioning. It may also require higher light levels than currently recommended for various visual tasks, potentially presenting a tradeoff in energy savings.

Photo credit: Shutterstock / ImageFlow

Office study

The Lighting Research Center (LRC) at Rensselaer Polytechnic Institute in Troy, N.Y., recently published the results of a major study demonstrating that office workers receiving high levels of circadian-effective light can experience better sleep and lower levels of depression and stress.

The LRC has conducted significant research and produced resources for circadian lighting implementation, notably the CS metric and circadian lighting design templates at CS expresses conversion of light into circadian response, or nocturnal melatonin suppression. It has an effective scale of 0.1 CS (response threshold) to 0.7 CS (response saturation). For circadian entrainment, the LRC recommends an exposure of 0.3+ CS for at least one hour in the early part of the day.

In 2017, the LRC conducted a study sponsored by the U.S. General Services Administration (GSA). This study involved 109 workers at five GSA-managed federal office buildings.

The goal was to measure the amount of CS received throughout the day as measured by an LRC-developed device called a Daysimeter, which participants wore as a pendant. Data was recorded for seven consecutive days in summer and winter months from 2014 to 2016. Using questionnaires, the LRC collected additional data about sleep and mood.

The LRC found that participants receiving a morning CS of 0.3+ demonstrated greater circadian entrainment than those with a morning CS of 0.15 or lower. They were able to fall asleep faster at bedtime, particularly in winter; experienced higher-quality sleep; and reported lower stress levels.

The LRC also found that, even though the spaces were designed to maximize daylight availability, a significant number of occupants were not receiving enough CS during the day. Even in open offices with many large windows, it was found that season, cloud cover, desk orientation, and window shade position combined to reduce the amount of CS the workers were receiving.

Circadian task lighting study

In response to the GSA study findings, the LRC theorized supplemental task lighting could be used to ensure office workers receive enough CS during the day. LRC tested the theory by installing circadian-effective task lighting at the White River Junction Veterans Administration Medical Center in Vermont and the Turner-Fairbank Highway Research Center in the Washington, D.C., suburb of McLean, Va.—where new overhead lighting was also installed.

The researchers also installed the same solution at two U.S. embassies in Reykjavik, Iceland, and Riga, Latvia, which experience a dramatic reduction in daylight hours and extended darkness in winter.

For the two-day intervention, lighting manufacturer Cree donated the overhead lighting, and the LRC designed and built the desktop task lights. As with the previous study, participants wore Daysimeters, which recorded CS levels before and during the intervention. The 68 office workers involved also completed questionnaires on their sleep habits, stress and subjective feelings of vitality and energy.

The study’s results showed that the new lighting delivered significantly higher amounts of CS. Self-reported sleepiness scores dropped during the intervention days, with participants reporting feeling more vital, energetic and alert during that time. These trends were consistent across all four office buildings.

“These results demonstrated that lighting systems delivering a high level of CS, especially early in the workday, can reduce sleepiness in office workers while also improving their mood,” said Mariana G. Figueiro, LRC director. “In addition to light’s synchronizing effect, light can also have an acute alerting effect, similar to that provided by a cup of coffee, at any time of day or night.”

While the study did not measure evening light exposures, which can be just as important, Figueiro said the results confirmed findings from previous LRC studies. Significant levels of circadian-effective light delivered to the eye during the day better aligns circadian rhythms and increases alertness. These results justify a re-examination of current office lighting standards, which focus almost entirely on illumination for visibility and comfort.

Moving forward

Circadian lighting is a new and possibly transformative trend, with the potential to play a major role in delivering healthier workplaces. Research such as these LRC studies and demonstration projects like the DOE Gateway project are critical to putting these ideas to the test.

While circadian lighting need not be prohibitively difficult, it often requires a change in design thinking with its emphasis on vertical light levels, layered lighting, and flexibility, and it may be challenging to commission.

About The Author

DiLouie, L.C. is a journalist and educator specializing in the lighting industry. Learn more at and





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