Jun 29, 2021

The authors make the case for a simpler approach that sharpens the distinction between day and night

By Robert Soler and Erica Voss

A Circadian PrescriptionOur biological rhythms are tied to light. However, living in the modern world, we spend more than 90% of our lives indoors, receiving too little light during the day and too much light at night. This has led to circadian desynchronization called social jet lag—a situation where our internal clock is out of sync with our social requirements. The near-term consequences of social jet lag are lack of energy during the day and poor sleep at night. But the long-term consequences can be much more severe, including metabolic disorders, cardiovascular disorders, cognitive impairments, detriments to mental health and even certain types of cancer.1

Lighting has the potential to play a significant role in creating healthier spaces and is increasingly a topic of interest for architects, lighting designers and engineers alike. While there is a lot of interest around the potential for circadian lighting to improve the quality of our indoor spaces, many believe circadian lighting requires complicated controls or complex calculations in order to meet existing circadian lighting thresholds. This is not necessarily true.

Another common misconception is that the benefits of circadian lighting are limited to 24-hour spaces or shift-work environments such as those for healthcare or memory care. What research actually tells us is that 87% of non-shift workers (that means average day-working people) have some level of circadian desynchrony and improper lighting is at the crux of the matter.2

In a nutshell, the problem is that indoor daytime light levels and nighttime light levels are too similar, causing circadian confusion. The best way to improve this situation is to provide a circadian daytime and nighttime lighting design. However, an even simpler approach can be used, which can still have a meaningful impact. By simply creating brighter days in “9-to-5” work spaces or in a home office, for example, designers can do a great deal to help occupants get a better night’s sleep.3

This article is designed to outline simple steps for applying circadian lighting, using straightforward strategies to achieve a meaningful circadian lighting design. Table 1 offers a comparison of circadian metrics, models and criteria. While each metric uses different terminology, Figure 1 clearly shows that they are in complete agreement at 3500K and warmer, with peak sensitivity between 485 nanometers to 490 nanometers. This makes 3500K with a peak in the blue region ~485-490 nanometers a very good starting point for daytime circadian lighting applications. A 490-nanometer peaked spectrum will reduce the criteria by 30% (i.e. at 3500K the vertical requirement from a standard LED is 24 footcandles. A 490-nanometer peaked 3500K would require 17 fc vertical).

Table 1. Comparing circadian metrics, models and criteria.
Circadian Strategy Simplified. Table 1. Comparing circadian metrics, models and criteria


Figure 1.
Figure 1. Converts the melanopic lux and CS criteria into visual design footcandles for standard LED sources. Here we see that the WELL criteria and the CS criteria are identical for 3500K and warmer. For cooler light sources, the CS criteria requires significantly more light.


Figure 2.
Figure 2. Diagram showing the difference between vertical and horizontal calculations.


Figure 3.
Figure 3. Elevation view of calculation plane for horizontal lux and vertical lux in AGi32.

Daytime Approach. The following steps outline a simple methodology for setting up circadian lighting calculations using AGi32:

  1. Determine which health and wellness daytime lighting criteria you are trying to meet.
    • This example focuses on the WELL Building Standard and an EML target of 150 for spaces that rely only on electric lighting. For reference, 150 EML equates to 135 melanopic EDI.
  2. Set Up AGi32 model and use vertical calculation planes.
    • Circadian light levels must be measured in the vertical plane at the observer’s eye (~18 in. above the task plane) or typically 48 in. above finished floor (Figure 2).
    • Vertical calc planes in AGi32 are similar to standard horizontal calc planes, the only difference is the orientation of the light meter (Figure 3).
    • For spaces where the viewing direction of the occupant is unknown (ie. break room, lounge, dining room), vertical light level measurements should be taken in four different directions (0 deg, 90 deg, 180 deg, 270 deg) and averaged to gain a clear understanding of the circadian lighting impact (Figure 4).
  3. Determine melanopic (m/p) ratio for light source needed to achieve target EML.
    • Run lighting calculations to obtain average vertical light level in lux (example: 180 vertical lux).
    • Determine the required m/p ratio needed to achieve circadian target of 150 EML using this equation [Target EML = (m/p) * Vertical Lux]
      • 150 EML = (m/p) * 180 vertical lux
      • m/p = 150 EML / 180 vertical lux = 0.83
      • m/p = 0.83 needed

Last, we offer a few tips for considering light spectrum. In general, white LEDs with 80 CRI have a low m/p ratio and consequently offer poor daytime circadian spectrum. Simply selecting LED sources with 90 CRI does not improve outcomes much and only boosts the melanopic (m/p) ratio by about 5-10%. Selecting LED sources which have been “spectrally optimized” is where you will get the biggest bang for your buck, boosting the melanopic (m/p) ratio by as much as 50% while also maintaining color quality. Using an LED source designed with both the circadian and visual
system in mind allows for effective circadian lighting design without compromising aesthetic.

Figure 4.
Figure 4. Plan view of calculation planes in four different directions (0 deg, 90 deg, 180 deg, 270 deg).

Nighttime Approach. While there are daytime circadian lighting thresholds, designing for nighttime can be more challenging because there is an established low-end threshold. The Lighting Research Center recommends a CS less than 0.1 and WELL requires an EML below 50. However, recent research 4 suggests that both of these values are too high.

Rather than focusing on a low-end threshold, designers can follow a more fundamental principle which has emerged from research studies—our brains need clear delineation between daytime light and nighttime light. If we use the WELL thresholds and provide 150 EML during the day and 50 EML at night, that is only a 3:1 ratio between our daytime and nighttime light signals. This is not a great enough contrast for day versus night delineation. What we should be striving for is a ratio of at least 10:1 (daytime to nighttime). Lighting designers will recognize this approach as thinking in layers of light for day and night.

Below are some holistic strategies for achieving a “darker” biological night, listed in order of priority.

  1. Intensity. Reducing light intensity is the simplest and best first step to reduce the impact of light at night. Dim down lighting at night, decrease the light levels as much as is acceptable while still meeting occupant needs.
  2. Spectrum. Use the warmest acceptable color temperature for nighttime based on your application. Interesting fact about LEDs: the 450-nanometer blue peak everyone cautions about is actually the most efficient blue light for our visual system. This means that LED already uses the least amount of blue light for a given color temperature. For light at night, spectral optimization is not really necessary and traditional LEDs with a low color temperature are a great way to reduce the impact of light a night. 5
  3. Direction. Research has shown that light coming from above the horizon has the strongest biological impact. 6 This means that for nighttime lighting, focus on placing light only where you need it, use task lamps and lower level lighting, and avoid lighting the ceiling and walls.

1 Sulli, G., Manoogian, E., Taub, P. R., & Panda, S. (2018). Training the Circadian Clock, Clocking the Drugs, and Drugging the Clock to Prevent, Manage, and Treat Chronic Diseases. Trends Pharmacol Sci., 39(9), 812–827. https://doi.org/10.1016/jtips.2018.07.003.
2 Roenneberg, T., & Merrow, M. (2016). Review The Circadian Clock and Human Health. Current Biology, 26(10), R432–R443. https://doi.org/10.1016/j.cub.2016.04.011
3 https://research.tue.nl/en/publications/cie-position-statementon-non-visual-effects-of-light-recommendin
4 Phillips, A. J. K., Vidafar, P., Burns, A. C., McGlashan, E. M., Anderson, C., Rajaratnam, S. M. W., … Cain, S. W. (2019). High sensitivity and interindividual variability in the response of the human circadian system to evening light. Proceedings of the National Academy of Sciences, 201901824. https://doi.org/10.1073/pnas.1901824116
5 6 Glickman, G., Hanifin, J. P., Rollag, M. D., Wang, J., Cooper, H., & Brainard, G. C. (2003). Inferior Retinal Light Exposure Is More Effective than Superior Retinal Exposure in Suppressing Melatonin in Humans, 18(1), 71–79. https://doi.org/10.1177/0748730402239678


Robert Soler

Robert Soler

Robert Soler is the vice president of BIOS... More info »
Erica Voss

Erica Voss

Erica Voss is the director of circadian design for BIOS where she works with specifiers, architects and engineers. She holds a Bachelor of Architecture from Rensselaer Polytechnic Institute and a master’s degree in lighting from the Lighting Research... More info »