The Fabric of our Society

October 2023

Bright Days and Dark Nights: From Science to Application 

Mariana G. Figueiro, PhD
Professor and Director
Light and Health Research Center, Icahn School of Medicine at Mount Sinai

In The Art of Scientific Investigation, W. I. B. Beveridge describes three phases in the reception of an original idea. First, the idea is derided as untrue, impossible, or useless. With the passage of time, the once doubtful community acknowledges that there may be something to the idea, but dismisses it as impractical. In the third phase, the community finally recognizes that the idea holds water but now declares it to be derivative and anticipated by others.

The discovery and acceptance of the intrinsically photosensitive retinal ganglion cell (ipRGC) in the mammalian retina followed this pattern to a T. British neuroscientist Russell Foster was met with ridicule when presenting his seminal work showing that blind animals (with no functional rods and cones) were nonetheless capable of responding to light, as measured by wheel-running activity. In 2002, Brown University scientist David Berson found the missing piece of the puzzle: he identified the ipRGC in the mouse retina. What was once ridiculed by vision scientists had become established as fact.

The ipRGC and lighting research
Following Berson's discovery, a series of studies teased out the ipRGC's response to light, and the nature of its interaction with rods and cones. We now know that the ipRGC responds to short-wavelength (blue) light. Further, we also know that, though there are only a few thousand ipRGCs per retina, they respond to light directly and receive processed input from rods and cones. The ipRGCs combine this light information received in the back of the eyes and transmit these processed light signals to the biological clock in the brain. The early studies by Foster and Berson laid the foundation for a paradigm shift in how lighting is measured, manufactured, specified, and applied to promote circadian entrainment, health, and well-being.

Subsequent research fostered the development of the circadian stimulus (CS) metric to quantify light’s impact on the circadian system, proposed through our work at the Light and Health Research Center (previously the Lighting Research Center). In addition, we also developed a wearable personal sensor that is calibrated to measure CS in the field, called the Daysimeter. We have been working with the National Institutes of Health, the General Services Administration, the National Institute of Occupational Safety and Health, the Swedish Energy Agency, the US Navy, and the Office of Naval Research, among others, to refine the Daysimeter and quantify the impact of CS on various populations. Finally, we have developed the CS calculator and lighting patterns that serve as tools for designers to learn how to implement 24-hour lighting schemes. The goal is to promote healthy circadian entrainment in the built environment.

Despite these advances, the lighting industry still seems locked in Beveridge’s second phase. We have embraced the knowledge that lighting isn’t just for vision, but we seem unable to fully realize the final step of actually implementing 24-hour lighting solutions that deliver effective lighting for the human circadian system.

Incorporating circadian stimulus into lighting design practice
Designers need to think holistically and broaden their horizons beyond considerations of workplane illumination and tunable spectra. There are a host of other lighting characteristics that can be manipulated when designing for circadian-effective light.

1. There’s only so much you can do with white light spectra

• Don’t get bogged down in choosing metrics, which mostly just cover spectrum anyway. And don’t think that you can only accomplish your goals using expensive systems installed in the ceiling. Just think, Bright days and dark nights. "Bright days" translates to roughly 350–400 lx at eye level (vertical illuminance).
• Don’t think you need to use “blue-enriched” light bulbs everywhere. Any correlated color temperature will do! You will just need to adjust the light level to deliver circadian-effective light with warmer light sources.
• Light levels and spectra are important considerations, but don't be constrained by polychromatic white light, correlated color temperature, or ceiling-mounted luminaires. A desktop lamp providing 30 lx of pure 470 nm (blue) light at a user’s cornea, for example, will have a circadian impact equivalent to 400 lx of a 6500 K source or 550 lx of a 2700 K source. Tuning an ambient lighting system's spectrum to deliver bluish white light will save some energy, but the 470-nm desktop lamp will provide the user with more than 10 times the amount of CS.

2. Think beyond the ceiling

• Vertical illuminance (light at the cornea) and not just horizontal illuminance (light on the workplane) should be a major consideration in your design, because light reaching the back of your eyes  is what stimulates the circadian system. Light does not have to come from the ceiling. 
• Providing light at the cornea can be achieved by facing a window. If a window is unavailable or the view is undesirable, consider using an actual light therapy box or self-luminous LED panel. But be aware that most commercially available light therapy devices typically deliver as much as 10,000 lx and might not be suitable for all users.
• Consider filling the designed environment with layers of light. Ceiling luminaires can meet the required horizontal illuminance for visual performance and energy efficiency, but layers of light can more effectively and comfortably deliver circadian-effective light.
• When using only ceiling luminaires, ensure the light's distribution hits a vertical to horizontal illuminance ratio of at least 7:10. For example, if the target workplane illuminance is 300 lux, the resulting vertical illuminance would be 210 lux or more.

3. Timing matters

• Designers also need to specify not only when to deliver circadian-effective light, but when to deliver darkness. Lighting controls, even simple dimmers, are imperative. Way beyond color tuning for aesthetics, successful circadian-effective lighting systems deliver and remove circadian-effective light at the right times. Timing is a robust dimension we need to consider when designing lighting for the non-visual system.

We’ve come a long way and now is not the time to sit on the sidelines. As researchers continue to show light's effectiveness for improving sleep and promoting well-being, we are counting on designers to do what they do best: be bold, be creative.

This direct-indirect office lighting system is supplemented by self-luminous blue light panel workstation dividers for providing high CS during the day.

A "lighting oasis" breakroom with blue wallwashers supplementing 2X2 troffers provides high CS in spaces that might lack access to daylight during the day.

This lunchroom uses recessed blue wallwashers to supplement 2X2 troffers and daylight, providing high CS during morning hours.