By Martin Moore-Ede, M.D., Ph.D. 

Electric light enabled us to conquer the night, but it has come at a significant cost to human health. Over the past 20 years, light exposure at night, and too

little light during the day, has been linked to dozens of serious health disorders caused by circadian rhythm disruption, including sleep disorders, depression, obesity, diabetes, cardiovascular disease and several hormone sensitive cancers including breast and prostate cancer. Exposure to the wrong light at the wrong time is now a well-established public health hazard. 

But what is the right light? If we knew the precise light wavelengths that synchronize the human circadian clock, we would have the key to designing healthy circadian lighting for day and night applications. 

The circadian disruption underlying the health effects occurs when a special type of photoreceptor in the eye called “intrinsically photosensitive retinal ganglion cells” (ipRGCs) detect too much light at night, and too little in the day and communicate this to the suprachiasmatic nucleus (SCN) - the biological clock which regulates our circadian rhythms. 

However, 20 years of research has yielded conflicting evidence on the responsible wavelengths. While the ipRGCs contain the photopigment melanopsin which has a peak sensitivity at 479 nm in the blue part of the visible light spectrum, other studies of humans exposed to different color lights showed peak circadian sensitivity to shorter wavelengths in the 457-464 nm range, and have suggested a broad spectrum of violet, blue and green light may impact circadian rhythms. 

Circadian Blue Light: The narrow band of blue light (between 438-493nm) that synchronizes our circadian rhythms during the day and disrupts them at night 

Our investigation revealed these prior attempts to define the circadian spectral sensitivity curve, had relied on short exposures to mostly monochromatic (single color) lights under dark-adapted conditions: 

  • Some scientists used 30, 40, 60 or 90-min light exposures into the eyes of previously blindfolded dark-adapted human subjects and found a broad violet-blue-green sensitivity. 
  • Other groups used camera flashes of light onto dark-adapted isolated retina, or human melanopsin photopigment in tissue culture. 
Both these methodologies were problematic since neither reflected how humans interact with light in the real-world. We spend the majority of our waking hours in a light-adapted state, exposed to white polychromatic light sources such as LEDs, and fluorescents. 

Furthermore, by relying on short exposure times, these studies failed to account for the transient effects of light on the circadian system as we emerge from a dark-adapted state. During the first 1-2 hours after being fully dark-adapted, there is temporary sensitivity to violet (400-429-nm) and green (500-560-nm) light mediated by cones and rods that feed into the ipRGCs. 

To test the hypothesis that human circadian clocks in normal light-adapted conditions might have a narrower, predominantly blue sensitivity, we took advantage of the fact that white light can be created using many diverse combinations of color wavelengths. We used six different polychromatic white LED ceiling light fixtures with diverse spectral power distributions to illuminate a conference room. Thirty-four healthy men and women were exposed to the same typical 50 ft candle tabletop (540 lux) workplace light intensity from each of the LED lights on different 12- hour night shifts. The effect on total nocturnal melatonin, was used as a marker of light-induced circadian disruption, 

Despite the same subjects being exposed to the same light intensity for the same length of time, very different salivary melatonin responses were seen depending on the spectral power distribution of the light. Some of the LED lights greatly suppressed melatonin; others had little effect. 

This enabled us to identify a narrow human Circadian Potency spectral sensitivity curve with a peak at 477 nm and a full-width half-maximum of 438 to 493 nm which impacts circadian rhythms under normal lighting conditions. We are calling this band of light “Circadian Blue.” 

The 477nm peak color of Circadian Blue not only is very close to the 479nm peak sensitivity of human melanopsin, but also has some very interesting significance in nature: 

  • Circadian Blue is the color (474nm) of the clear blue sky 
  • Circadian Blue is the color (475 nm). which most deeply penetrates the ocean as all other wavelengths are absorbed by seawater before 200 meters depth, Circadian Blue has thus indicated whether it was day versus night for ocean life for billions of years. 
This discovery of Circadian Blue spectral sensitivity has permitted the development of spectrally engineered LED light sources to minimize circadian disruption and address the health risks of 

light exposure at night in our 24/7 society. By alternating between Circadian Blue rich white light during the day and LED lights devoid of Circadian Blue at night we can provide ourselves with good quality white light at night, while still maintaining the ancient signal that distinguishes night from day. 


Martin Moore-Ede M.D., Ph.D. 

For over 30 years, Dr. Moore-Ede has been a leading expert on circadian clocks. and was one of the first to define the challenges of living, working and sleeping in a 24 hour a day, 7-day a week world. As a professor at Harvard Medical School (1975 – 1998), he led the team that located the suprachiasmatic nucleus, the biological clock in the human brain that controls the timing of sleep and wake, and pioneered research on how the human body can safely adapt to working around the clock. Dr. Moore-Ede is Director of the Circadian Light Research Center and CEO of CIRCADIAN® ZircLight developing LED light fixtures and bulbs with medically-optimized light spectra. Dr. Moore-Ede graduated with a First-Class Honors degree in Physiology from the University of London, and received his medical degrees from Guy’s Hospital Medical School, and his Ph.D. in Physiology from Harvard University. He has published 10 books, and more than 170 scientific papers on circadian physiology and its applications to human health, safety and productivity.
Share this article


Circadian Light Logo