Executive Summary 

Electric light enabled us to conquer the night, but it has come at a significant cost to human health. Over the past 20 years, excess blue-rich 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? Modern fluorescent and LED lighting provides too much blue-rich light exposure during the night, and too little blue-rich light exposure during the day. Therefore, it is important to design and specify effective circadian lighting which automatically switches between blue-rich daytime lighting to synchronize and strengthen circadian rhythms, and blue-free evening and nighttime lighting to prevent the harmful effects of light at night. This paper explains:

  1. The key metric for assessing circadian lighting is the blue content of the light entering the cornea of the eye, and not desktop illumination in lux or foot-candles or the color temperature (CCT) or CRI used in traditional lighting design.
  2. The specific wavelengths of blue light (“circadian blue”) that must be controlled to strengthen and protect circadian rhythms and good health are 438-493nm.
  3. The minimum threshold levels of blue content required for effective daytime lighting (sunrise to sunset) is 20 µW/cm2 of 438-493nm circadian blue light.
  4. The maximum safe threshold levels of blue content for evening and night lighting (sunset to sunrise) is 2 µW/cm2 of 438-493nm circadian blue light.

In practical terms, assuming typical IES lighting illumination standards of 300-500 lux at the tabletop, circadian lighting fixtures should emit less than 2% circadian blue content during the evening and night hours, and more than 20% circadian blue content during daytime hours. Lighting products which meet this standard for effective and safe circadian lighting at night can be recognized by the UL verification mark


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