• Humans evolved to take advantage of the day-night cycle 
  • Nighttime darkness triggers the release of hormones important to physical and mental health and development
  • Artificial light at night interferes with the body’s ability to repair & rejuvenate itself
  • Light from the blue-indigo-violet part of the spectrum is especially disruptive to body clock synchronization
  • Outdoor lighting should be kept as dim and as low color temperature as possible

Artificial Light At Night, LED Street Lighting, and Health

When we looked at safety we learned that, according to the nation’s lighting experts, we can choose the color of the light without sacrificing safety. What should we do with that freedom? Let’s talk about public health. 

The American Medical Association Weighs In

The American Medical Association (AMA) has issued two major policy statements regarding the public health effects of night lighting. The first, issued in 2012, was in response to evidence that artificial night lighting was having a significant adverse effect on public health and safety. The second, issued four years later, focused specifically on the evidence of serious public health and safety hazards associated with high color temperature commercial LED street lighting.

But someone showed me a study . . . 

Important medical questions are typically studied multiple times by different research teams in different locations and in different ways. Individual studies typically only tackle a small part of a larger question. Results of individual studies are often diverse and conflicting, owing to their small size, lack of participant diversity, and different study designs. Each of the AMA statements was based on a  meta-analysis: a rigorous and systematic survey of all the relevant scientific literature available at the time. Meta-analyses are able to reach conclusions that are stronger and more reliable than any individual study.

Why is the dark important?

Humans evolved to take advantage of the day-night cycle. Alert during the day, the body works to repair and rejuvenate itself at night.   

Our eyes sense the dark, triggering the release of hormones that change the way different body systems function. The brain shifts gears from outward-focused alertness to process memories and emotions; DNA damage in the brain and other body organs is repaired; the heart slows and blood pressure falls; the immune system recharges itself; etc. Sleep at night is part of the process by which the body focuses its energy on these redemptive functions. 

Light at night interferes with the body’s sense of time and its ability to repair itself and prepare for the day to come.

What makes high color temperature LED light special?

To answer this question we need to talk just a bit about the relationship between color temperature and light color in commercial LED street lights  

Color Temperature measures the Ratio of “Blue To Red”

If you pass light through a prism and you’ll get a rainbow: red to orange to yellow, green, blue, indigo, and violet. The intensity of the different colors in this continuum is another way of describing the character of the light. Scientist and engineers call the intensity of light at the different rainbow colors the light’s spectrum. Water droplets in the air act like prisms to create a rainbow. The intensity of the light along the rainbow is sunlight’s spectrum. 

The color temperature of light is describes the balance of light from the blue-indigo-violet part of the spectrum to the light from the red-orange-yellow part of the spectrum: a higher temperature means a greater fraction of the light is from in the blue-indigo-violet. 

The Body Synchronizes Itself To Blue Light

Light that enters the eye stimulates light-sensitive cells, which in turn report to the brain. Some of those cells report to a part of the brain – the visual cortex – where they are interpreted as vision. Other cells report to a different part of the brain, which is responsible for signaling the onset of night and synchronizing the different body clocks. These cells are very sensitive to blue-indigo light, and very insensitive to red-orange-yellow light. 

In other words, the brain recognizes day and night by the presence or absence of blue-indigo light.

Commercial LED Streetlights are excessively blue

On the right are examples of the different spectra  you would get if you passed light from commercial LED streetlights of different color temperature through a prism. For comparison the light from a corresponding incandescent lamp, of the same color temperature, is also shown.

Notice that as the color temperature increases the LED spectrum develops a bright blue-indigo, which is not present in the incandescent light spectrum. For this reason we say that light from commercial LED streetlights is excessively blue.

It happens that the location of the blue-violet peak in the commercial LED street lamp spectrum is at just that blue-indigo color where the cells that control the body’s clock are most sensitive.