I've written several times on the link between artificial light at night and breast cancer.  

While people usually think that it is 'interesting', most don't find the increased cancer risk a compelling reason to block out artificial light at night.

I think the apathy is because there are lots of headlines screaming at us all the time about how this or that will give us cancer.  We have become inured to the hype and no longer care, even when there is real science showing a real problem.

With the topic of light at night and cancer, a lot of people assume the association is probably based on something else – such as people who are in areas of more light (urban areas) are also exposed to some probably unknown toxin that is causing the cancer.  While this probably has a lot of you nodding and thinking, "yep, gotta be something other than light" - it turns out that most of the epidemiological studies are controlling for tons of variables including toxins and socioeconomic factors. The cancer researchers aren't dummies, and they also have thought about those other possible factors.

I'm going to dive into and explain a new study that directly looks at the probable cause of why light at night is linked to an increase in breast and prostate cancer.

Spoiler alert: Melatonin is the link.

Light in the blue wavelengths at night decreases melatonin production. Lots and lots of studies show this. Melatonin, which most people think of as the 'sleep hormone', is a powerful intercellular antioxidant, protecting cells from DNA damage.  

Animal studies in which the animal is grafted with human breast cancer cells in order to form tumors have shed a lot of light on this topic. Light at night has been shown to significantly accelerate tumor growth. (study) Melatonin, in addition to being an antioxidant, also regulates both the expression of the estrogen receptor in breast cancer cells as well as the enzymes involved in estrogen metabolism. One study on light at night (LAN) concludes:

Experimental evidence in rats and humans indicating that LAN-induced circadian disruption of the nocturnal melatonin signal activates human breast cancer growth, metabolism, and signaling provides the strongest mechanistic support, thus far, for population and ecological studies demonstrating elevated breast cancer risk in night shift workers and other individuals increasingly exposed to LAN.

If a lack of melatonin (due to light at night) is part of the cause of the cancer growth, can melatonin supplements block the tumor growth? Animal studies do show this to be true. Mice exposed to blue/white light at night had increased tumor growth and metastasis formation, along with decreased melatonin production, when compared to mice exposed to an incandescent yellow bulb. Treating mice with melatonin in their drinking water inhibited tumor growth and reduced metastasis, even under blue light exposure at night. (study)

Problem solved. We just need to add melatonin to our drinking water supply (along with the fluoride and chlorine).  But only at night. And just in the right amount. Plus we would need to drink it while we sleep.

So that won't work...

Let's go back to the study on light and cancer.  This new study broke down the 'light at night' into light from four different types of light bulbs: compact fluorescent, halogen, incandescent, and LED.  The mice were given human breast cancer cells so that tumors would form.  And half of them were also given supplemental melatonin.  The endpoints measured were tumor volume, number of metastasis, and survival rate.

Of the bulbs listed above, the incandescent bulb gave off the least amount of light in the blue wavelength. The halogen and LED bulb contained the most light in the blue wavelengths (which suppresses melatonin).   The tumor volume in the mice exposed to the incandescent light was quite a bit less than either of the other three lighting sources (~1800 vs ~3300 mm^3). The number of metastasis with the incandescent bulb was half that of the other three bulbs.  This all tracked with melatonin levels as well.

The mice exposed to the different light source that were also given supplemental melatonin all showed a decrease in tumor volume and metastasis relative to their counterparts in the same lighting group.  The group with incandescent lighting and melatonin had the best outcome, and the group with melatonin and halogen or CFL bulbs showed only a slight increase in tumor volume reduction and survival rate.

What this study didn't include was a control group that wasn't exposed to light at night.

Actual solution:
All of these studies on melatonin suppression due to light at night and cancer lead to the conclusion that we all need to shut off the lights at night.  Realistically, that isn't going to happen – even though astronomers and stargazers everywhere wish it would!

A decade ago, the government called for the phase-out of incandescent light bulbs because they used more electricity than CFL bulbs. That lead to all of us buying CFL bulbs - and more recently to buying LED bulbs.

Great, we are all saving electricity... But the decrease in melatonin is linked to not only breast and prostate cancer but also depression, obesity, Alzheimer's and heart disease.

On an individual basis, we can make changes and choices to solve this problem. Blue-blocking glasses will protect us individually from the excess blue light at night from TV/computer/cell phones. Switching out our light bulbs at home for either incandescent bulbs or LED bulbs that are orange will also help at night. We need lots of bright light and blue light during the day, so those 'daylight' LED bulbs are great for daytime use.  We just need to turn them off at night and switch on lamps with orange or red bulbs at night.

On a city-wide basis, we need to convert from the white/blue tinted LED streetlights to LED lights that don't give off any light in the blue wavelengths. Yep, these exist. And they are marketed as animal-friendly and sold for parks and other outdoor areas. We need to wise up – not only do the rest of the animals need the circadian-friendly LED street lights, but all of us humans do as well!

What can you do personally to help out your neighbors and friends?

  • Change out your outdoor light bulbs to be circadian friendly. You can get an LED orange bulb for less than $8 on Amazon. They are marketed as wildlife / turtle friendly.
  • Talk with your city planners and county councilman to promote switching over to circadian friendly streetlights in your area. It probably won't happen overnight, but raising awareness of the problem is the first step.
  • Also talk with local businesses or apartment complexes about changing out their white outdoor lighting for ones that are more circadian friendly.
  • Pass this article on through your social media accounts.