Why light at night increases the risk for cancer

The World Health Organization listed ‘light at night’ as a possible carcinogen in 2007.  Let’s let that sink in for a minute... On the same list of possible carcinogens that includes formaldehyde, aflatoxin, and the HPV virus is something as innocent as artificial light at night.[ref]

From streetlights and outdoor accent light to the glow of TV’s, lamps, and cell phone… artificial light at night is ubiquitous.

Light at night and cancer

How can light increase the risk of cancer?

Will turning on the TV or an overhead light after dinner suddenly cause you to sprout a tumor?

To approach this topic, let's start with the background science on the biological pathway through which light at night impacts cancer risk.

From there, we will move on to the epidemiological evidence and then into the genetic connections.

Finally, we'll wrap up with putting the risk into perspective, because - no, turning on the TV one night after dinner will not cause you to sprout tumors. Instead, this is a more complex picture involving a fundamental change to our society with the invention of electric lights.


Melatonin and cancer prevention:

Melatonin, which most people think of as 'the sleep hormone', rises at night and falls during the daytime. Maintained in almost all animals, this is a circadian rhythm governed by the light hitting the retina of the eye.

Melatonin, though, is not just a sleep hormone. In addition to circadian signaling, recent research shows that melatonin acts as an antioxidant within cells, helping to combat oxidative stress in our cells at night. While we sleep, our cells go into rest and repair mode, cleaning up the waste from the active period during the day. When cells are exposed to too much oxidative stress, it can lead to DNA damage.

There are two factors governing melatonin production overnight:

  • exposure to light during the day - and -
  • absence of light at night.

First, let's look at light exposure during the day:
Melatonin levels are affected by the amount of light (specifically in the blue wavelengths) that you get during the daytime.  An animal study showed that increasing blue wavelengths (465-485) during the day caused a 7-fold increase in nighttime melatonin levels compared to the animals kept under standard fluorescent lighting. The animals exposed to the blue-enriched light during the day also had markedly reduced tumor growth. The increased melatonin production leads to a decrease in the Warburg Effect, which is the shift to glycolytic metabolisms that cancer cells exhibit.[ref]

Other studies have shown similar results -  increased melatonin causing decreased tumors (prostate, oral, breast).[ref][ref][ref]

The second factor impacting overnight melatonin production is exposure to light at night:
The rise in melatonin at night is governed by the decreasing amount of blue light hitting our eyes in the evening hours. Researchers refer to this rise of melatonin in the evening as ‘dim light melatonin onset’.

While suppressing melatonin with lots of blue light during the day is good, the opposite needs to occur after the sun sets.  At night, we need melatonin levels to rise as much as possible.

Studies over the last twenty-plus years have made it clear that light at night (dim or bright) causes a decrease in melatonin levels.

Animal studies clearly show decreased nighttime melatonin levels increase the risk of certain types of cancer.[ref][ref][ref][ref][ref]

Stress hormones also play a role:

In addition to affecting melatonin levels, light at night also increases cancer risk through the activation of stress hormones.[ref]

The connection between an increased risk for hormonal cancers and salivary cortisol levels has been well established, and disruption to the normal circadian rhythm of cortisol is linked doubling the risk of death in breast cancer.[ref]

So there is a bit of a double whammy here: light at night decreases melatonin (a cancer preventative) and increases stress hormone levels (cancer-causing).


What do epidemiological studies show about cancer and light at night?

Epidemiological studies on cancer risk are often based on correlating environmental factors with an increase in risk. In most studies, researchers do their best to account for risk factors such as socioeconomics, exposure to pollutants, and ethnicity.

Let me start with two recent - and contradictory - studies on exposure to artificial light at night.

The first example is an Israeli study that investigated breast cancer incidences in relation to light at night in a unique way.  The study used spectral imaging – a satellite mapping method looking at the color spectrum of light – to investigate a link between the wavelengths of light at night and breast cancer. When looking at artificial light from a streetlight, the color of the light can change from the yellow glow of sodium-vapor light to the blue-white of LED lights. The researchers theorized that the light spectrum would matter based on animal studies showing light in the shorter wavelengths (blue light) increases breast cancer incidence. Additionally, the researchers factored in differences in socio-economics, age, ethnicity, and other environmental factors which could increase the risk of breast cancer.  The results showed light at night increased breast cancer risk, but only areas of the country with more light in the blue wavelengths at night.[ref]

Not all studies agree that light at night impacts cancer risk, of course. But when looking at epidemiological studies, you really need to examine whether the methods make sense. Take for example a study published in the British Journal of Cancer showing exposure to light at age 20 didn’t increase the risk of breast cancer. To come to this conclusion, researches asked women who were currently in their mid-40’s to report on how much light they were exposed to at night when they were 20.[ref] (Do you remember where all you lived in your 20s and how much light crept around the curtains at night?)

Studying light exposure at night is difficult, which makes it hard to prove causation for cancer. The majority of the world is exposed to more and more light at night, and measuring the impact of that exposure over years or decades is imprecise, to say the least.

One group of people exposed to bright light all night long, though, is night shift workers.

Night shift workers have an increased risk of breast cancer.

Evidence from studying shift workers (mainly nurses) showed varying results for the increased risk of breast cancer.

  • One large study found a 79% increase in breast cancer risk for women working the night shift for 20 years.[ref]
  • Another study look at the combined data from the Nurses Health Studies found that for women exposed at younger ages to night shift work (light at night) there was a more than doubled risk of breast cancer.[ref]
  • Not all studies show such a large risk, with one study estimating only a 7 – 21% increase in risk.[ref]
  • A meta-analysis combining data from 28 studies found the increase in relative risk from night shift work to be 19%.[ref]

Dim light at night and cancer:

What about the risk from general light at night (street lights, lights at home)?

It turns out that you don’t have to work the night shift to have an increased risk of cancer due to light at night.  A California study of over 100,000 women found a 34% increased risk of breast cancer for premenopausal women exposed to higher amounts of light at night.[ref]

There are quite a few smaller studies on breast cancer and light at night with similar findings to the larger ones – with a few interesting tidbits thrown in.

  • One study found a 51% increased risk with higher ambient light at night.[ref]  It also found that sleeping longer (thus more melatonin) cut the risk of breast cancer in half.
  • Closing the shutters at night (shutting out the streetlights) was also associated with a significant decrease in cancer risk.[ref]

Why does a dim light at night matter?

You may wonder why dim light matters when you have your eyes shut at night. It turns out that light in the blue wavelengths passes through your eyelids fairly well.

A study from a few years ago tested a light device to see if they could shift melatonin levels while the participants were asleep. The researchers used sleep masks with different colored led lights built into them; the lights turned on for two seconds every minute for an hour -- while the study participants slept! The results showed that exposure to blue wavelengths through their closed eyelid affected melatonin. It significantly shifted the time that melatonin onset began the following night.[ref]

How much light is too much?

The answer may surprise you…  Even 0.2 lux (way less than a nightlight) was found to affect cancer rates in rats.[ref] People are perhaps not as sensitive as rats, and most studies on dim light at night use 5 lux as a test amount. This would be about the amount of light from having a nightlight shining out in the hall near your bedroom. To put this in perspective, on a sunny day the outdoor illuminance can be as high as 120,000 lux, and a cloudy day is about 1,000 lux. Contrast this with a moon-lit night which ranges from .002 lux to .25 lux (quarter moon vs full moon).


Genetics, Circadian Rhythm, and Cancer:


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