How light at night could double your risk of cancer.

The World Health Organization listed ‘light at night’ as a possible carcinogen in 2007. That is an eye-opening statement for something that affects almost all of us. From streetlights to the lamp in the living room, from accent garden lighting to the glow of TV’s and cell phone… artificial light at night is truly ubiquitous.

An often stated fact is that 80% of people in North America cannot see the milky way at night.  What was more surprising to me was that the Milky Way was supposed to be visible!  Who knew?  Oh, wait – people with no light pollution know…

So how can light possibly be a carcinogen? Will turning on the TV or a light in the living room after dinner suddenly cause cancer?  Let me start with two recent, contradictory studies, and then I’ll get into the science of why I think that artificial light at night is a fundamental health problem. 

A few weeks ago a study was released that looked into breast cancer incidences and light at night in Israel.  The study used spectral imaging – a satellite mapping method that looks at the color spectrum of light – to investigate a link between wavelengths of light and breast cancer.  This was based on animal studies that showed that shorter wavelengths (the blue end of the light spectrum) were associated with increased breast cancer. The researchers factored in the socio-economic impact, age, ethnicity, and other environmental factors that increase the risk of breast cancer.  The study showed that rather than all light at night being associated with increased breast cancer risk, only areas with more light in the blue wavelengths were at an increased risk. [ref]

Not all studies agree that light at night impacts cancer risk, of course.  Another study published in February 2018 showed that exposure to light at age 20 didn’t increase the risk of breast cancer. I did see a few flaws in the study…  The participants were women in their mid-40’s who were asked to report on how much light they were exposed to at night when they were 20.[ref] I’m a woman in my mid-40’s, and I have no idea what my light exposure at night was when I was 20.  I probably would have said it was dark at night.  In fact, up until a year or so ago when I finally installed blackout curtains, I had no idea what sleeping in the true dark really felt like.

Studying light exposure at night is difficult and hard to prove causation for cancer. The majority of the world is exposed to more and more light at night. Ubiquitous. While hormone-related cancers have risen over the last fifty years,  a number of other things we are all exposed to – like endocrine disrupting chemicals – can also be linked to cancer.

Let me dig into the research a little more and explain why light at night is most likely a carcinogen, and then I’ll put it into perspective, showing how large the risk is so that you can decide if it is something you should actually do something about.

Melatonin and cancer:
Melatonin, which most people think of as ‘the sleep hormone’, rises at night and falls during the daytime. This is a circadian rhythm that is maintained in most animals, and it is governed by light hitting the retina of the eye. Even in nocturnal animals, melatonin will rise at night.

Melatonin, though, is not really a just sleep hormone. In addition to circadian signaling, recent research shows that melatonin acts as an antioxidant, helping to repair 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.

Two factors govern melatonin: light during the day and absence of light at night.

Melatonin levels are affected by the amount of light (specifically in the blue wavelengths) that you get during the daytime.  A June 2018 animal study on liver cancer found that increasing blue wavelengths (465-485) during the day increased nighttime melatonin levels by 7x compared to the animals kept under standard fluorescent lighting. The animals exposed to the blue-enriched light also had markedly reduced tumor growth. The daytime blue light exposure and increased nighttime melatonin decreased the Warburg Effect, which is the shift to glycolytic metabolisms that cancer cells exhibit.[ref] Other studies have shown similar increases in melatonin and decreases in tumors (prostate, oral, breast).[ref][ref][ref]

The rise in melatonin at night is governed by the lessening of the blue light hitting our eyes in the evening hours. As the sun heads towards the horizon, we get more of the red end of the light spectrum. The golden hour.  In studies, the rising of melatonin levels is referred to as ‘dim light melatonin onset’.

While suppressing melatonin with lots of blue light during the day is good, at night, we need melatonin levels to rise so that our body can clear out bad cells and fight off cancer.

Studies over the last twenty plus years have made it clear that light at night (dim or bright) causes a decrease in melatonin levels, and animal studies show without much doubt that the decreased nighttime melatonin levels increase the risk of certain types of cancer. [ref][ref][ref][ref][ref]

Human studies for cancer are based on looking at the environmental factors (in this case either light at night or lack of light during the day) and then correlating them with an increase in risk. We, of course, can’t do human trials to intentionally test a condition to cause cancer, so there always seems to be a little wiggle room to be hopeful and say “maybe light doesn’t affect humans like it does all other mammals”. The overwhelming evidence of a link between light at night and cancer, though, really means that we need to pull our heads out of the sand on this topic and take a real look at the impact on our health.

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]   while another 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] But not all studies show such a large risk, with one estimating only a 7 – 21% increase in risk. [ref]

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 have your eyes closed when you sleep, right.

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 blue wavelengths through their closed eyelids had an effect on melatonin – actually shifting 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]   Most studies on dim light at night use 5 lux, which would be about the amount of light from having a nightlight 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).

Beyond melatonin:
In addition to affecting melatonin levels, light at night also increases cancer risk through 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 (cancer preventative) and increases stress hormone levels (cancer-causing).

Quantifying the risk:
So after all of the studies (and there are hundreds more than I’ve referenced here), what is the consensus for the cancer risk from light at night?  The World Health Organization and the American Medical Association both place light at night as a probable carcinogen.  “Probable”, though, is a word with wiggle room.

The problem with quantifying the impact of light at night is that it is pretty much everywhere.  There are a few darker spots left, though, around the world. A  study looking at artificial light levels in protected or natural areas (such as forests, conservation areas, etc)  in 158 different countries and compared the cancer incidences to areas with high light. The results of the study, when all confounders were taken into account, showed that colorectal, prostate and breast cancer risk increased by up to 35% with light at night. [ref]

What does a 35% or 79% or 2-fold increase (depending on the study!) in breast cancer risk really mean? The lifetime risk in the US of breast cancer for women is 1 in 8, with the risk increasing with age. [ref]  So a 35% increase is going to change that risk to about 1 in 6, while a 2-fold increase would increase the lifetime risk to 1 in 4.  Putting this into perspective against other known breast cancer carcinogens: hormone replacement therapy increases the risk of breast cancer by 75% (~1 in 5) [ref]; BRCA1/2 gene mutations can increase risk of breast cancer to about 1 in 2.[ref]  So light at night is not as risky as carrying the BRCA mutations, but possibly as risky or riskier than hormone replacement therapy.

Why is no one else talking about this?
There are a few articles here and there in the mainstream media on the topic of cancer and light at night, mostly when a new study comes out. And there are a few health gurus that have started talking about the impact of light at night.  The problem is that the topic gets lost in the swirl…

This topic isn’t sexy or exciting, and, quite frankly, blocking blue light at night is inconvenient. No one can make a profit by telling people to turn off their lights and go to bed.  No one wants to listen to that – sounds like just some hippie-dippie wacko stuff.  But there is more research on this topic than pretty much anything else that I’ve blogged about. The science is real. And it is time to take it seriously.


Simplest: get as much light as possible during the day.  Go outside in the morning when you get up.  Have your cuppa tea or coffee outdoors.  A recent study also showed that blue light emitting bulbs during the day help increase the production of melatonin at night. [ref]

Also simple, but sometimes kind of geeky/not cool: wear blue-blocking glasses in the evening starting a couple of hours before bedtime. Many studies show that wearing blue-blocking glasses in the evening increase melatonin production and sleep quality and quantity. [ref][ref]

An alternative to blue blocking glasses: shut off all your lights at night and go back to using candlelight.  Yeah — probably not realistic for most.  So get yourself some blue blocking glasses and join the dorky glasses club.

Curtains: Get some curtains and sleep in the dark. True dark is needed, so get blackout curtains or perhaps curtains over blackout shades.  Also get rid of all the little lights in your bedroom from LED indicator lights.  A little bit of black electrical tape will block them out.

A cell study found that curcumin in combination with melatonin killed bladder cancer cells.  So perhaps taking curcumin at night before bed would increase its cancer prevention ability.[ref]

Resveratrol, in combo with melatonin, was somewhat effective in reducing tumors in rats. Taking resveratrol at night may boost its benefits.[ref] Resveratrol and melatonin both boost sirtuin 1, an enzyme vital to cellular function and longevity.[ref]

Melatonin pills:
If melatonin is so great, why not just take a pill instead of producing it yourself naturally? Melatonin as an adjuvant to chemotherapy has been shown to be helpful and something to talk with your oncologist about if you are currently doing chemo.[ref][ref]  The problem with just popping a melatonin pill for the rest of us is that a big single dose isn’t what your body needs/expects. The natural rise and fall of melatonin that your body produces without light disruption is just plainly better, so your better option is to block out the blue light. Additionally, people react to the hormone differently, and your natural production of melatonin may decrease with supplementation.

Food sources of melatonin:
Almost all plants contain small amounts of melatonin which acts as an antioxidant in the plant. Pineapple, oranges, and bananas all have been shown to significantly increase serum melatonin levels.[ref] The flip side of this is that you really shouldn’t eat at night due to a circadian drop in insulin sensitivity.

Author Information:   Debbie Moon
Debbie Moon is the founder of Genetic Lifehacks. She holds a Master of Science in Biological Sciences from Clemson University. Debbie is a science communicator who is passionate about explaining evidence-based health information. Her goal with Genetic Lifehacks is to bridge the gap between scientific research and the lay person's ability to utilize that information. To contact Debbie, visit the contact page.