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 every person on Earth. From streetlights to the overhead lights in your living room, from accent garden lighting to the glow of TV’s and cell phone… artificial light at night is ubiquitous.

Light at night and cancer

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 – and 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 recent study investigated breast cancer incidences and light at night in Israel in a unique way.  The study used spectral imaging – a satellite mapping method that looks at the color spectrum of light – to investigate a link between the wavelengths of light and breast cancer. This is based on many previous animal studies showing that light in the shorter wavelengths (the blue end of the light spectrum) increases breast cancer incidence. While animal studies are easier since researcher can control for many variables, real-life human studies are more difficult. Thus, the researchers factored in all of the socio-economic impact, age, ethnicity, and other environmental factors that increase the risk of breast cancer.  The results showed that 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, and many people point out these negative studies as a way of casting doubt on the science. When you look at how the studies were done, not all of them approach the topic in a scientifically-valid manner.  Take for example a study published in February 2018 which showed that 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] (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. 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, which makes it hard to prove causation for cancer. The majority of the world is exposed to more and more light at night. Ubiquitous.

Let me explain some of the background research on how light at night can be a carcinogen.   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 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.

Two factors govern melatonin production overnight:

  • exposure to 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 caused a7-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 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 decreasing amount 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, this rise of melatonin in the evening is referred to as ‘dim light melatonin onset’.

While suppressing melatonin with lots of blue light during the day is good, the opposite needs to occur when the sun sets.  At night, we need melatonin levels to rise so that our body can clear out bad cells and fight off cancer (along with numerous other positive effects from melatonin at night).

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 that the decreased nighttime melatonin levels increase the risk of certain types of cancer. [ref][ref][ref][ref][ref]

What do human studies show?

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”.

In my opinion, 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 may wonder why dim light matters – light coming in from streetlights or urban skyglow- when you have your eyes shut at night. It turns out that light in the blue wavelengths passes through your eyelids.

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 eyelids had an effect on 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] 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).

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 (cancer preventative) and increases stress hormone levels (cancer-causing).

Quantifying the risk:
So after all of the studies (and there are hundreds more than are 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.

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]

The range of increased relative risk of breast cancer,  according to different studies, is anywhere from  35% to 79% to 200%.

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.

Let’s put this into perspective against other known breast cancer carcinogens: hormone replacement therapy increases the risk of breast cancer by 75% (~1 in 5) [ref]; BRCA 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.

This topic isn’t sexy or exciting, and, quite frankly, blocking blue light at night is inconvenient and boring. 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 an overwhelming amount of research on this topic.

The science is real, and it is time to take it seriously.


Simply go outside:
Get as much exposure to full-spectrum light as possible during the day.  Go outside in the morning when you get up.  Have your cup of 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]

Block blue light at night:
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 increases 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.

Block out dim light while you sleep:
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 ad-on to chemotherapy has been shown to be helpful in some situations. This is definitely 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. There are low-dose, timed-release melatonin supplements available now, so it may be something to consider in addition to blocking out light at night.

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]


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.