Lifestyle and exercise

Does light at night pose a health risk?

"Britons should fit blackout blinds and ban electronic gadgets from the bedroom to avert the risk of diseases such as cancer," the Mail Online warns.

This alarmist advice is prompted by a review looking at the theory that electrical light at night disrupts our normal body block and could therefore pose a risk to our health.  

In the review, researchers looked at various studies, including research linking night-shift work with breast or colon cancer, and light levels in the bedroom being linked to depression and obesity.

As the authors of this review acknowledge, the main problem with this type of evidence is that much of it is circumstantial, and may be influenced by bias and confounding from other factors.

Another drawback is this study does not appear to be systematic. The researchers provide no methods for how they identified the studies they discuss, and we do not know that all relevant studies have been included.

This effectively makes the review an opinion piece, albeit with lashings of supporting evidence. This means there is the risk that the authors have cherry-picked evidence that backs up their claims, while ignoring research that doesn't fit in with their theories.

The potentially large public health impact of even a small increase in disease risk linked with light at night seems worthy of further study. But this study doesn't prove that light at night harms our health.

Regardless, getting a good night's sleep is important. Read more about how to have a restful night.

Where did the story come from?

This opinion piece was written by two researchers from the University of Connecticut and Yale University in the US, and was jointly funded by the two universities.

It was published in the peer-reviewed Philosophical Transactions B on an open-access basis, so it is free to read online or download as a PDF.

The Mail appears to have taken the study at face value, recommending that Britons need to use blackout blinds on their windows, and clearly hasn't considered some of the drawbacks of this particular piece of research.

Because this wasn't a systematic review, we can't be certain that the studies used to inform the authors' conclusions are representative of the literature on the subject, and could also be of questionable quality. 

What kind of research was this?

This was an evidence-informed opinion piece, or narrative review, where the researchers discussed the theory that electrical light, especially at night, disrupts our normal body block. They consider whether this poses a risk to our health.

This narrative discussion is referenced throughout, but no methods are provided. It does not appear to be a systematic review, where researchers search all the available evidence to identify studies related to the issue of the effects that electrical light may have on the body clock.

This means we do not know that all the relevant studies related to this issue have been identified. As such, this review must largely be considered to be an article outlining the researchers' opinions, as informed by the evidence they looked at.

What do the researchers discuss?

The researchers present sleep deprivation or disruption at night as a result of exposure to electrical light as being a burden of modern life.

While they say light at night has been linked to sleep disruption, "What has not been 'proven' is that electric light at night causally increases risk of cancer, or obesity, or diabetes, or depression."

They say these links are plausible given that disturbed sleep can have an effect on cellular processes and DNA repair. The problem, they say, is that much of the evidence linking disturbed sleep and light at night to these diseases is circumstantial. They then describe what this circumstantial evidence looks like.

What do they say about light at night and disease risk?

The researchers discuss the issue of light at night and disease risk, supported by various studies.

They first discuss studies that have linked night-shift work in women with an increased risk of breast cancer, thought to possibly be a result of the influence of melatonin on oestrogen levels.

Melatonin is a sleep hormone, while high oestrogen levels are linked to breast cancer development.

Similarly, a handful of studies have linked shift work or sleep disruption with bowel cancer in both sexes, and with prostate cancer in men, as discussed in our special report on shift working and health last year.

But the researchers fail to mention that these studies may be influenced by various confounders.

The International Agency for Research on Cancer (IARC) currently defines the confidence that something may cause cancer as:

  • 1 – human carcinogen
  • 2a – probable carcinogen
  • 2b – possible carcinogen
  • 3 – inadequate evidence
  • 4 – probably not a carcinogen

In 2007 the IARC classified shift work that involves circadian disruption as a class 2a probable carcinogen, putting it in a category alongside anabolic steroids, vinyl fluoride and mustard gas.

This categorisation was based on a "compelling animal model", but limited epidemiological studies, where signs were consistent with a causal relationship but probably influenced by bias and confounding.

The researchers then discuss other observational studies linking light level in the bedroom (either self-reported or measured) with depression and obesity risk.

They acknowledge a risk of bias and confounding in these studies, but say that, "If these reported associations are causal, then there would be obvious and easy interventions, such as to use black-out shades and elimination of all light sources in the bedroom, no matter how minute."

The researchers go on to present other small experimental studies where participants were exposed to different amounts of light at night. The effects on body chemicals were then measured, including the sleep chemical melatonin.

Some of the broad conclusions were:

  • blue light has the greatest effect upon sleep disruption; red the least
  • there is a dose-response relationship
  • light exposure during the day influences night-time sensitivity
  • individuals have different levels of sensitivity to light
  • even through closed eyelids, a very bright light can suppress melatonin levels

The researchers go on to discuss the possible effect light has on genes involved in the control of the body clock, and how these could potentially be linked to cancer.

How did the researchers interpret the results?

In response to their overall question of whether electrical light exposure at night is a risk factor for our health, the researchers say this "cannot yet be answered with assurance, but is important to ask".

They say that, "It must be stressed that there is ample evidence for the disruptive effect of electric light on physiology in short-term experiments in humans.

"There is some epidemiologic evidence on the long-term impact on disease, but this evidence is not yet adequate to render a verdict."

However, they stress this is "an urgent issue given the increasing pervasiveness of electric lighting in our built environment."


This opinion piece discusses the evidence related to whether exposure to electrical light night is a health risk.

Much of the article considers various experimental studies where small numbers of participants were exposed to different light levels at night, as well as observational studies reportedly linking night-shift work with cancer, including breast and colon cancer.

The researchers also identified some studies linking self-reported or measured light in the bedroom with depression and obesity.

But this study has two prominent limitations. It does not appear to have been a systematic review. No methods are provided, and we do not know whether the researchers have searched the entire global literature on the subject to identify all relevant studies.

We also do not know whether studies linking light at night with disease could have been preferentially discussed as examples, while other studies that did not find any links were either not identified or not discussed in this review.

As such, this review must largely be considered to be the opinion of the researchers as informed by the evidence they looked at.

The second limitation is the strength and quality of the evidence linking light exposure at night to disease.

Most of the experimental studies discussed, where people were exposed to different light levels at night, were very small (one included 12 people, another eight).

These results are specific to the small sample included. This means they may be heavily biased and confounded by the characteristics of the participants, and therefore not apply to wider populations.

The small sample size may also fail to identify any real differences because of a lack of statistical power.

And just measuring body chemicals after a couple of nights of artificially manipulated light levels may not give us reliable evidence of health effects that would be seen with longer-term patterns.

Much of the evidence looked at is also circumstantial and based on observational studies. Though the design and quality of these underlying studies was not examined as part of this appraisal, it is likely the studies may be subject to various sources of bias or confounding, making it difficult to establish direct cause and effect.

The IARC study reportedly classified shift work that involves circadian disruption as a probable carcinogen. But the organisation acknowledged that this is based on limited epidemiological studies that may have been influenced by bias and confounding factors.

Overall, the possible links between electrical light exposure at night and disease is definitely worthy of further study. But, for now, people should not be overly alarmed by these findings and feel the need to rush out to buy blackout blinds for their bedroom windows.

That said, creating a calming environment in your bedroom, free from visual and audio distractions, can enhance the quality of your sleep.

NHS Attribution