What the Science Actually Says

By Dr. Martin Moore-Ede, MD, PhD. Co-discoverer of the suprachiasmatic nucleus, the master biological clock in the human brain. Founder of the Laboratory for Circadian Physiology at Harvard Medical School.

Women who work night shifts under bright electric light have higher rates of breast cancer than women who do not, and the World Health Organization has classified the kind of circadian disruption that night work causes as a probable human carcinogen. A key reason is a critical hormone. Light at night suppresses melatonin, and melatonin is one of the body’s natural defenses against the growth of cancer tumors. Take the melatonin away, night after night, and you remove part of that defense.

I want to walk you through what we actually know, carefully, because this is a subject whereboth the fear and the dismissal tend to run ahead of the evidence.

Why I take this personally

Years ago, my wife’s best friend, Teresa, died of breast cancer in her forties.

I still remember the wedge-pattern soles of her shoes, the distinctive track marks they pressed into our carpet, so that I always knew when she had been to visit. There is nothing more devastating to a family than a young mother lost this way. Children miss her loving presence through the years they need her most. It was watching tragic cases like Teresa’s, and asking why breast cancer was climbing in exactly the parts of the world that had the most electric light, that pointed me and many other scientists toward a question almost no one was asking.

The light, melatonin, cancer pathway

In 1987 an epidemiologist named Richard Stevens, whom everyone knew as Bugs, became the first person to suggest a link between electric light at night and breast cancer.

At the time it sounded, in his own framing, like a way-out idea. The leading explanations for rising breast cancer rates were things like diet, age at first childbirth, and racial differences, and one by one the evidence ruled them out. Stevens proposed something different, and he laid out a clear chain. Light at night suppresses the production of melatonin, the hormone made by the pineal gland after dark. Melatonin, in turn, has powerful effects in holding back the growth of cancer tumors. So when electric light switches on after sunset and melatonin falls, tumors that the body would otherwise have kept in check are free to grow.

In the decades since, a large body of work in both humans and animals has tested that pathway and held it up. The classic laboratory studies came from Professor David Blask and his colleagues, who showed in detail how light at night and the loss of melatonin let tumors grow faster.

What the human studies found

A causal pathway in the lab is one thing. Whether it shows up in real women’s lives is another, and that is the question the epidemiology had to answer.

The way you answer it is with large, controlled studies, comparing many women who have worked nights against many who never have, balanced for the other things that affect breast cancer risk, age, smoking, number of pregnancies. In 2001 the first large-scale studies of this kind were published. Dr. Johnni Hansen at the Danish Cancer Society was among those who found the effect was real and not just anecdotal. By 2007 enough evidence had accumulated that the WHO’s International Agency for Research on Cancer classified circadian disruption from night shift work as a probable human carcinogen. The Danish government went on to pay compensation to some night shift workers who developed cancer.

I want to be precise about the language here, because it matters. Probable carcinogen does not mean light at night is proven to cause every case, and night work does not doom anyone. It means the evidence is strong enough that serious scientific bodies treat the risk as real and act on it.

How large is the risk?

This is where care is most important, because the honest answer has a wide range.

Among night shift workers specifically, studies have found breast cancer rates roughly fifty percent higher than in comparable day workers. Looked at across the whole sweep of human history, the contrast is far larger: women who never encounter electric light very rarely develop breast cancer at all, while women regularly exposed to bright light after sunset have rates many times higher. The comparison I find hardest to dismiss is with tobacco. Just as only about a fifth of lung cancers occur in people who never smoked, breast cancer is uncommon in women with no exposure to electric light at night. Based on everything we know today, blue-rich electric light at night may turn out to be the tobacco of breast cancer.

What this means if you work nights

None of this is a reason for fear, and it is certainly not a reason to leave a job you need. It is a reason to take the light seriously.

If you work nights, the goal is to protect your melatonin where you can. That means blue- depleted lighting in the spaces where you work at night, rather than the bright, blue-rich white light most workplaces install. It means getting real daylight during your waking hours to keep your clock as strong as possible. And it means giving yourself genuine darkness when you do sleep, even though you are sleeping against the sun. These are not perfect fixes. Night work fights the body’s clock by its nature. But the difference between hostile lighting and protective lighting is real, and it is within reach.

The evidence took decades to assemble, and it is still being refined. But the direction has been clear for a long time. The lighting we have built our nights around was never tested for what it does to the body, and the people who carry the most risk are the ones keeping our hospitals, factories, and cities running while the rest of us sleep.

About the Author

Dr. Martin Moore-Ede, MD, PhD co-discovered the suprachiasmatic nucleus, the master circadian clock in the human brain, and founded the Laboratory for Circadian Physiology at Harvard Medical School. His research on circadian disruption and night shift work has informed health and safety practice across many of the world’s largest employers. He is one of 248 scientists who contributed to the Frontiers in Photonics consensus paper on the health risks of blue-rich LED lighting. For more on the science of light and health, visit circadianlight.org. His book The Light Doctor: Using Visible and Invisible Light to Boost Health, Improve Sleep, and Live Longer explores these questions in full.

Last reviewed: June 2026
This article is for general education and is not medical advice. If you have concerns about your personal cancer risk speak with your doctor.