Night Lights

Night Lights © 2012 Clipart.com

Experts have long touted the importance of good outdoor lighting as a deterrent to crime—hence, parking lots that are lit as bright as day and glaring store marquees that are on all night. But lights that are too bright only waste electricity without increasing safety. And in this Science Update, you'll hear why bright nighttime lights could also be bad for women's health.


Cancer's heart of darkness. I'm Bob Hirshon and this is Science Update.

American women are five times more likely to develop breast cancer than women in non-industrialized countries. George Brainard, a neurologist at Thomas Jefferson University in Philadelphia, believes bright, night-time lighting may be one reason for this increased risk.

He says while much of the light that enters our eyes is used for vision, some is sent to a part of the brain that controls production of the hormone melatonin. Melatonin regulates our daily biological rhythms, but it's also been shown to halt the growth of breast cancer tissue.


There are cell culture studies. And certain lines of human breast cancer tissues have their growth actually stopped by the addition of melatonin to the culture medium.

Brainard says darkness signals the brain to make the hormone, and light stops it. So when bright light interrupts the normal night-time production of melatonin, the body loses one of its defenses against breast cancer.


There's a lot of street lighting, there's a lot of light glare, that may in fact be influencing human hormone production and ultimately be a risk factor. But I want to stress, this is a hypothesis, it needs further testing, and it is far from proven. But I think it's an interesting and important area to do studies on.

For the American Association for the Advancement of Science, I'm Bob Hirshon.

Making Sense of the Research

Modern humans live in a very unnatural environment. Many of the technologies that we've developed to help us with our daily lives also run against the grain of thousands of years of evolution. The drawbacks of our modern lifestyle range from exposure to pollutants that our ancestors never encountered, to the high rates of obesity, diabetes, and heart disease that have resulted from our virtually unlimited food supply and abundance of labor-saving devices.

Here, we see the surprising potential consequences of another unnatural aspect of 21st century life: the 24-hour day. For thousands of years, humans were limited in their activities by the availability of nighttime lighting. Although fires, candles, and oil lamps allowed our predecessors to continue some functions after dark, these light sources lacked the power and the universal access to erase the difference between night and day. As a result, for 99% of human history, people have generally wound down at night.

If you've ever been in New York's Times Square at midnight, you know how radically things have changed. But nighttime lighting isn't just a big-city issue, although it's more influential there. Even staying up late in your home with electric light bulbs burning is a big change from what your great-great-grandparents experienced.

Brainard presents evidence that all this lighting may be having serious, unintended consequences. His circumstantial case is based on four main pieces of evidence: (1) breast cancer rates in the U.S. are five times higher than in undeveloped countries, where nighttime light is minimal; (2) melatonin is known to be produced in dark periods, and melatonin production is inhibited by light; (3) melatonin slows the growth of breast cancer tissue in animals; and (4) melatonin has been shown to stop the growth of breast cancer tissue in the laboratory.

All this doesn't prove that nighttime lighting is responisible for our high breast cancer rates. And even if it does, many unrelated factors may play a role as well. However, this combination of observations does suggest that our night-owl culture makes it easier for breast tumors to grow. And there are certainly many other consequences to manipulating our biological rhythms—from sleep disorders to mental illnesses like depression—that we are only just beginning to understand.

Now try and answer these questions:

  1. What is melatonin? What is its relationship to light and darkness?
  2. What evidence suggests that nighttime lighting may contribute to breast cancer?
  3. Suppose you wanted to test Brainerd's hypothesis. Design a study that would set out to explore further the question that he raises. Keep in mind the ethical considerations that would go into any study in humans.
  4. What other consequences might disrupting our day-night cycle have? What other aspects of our modern lifestyle run counter to the way our bodies are designed?

For Educators

This detailed Melatonin and Sleep Fact Sheet is published by the National Sleep Foundation.

The International Dark-Sky Association is an organization devoted to curtailing "light pollution" and its consequences.

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