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On Exposure to Light at Night

Posted by medconsumers on September 1, 2008

On Exposure to Light at Night, Cancer…and Rethinking Normal Sleep

There is no scientific consensus about the cause of breast cancer, according to Richard G. Stevens, PhD, Professor and Cancer Epidemiologist at the University of Connecticut Health Center, Farmington. That’s as true today as it was in 1987 when Dr. Stevens began to publish a new theory about exposure to electric light at night and how it may trigger disruptions in the body’s circadian rhythm—that is, the cyclical changes that recur regularly over a 24-hour period.

These disruptions, in turn, can cause hormonal disturbances implicated in the development of breast cancer. Central to this theory is melatonin, a hormone that helps regulate the sleeping/waking cycles. Melatonin is primarily produced by the pineal gland at night and is suppressed by exposure to light.

Dr. Stevens has co-authored studies showing an increased risk of breast cancer among women who work the graveyard shift. And he has co-authored another study showing a decreased risk among women who sleep nine hours or more each night. He also challenges the prevailing medical wisdom about normal sleep in the following interview conducted by Maryann Napoli, Center for Medical Consumers.

MN: In just 20 years, you went from proposing what might have seemed like a crackpot idea at the time to having it validated by the International Agency for Research on Cancer, based in Lyon, France. In 2007, that agency, which is part of the World Health Organization, declared shift work to be a “probable human carcinogen.” What made you initially suspect the link between breast cancer and exposure to electric lighting at night?

RGS: I’ve been interested in breast cancer for a very long time. It’s a mystery why it is so common in industrialized countries and why it becomes common as countries become industrialized. Up until the mid-1980s, we [researchers] thought it had to be due to the Western diet that is high in fat, but the really good, big cohort studies of diet that followed hundreds of thousands of women for 10-30 years – all came up negative. There is no relationship between fat consumption in adulthood and the risk of breast cancer.

MN: So you began thinking of other possible causes.

RGS: Yes, I was lying in bed awake in the middle of the night with the light from the street lamp coming into my room—and I’m not suggesting that streetlights alone are the problem. [It was so bright] I could almost read in my bedroom, and I thought this is something that has changed since society has become industrialized. I wondered if that had anything to do with cancer. Then I found out about what light can do and the hormones and the melatonin connection. Then I started publishing on the topic.

MN: You took off from the idea that we humans have evolved over the course of three billion years, but we have dramatically changed the environment only in the last 130 years with the introduction of electricity. How did you start to prove your theory?

RGS: The way we gather evidence is to make predictions. For example, if shift workers get lots of light at night and their circadian rhythms are disrupted, then, according to the theory, they should be at higher risk of breast cancer. Another prediction was that blind women would be at a lower risk [because their melatonin is not suppressed by exposure to light]. Another is the association between long sleep duration and a lower risk of breast cancer.

MN: You co-authored a study of shift workers that was published in the Journal of the National Cancer Institute in 2001. It showed an increased risk of breast cancer among women who worked long hours and many years in the graveyard shift, compared with those who didn’t do shift work. But it was only a modest increase in risk, wasn’t it?

RGS: That’s correct. But keep in mind that the comparison group, though it was comprised of women who didn’t do shift work, was getting plenty of the usual exposure to electric light at night. The strongest evidence is for the shift workers, but we don’t have a lock on it yet. There are also the four or five studies of breast cancer in blind women, and they all reported a lower risk of breast cancer as predicted. There are three good studies of sleep duration, two of which found that women who sleep a long time have a lower risk of breast cancer. So you get the idea—it looks as though the more time you spend in darkness, the lower the risk of breast cancer, although there is no scientific consensus on this. We need more studies consistent with those we have if we are to get to that point.

MN: What other ill effects might be associated with circadian disruption?

RGS: Three studies about prostate cancer were published in the last few years. Two showed that shift-working men were at a higher risk of prostate cancer. And another, just published in the British Journal of Cancer, was about sleep duration—the longer the sleep, the lower the risk of prostate cancer.

MN: What about interrupted sleep? People who wake up in the middle of the night will read or watch TV. Or they turn on the light briefly just to go to the bathroom.

RGS: There is evidence that those vanity lights in the bathroom [with light bulbs all around the mirror] are bright enough to start lowering the melatonin almost immediately.

MN: Is it OK to keep a night light in the bathroom?

RGS: We don’t know yet whether this is relevant to breast cancer, but just in terms of circadian health, the best thing to install in the bathroom is a night light with a red bulb.

MN: Why red?

RGS: The wavelength with the maximum ability to affect melatonin is blue. Red is much less efficient. You need a lot of red light to affect melatonin. So a dim red bulb will almost surely have no effect on your melatonin.

MN: My husband has a very bright digital alarm clock on his side of the bed. It’s irritating to me when he’s not in the bed to block the light.

RGS: What color is it?

MN: Green.

Throw it away and get a red one. We don’t know whether that green light is bright enough [to cause a problem], but if it irritates you, throw it away.

MN: So the point is to keep the bedroom as dark as possible.

RGS: You don’t have to be asleep for melatonin rhythm to be fine, but you do have to be in the dark.

MN: So, if you can’t sleep, you’re better off just staying in bed in the dark, perhaps listening to your radio or iPod?

RGS: I don’t think there are experts on this anymore. What we have been told by the sleep doctors for some time now is that you’re supposed to be getting eight straight hours of sleep. And if you wake up in the middle of the night, you should leave the bedroom and read or watch television. But that view runs counter to our evolution and is being seriously questioned by many sleep researchers.

MN: Why?

RGS: We have evolved in 12 hours of dark—a biphasic sleep—a first sleep and a second sleep. There’s an historian who points out that there are many references to the first sleep and the second sleep in medieval literature, in Shakespeare and in Chaucer. What it means is that it was normal—up to the invention of electricity—to go to sleep fairly close after the sunset, sleep for a few hours and then wake up and be awake for maybe an hour or two in the dark. (See Recommended Reading below).

MN: But isn’t this is a form of insomnia?

There is a growing group of sleep researchers who are challenging that idea. I think it’s fine to be awake in the middle of the night. Enjoy the period of quiet wakefulness. But if you do get up and turn on the lights, you are suppressing melatonin, you are disrupting circadian rhythms and that, to me, may be increasing your risk of breast cancer and perhaps other maladies.

MN: What about taking melatonin supplements?

RGS: People often say to me, “Melatonin is good for you because it fights cancer, so I’m going to take a melatonin tablet every day.” I would certainly not do that. It has become clear with experiments in people taking melatonin tablets in the evening that this will actually change circadian rhythm and that can’t be good.

MN: So in summary: If you have insomnia, you shouldn’t take melatonin supplements or get up to watch TV or read. You should just lie there in your bedroom that is as dark as possible. How do you think that advice will go over with the millions of pill-popping insomniacs out there?

RGS: Change your attitude. That period of quiet wakefulness in the middle of the night is a time to wander peacefully in your mind and spirit.

Recommended Reading:

“Acknowledging Preindustrial Pattern of Sleep May Revolutionize Approach to Sleep Dysfunction” by Walter A. Brown, MD, May 26, 2006, Applied Neurology.

Maryann Napoli, Center for Medical Consumers ©

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