A link between light at night and breast cancer is not yet proven, but there is a growing body of evidence for the correlation. A 2001 study from the Fred Hutchinson Cancer Research Center found that women in Seattle who work the graveyard shift face up to 60 percent increased risk of breast cancer. Other studies have shown a similar pattern. And the risk of breast cancer is up to five times higher in industrialized nations (where exposure to nighttime illumination is more common) than it is in undeveloped countries. About half of those breast cancers cannot be accounted for by conventional risk factors.
So what's going on?
Back in 1987, a researcher named Richard Stevens, then at Pacific Northwest Laboratories in Richland, Washington, hypothesized that even brief exposure to nighttime illumination suppressed the pineal gland's production of melatonin, a neurotransmitter with strong anti-cancer properties.
Recently, that hypothesis came closer to confirmation. In April, I had the privilege of listening to a presentation by David E. Blask of the Bassett Research Institute in Cooperstown, NY. In research published in 2005, he found that human breast-cancer tumors grafted to rats grew more quickly in the presence of melatonin-depleted blood. The growth was fueled by rapid uptake of linoleic acid, a proinflammatory fat that's abundant in processed food, especially fried snack foods.
So far, light at night has not been linked to other kinds of cancers, perhaps because melatonin suppression boosts estrogen production by the ovaries, which in turn can support cancerous cells in a woman's breast. In other words, diminished nighttime melatonin production may boost breast cancer risk by two different mechanisms: speeding cancer cells' linoleic acid uptake and boosting estrogen levels. But I would not be surprised if light at night is eventually correlated with other cancers as well.
What should we make of this? The clear message is that we ignore our evolutionary development at our peril. Proper regulation of hormones requires us to be bathed in full light for at least part of each day, and to experience a natural level of darkness (with no more illumination than is provided by the moon and stars) each night. It makes perfect sense to me that spending our days in relatively dark interiors and our nights under bright lights opens the way for disease.
The solutions are straightforward: try to get at least 30 minutes of full sun each day, and to sleep at night in a room that's as dark as you can make it. In the modern world, it is not always possible to match wake/sleep cycles with the sun, but the closer you can come to doing so, I think the better off you will be. Personally, I am usually in bed before 10 p.m., and tend to get up with the sunrise. If you need to get up at night to urinate, use the dimmest nightlight possible to avoid shutting down your pineal gland's production of melatonin. One fascinating tidbit from recent research is that blue-hued light tends to strongly suppress melatonin synthesis, while red lights have much less effect, so it might be prudent to switch to a red-hued nightlight in the bathroom. And while it's a good idea to avoid fried foods generally, make a particular effort to avoid eating them during late nights under bright lights. In other words, don't flip on the kitchen light and break into a bag of chips.
Andrew Weil, M.D.