Much attention has been paid to the vitamin-D boosting benefits of sunlight. But darkness, it seems, can also keep you healthy. A new study finds that women who live in places with bright illumination at night (think Times Square in New York) are more apt to develop breast cancer than those who can gaze up at the night sky and see a full array of stars. Other research suggests that rotating shift workers have higher rates of breast and prostate cancers, heart disease, diabetes, and depression. U.S. News called on experts to explain the findings and how people should respond to them.
Can nighttime exposure to light really increase the risk of breast cancer?
Although the latest research doesn't prove this, the link is certainly plausible. Animal studies have shown that having abnormally low levels of the hormone melatonin, which is typically most abundant at night, increases the rates at which cancers grow. "There are, though, still a lot of uncertainties," says melatonin researcher George Brainard, a professor of neurology at Jefferson Medical College in Philadelphia.
How could light at night lead to cancer?
The leading theory is that getting too much light at night disrupts the body's nocturnal release of melatonin into the bloodstream, causing a cascade of negative health effects like inflammation, tumor cell growth, and a suppressed immune system. The brain's pineal gland produces that hormone in the absence of light—but suppresses its production when light travels through the eyes. No one knows if there's an optimal level of melatonin or whether what matters is that the hormone's release comes at the same time each day, since it helps drive the body's circadian rhythm. "It could be a combination of all these factors," Brainard says. "We just don't know."
Should people start popping melatonin pills to prevent cancer?
"Nobody is suggesting that," Brainard emphasizes. "But there are appropriate times to take melatonin." Night shift workers might consider taking a supplement right before they go to sleep in the morning. Those who travel internationally can try it to reset their body's clock and avoid jet lag. "If I'm flying from Philadelphia to Paris," Brainard says, "I would take a dose in the evening right before takeoff to help me go to sleep earlier, and then when I arrive, I'd take it at the local bedtime." The appropriate dose ranges from 1 mg to 5 mg. Check with a doctor before taking it.
Should light exposure be minimized at night?
Experts think that's probably a good idea, though you certainly don't need to go to extremes, like wandering around a darkened house after sundown. You should, though, try to avoid bright lights, the kind you need for knitting, jigsaw puzzles, or other hobbies within three hours of bedtime, recommends Mark Rea, director of the Lighting Research Center at Rensselaer Polytechnic Institute in Troy, N.Y. And keep computer time at night to no more than 15 minutes. Sitting in the glare from the screen for prolonged periods of time could suppress the release of melatonin. On the other hand, he says, watching TV is fine since the amount of light exposure you get sitting several feet away is minimal. In the bedroom, use room-darkening shades or curtains to block out the glare from streetlights. Plus, you might want to leave on a night light in the bathroom for those 4 a.m. visits, since flipping on the main bathroom light for even a few minutes, Brainard says, could disrupt melatonin production. "I recommend using one with a red bulb," he adds, "since light at that wavelength has a lower intensity and is less likely to interfere with melatonin."
What about daytime exposure to light?
Natural daylight can be just as important as nighttime darkness in maintaining a normal circadian rhythm. Rea recommends getting outside for 15 minutes at the same time each day, preferably in the morning, so your body gets a clear signal that it's daytime, not night. "It's the regularity that's key," he says. Also, avoid sitting in a dimly lit office all day. Rea says that can confuse the body's circadian rhythm just as much as getting bright light at night.