We hear a lot about the effects of light on sleep. Light in the evening—especially light emitted by devices with screens—blocks secretion of the hormone melatonin, causing symptoms of insomnia. Low lighting during the day also delays melatonin onset and shortens the night. Shift work, where workers are routinely are exposed to light at night and must sleep during daylight hours, is so likely to disturb people’s sleep that the problem has its own name: Shift Work Sleep Disorder.
Not only do unnatural lighting conditions interfere with sleep, say epidemiologists Richard G. Stevens and Yong Zhu. More and more evidence suggests that artificial lighting is behind the uptick in modern diseases, the topic of their recent opinion piece in Philosophical Transactions of the Royal Society of London B.
“It’s a new analysis and synthesis of what we know up to now on the effect of lighting on our health,” said Stevens in a University of Connecticut press release. “We don’t know for certain, but there’s growing evidence that the long-term implications of this have ties to breast cancer, obesity, diabetes, and depression, and possibly other cancers.”
In the Beginning
Humans evolved in an environment where the sun was the sole light source influencing internal circadian rhythms. The rising and setting of the sun determined when they slept and ate; it also set the rhythm for fluctuations in body temperature, gene expression, and hormone production. Sunlight in the daytime and darkness at night kept these systems cycling on a 24-hour day.
But the artificial lighting we spend our time in now is dim and poorly timed. Too little light during the day and too much at night lead to the disruption of circadian rhythms, insomnia, and increased susceptibility to disease.
Artificial Lighting and Cancer
Research suggests there is a relationship between exposure to light at night and breast cancer. Several studies show that women who work at night are more susceptible than women who work during the day. Night work is associated with melatonin suppression and, in women, higher levels of circulating estradiol, a reproductive hormone linked to breast cancer. Changes in the lymphatic system have also been observed.
Population studies, too, suggest a connection between breast cancer and exposure to light at night. Researchers in two studies used ambient light as measured by satellite at night to compare breast cancer risk across communities. They found a correlation between ambient light levels and breast cancer. “This has been tested and confirmed within Israel,” Stevens and Zhu write, “and among 164 countries of the world.”
Studies in molecular epidemiology also point to a relationship between circadian disruption and breast cancer. Long-term exposure to shift work alters patterns of gene expression throughout the body in ways that may increase breast cancer risk. Shift work may be a causal factor in colon and prostate cancers as well.
Do What You Can to Protect Your Health
You may be able to reduce your risk of developing these and other light-related health problems:
- Get a healthy dose of sunlight during the day. If you can’t, buy a light box and use it.
- Cut down on screen time at night. If you have a choice between an e-reader and a book, go for the book. Incandescent light is better than the blue light emitted by screens.
- Use heavy curtains to block out all light sources when you’re sleeping.
- Use red light if you must have light at night. (See my suggestion for red nightlights.) Firelight is circadian friendly for the same reason: it contains an abundance of red wavelengths.
- Cut down on red meat to reduce the risk of colon cancer. Epidemiological studies show there is a correlation between eating red meat, which is high in iron, and developing the disease.