Since the advent of the light bulb, we’ve been able to brighten our world with the flick of a switch. And what a breakthrough that has been: Artificial light has given us more time to read and learn, work and play. But the ability to keep lights burning 24/7 has also come with a dark side.
Our body’s natural rhythms are connected to and affected by the natural cycles of light and dark. For millennia, humans stayed awake when the sun was up (to hunt for food and protect themselves from enemies), and slept when it set. So, today, while artificially extending our exposure to “daylight” means we can accomplish more, it also puts us out of sync with natural cycles of light and dark — the very cycles the human body got used to during centuries without manmade sources of illumination.
“Our bodies evolved to expect 12 hours of light and 12 hours of dark,” says University of Connecticut epidemiologist Richard Stevens, PhD. “Since the advent of electricity, we haven’t been exposed to enough darkness.” Meanwhile, by spending most of our waking hours indoors and away from natural sunlight, we’re also often underexposed to the wavelengths of light that stimulate our natural rhythms.
That might not seem like such a big deal, but research has shown otherwise. A study published in the December 2005 issue of Cancer Research showed that open-eye exposure to artificial light at night stimulated the growth of breast tumors. Stevens and colleagues first observed the relationship between breast cancer and aperiodic (irregular) exposure to natural cycles of light and dark in 2001, when a study he conducted found that the incidence of breast cancer in night-shift nurses was 30 percent higher than in the general population. “Artificial light erodes the night,” he says, and that has long-term health consequences.
Stevens and Itai Kloog, an environmental epidemiologist at the University of Haifa in Israel, also found a correlation between excessive environmental-light exposure and prostate cancer in men. Their study, published in the January 2009 issue of Chronobiology International, looked at nighttime global-satellite images of 164 countries and noted that the incidence of prostate cancer was nearly double the average rate in areas exposed to the most light at night (LAN).
Evidence linking LAN with certain cancers is now considered so conclusive that the World Health Organization has classified night-shift work as a probable carcinogen.
Studies also show that exposure to aperiodic light and disrupted natural rhythms can affect our sleep patterns, mood, weight and susceptibility to other “lifestyle” illnesses. And the damage can happen early. Researchers at Vanderbilt University, in Nashville, Tenn., have found that excessive exposure to bright light early in life — such as in neonatal intensive care units — may contribute to depression and other mood disorders in adults.
By better understanding our bodies’ natural rhythms and their relationship to natural cycles of light and dark, we can make more health-promoting and health-sustaining decisions. We can use what we know about our natural, biological rhythms to improve our sleep, bolster our mood, regulate our metabolism and reduce our risk for chronic diseases.
Next: Life Rhythms