Night Shift’s Possible Link to Cancer

There’s a surprise waiting at the bottom of a review of 20 possible human carcinogens published in the October 2010 issue of the journal Environmental Health Perspectives. Number 20 on the list isn’t a chemical compound like the rest of its colleagues. It’s night shift work, which researchers are now intensively studying for its apparent link to breast cancer in women.

Hormone levels follow regular, 24-hour day-night cycles called circadian rhythms. Staying awake and working at night under bright artificial lights disrupts that cycle and in women, releases extra estrogen from the ovaries, which may increase their risk for breast cancer. Night shift work might also affect other cancers in which hormones play a role, including prostate cancer, according to the journal article.

The genes that control how healthy cells divide and how DNA repairs itself of any breaks or damage are also circadian. Healthy cells turn into cancer cells when their DNA gets damaged and they start dividing uncontrollably, so people who work at night may be more vulnerable to any other carcinogens they encounter, according to the review. Right now, the International Agency for Research on Cancer classifies shift work as a Group 2A agent, which means it’s “probably carcinogenic to humans,” one step below the known carcinogens in Group 1.

The thirty-three scientists who wrote the new review were trying to pinpoint which 2A agents best deserve more study, so they selected their 20 carcinogens based on how widespread they are and how strong the scientific evidence is against them—both excellent criteria. I think there’s another reason to put more research into shift work and some of the other possible cancer agents in the review. These are mostly agents that people encounter in the workplace, in jobs they often wouldn’t choose if they didn’t have to. I still remember my dad working night shifts at a printer factory when I was little, so he could take care of me during the day while my mom went to the local technical school to learn English. That was not long after my parents arrived in the U.S. from Vietnam; they couldn’t afford daycare. Potential carcinogens like shift work are risks people take when times are tough—I’d like to see that risk fully studied.