Many people are attracted to jobs that include some night shifts, in part because those shifts tend to pay more and can make for a more flexible schedule. But research in recent years has raised concerns that rotating night shifts (in which a person works several or more night shifts a month in addition to day and evening shifts) may increase the risk of some cancers. In 2007, in fact, the World Health Organization’s International Agency for Research on Cancer went so far as to declare that shiftwork that disrupts a person’s circadian rhythm is “probably carcinogenic,” based on animal studies and small population studies in humans (mainly nurses and flight attendants).
Now a large 2017 study in the American Journal of Epidemiology has added some support to that notion. The researchers used population data from the long-running Nurses’ Health Study and Nurses’ Health Study II, which involved nearly 200,000 female registered nurses who answered detailed questionnaires every few years about their work and lifestyles. The investigators defined night shift work as three or more night shifts per month in addition to day or evening shifts.
In the Nurses’ Health Study II cohort, who were younger than the women in the initial Nurses’ Health Study, the researchers found that those who worked for 20 or more years on rotating night shifts were more likely to develop breast cancer, especially if they started the shift work early in adulthood. (Women in the Nurses’ Health Study II cohort who did less than 20 years of night shift work did not have an increased breast cancer risk—nor, interestingly, did women in the original Nurses’ Health Study who did such work for years.) Scientists have hypothesized that shift work may induce a combination of hormonal and melatonin changes that somehow fuel tumor growth.
Of course, nurses are a specific population group and not necessarily representative of the general public. And since this was an observational study, it cannot prove cause and effect. But if you currently work a rotating shift schedule, or are considering doing so, it may make sense to avoid doing it continuously over many years if possible—especially since earlier research has also linked night shift work to negative cardiovascular effects. That advice may be especially important if you’re a younger woman.
Also see What’s a Biological Clock?