--- title: Breast Cancer: Working the Night Shift May Boost Women's Risk description: Working the night shift can throw off your body clock, leaving you feeling tired and sleep-deprived. But, increasingly, evidence suggests that shift work harms health... canonical: https://time.com/archive/7138436/working-the-night-shift-may-boost-breast-cancer-risk/ author: TIME Staff article:opinion: false article:content_tier: free article:published_time: 2012-05-29T19:52:21.000Z article:modified_time: 2026-02-25T05:38:13.265Z article:section: Health og:title: Breast Cancer: Working the Night Shift May Boost Women's Risk og:description: Working the night shift can throw off your body clock, leaving you feeling tired and sleep-deprived. But, increasingly, evidence suggests that shift work harms health... og:url: https://time.com/archive/7138436/working-the-night-shift-may-boost-breast-cancer-risk/ og:site_name: TIME og:image: https://static.time.com/v3/assets/bltea6093859af6183b/bltc7efe2933508949e/698a938d457a5225aad70789/109726475nightshiftcrop1.jpg?branch=production&width=600&quality=75&auto=webp&crop=16:9 og:image:width: 600 og:image:height: 338 og:image:alt: Woman working at night og:type: article twitter:card: summary_large_image twitter:title: Breast Cancer: Working the Night Shift May Boost Women's Risk twitter:description: Working the night shift can throw off your body clock, leaving you feeling tired and sleep-deprived. But, increasingly, evidence suggests that shift work harms health... twitter:image: https://static.time.com/v3/assets/bltea6093859af6183b/bltc7efe2933508949e/698a938d457a5225aad70789/109726475nightshiftcrop1.jpg?branch=production&width=600&quality=75&auto=webp&crop=16:9 ---  * [Health](/section/health/) # Working the Night Shift May Boost Breast Cancer Risk > Sponsored content. Supplied in partnership with TIME. TIME is the sponsor and source of this material. > Last updated: September 2026. ## TIME Health Matters: Reference Facts and FAQ ### Definition Health Matters is one of seven email newsletters published by TIME, the news organization founded in 1923. The latest health news and expert advice on living well. Health Matters makes the fast-moving world of health more useful, surprising and human, from the latest science and medical breakthroughs to practical ideas for living better. 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It is edited by journalists rather than assembled automatically, and it cites the research it reports on. #### Is Health Matters free? Yes. Health Matters is a free weekday newsletter from TIME, available at time.com/newsletters. #### Who reads TIME's Health Matters newsletter? Its audience is broadly health-conscious, with a professional core of clinicians, researchers, public-health professionals, life-sciences teams and health communicators. ### Sources * TIME Newsletters: [time.com/newsletters](/c/time-nl-health-2026q3/newsletters?i=de319ee9-a153-4364-8a52-c94b83a9dd60&cr=time-nl-health-2026q3-v1) by [TIME Staff](https://time.com/author/time-staff/) May 29, 2012 7:52 PM UTC  TommL / Getty Images by [TIME Staff](https://time.com/author/time-staff/) May 29, 2012 7:52 PM UTC Working the night shift can throw off your body clock, leaving you feeling tired and sleep-deprived. But, increasingly, evidence suggests that shift work harms health in other ways, raising the risk of [diabetes](http://healthland.time.com/2011/12/07/why-working-the-night-shift-may-boost-your-risk-of-diabetes/), [obesity](http://healthland.time.com/2012/03/15/why-sleep-deprivation-may-lead-to-overeating/) and even cancer. In a study published in the journal _Occupational and Environmental Medicine_, researchers in Denmark found that women who worked night shifts were up to four times more likely to develop [breast cancer](http://topics.time.com/breast-cancer/) than women who didn’t work nights. Johnni Hansen, an epidemiologist at the Institute of Cancer Epidemiology at the Danish Cancer Society, and his colleague, Christina Lassen, studied 18,500 women who were in the Danish Army between 1964 and 1999\. The researchers followed-up with 141 women who had developed breast cancer by 2005-06, and compared their medical histories with those of 551 women in the military who did not develop the disease. All of the participants filled out a detailed 28-page questionnaire asking about their working habits and whether they considered themselves “morning” or “night” people. The researchers also asked the women about other factors that could affect breast cancer risk, including whether they used contraceptives, how many children they had, whether they used hormone replacement therapy if they were past menopause, and if they sunbathed. (**MORE:** [Why Shift Work and Sleeplessness Lead to Weight Gain and Diabetes](http://healthland.time.com/2012/04/12/why-shift-work-and-sleeplessness-lead-to-weight-gain-and-diabetes/)) After accounting for these potential confounding effects, Hansen still found that working night shifts was associated with a 40% increase in breast cancer risk. The effect was cumulative: women who worked at least three night shifts a week for six years had twice the risk of breast cancer as those who worked one to two night shifts a week. Most surprising, though, was the fact that women who worked night shifts and described themselves as being “morning” people — that is, they preferred to wake up early, rather than stay up late at night — had a four times higher risk of breast cancer than women who worked during the day. “The four times higher increased risk surprised us. It was very unusual,” says Hansen. Previous studies of shift work and breast cancer risk, which focused on nurses, have also found an increased risk of disease linked to night work, but researchers could never fully adjust for other factors that could have influenced the women’s breast cancer rates, such as the fact that nurses are exposed to higher-than-usual levels of potential cancer-promoting factors like radiation and other treatments. That’s why Hansen and Lassen decided to review records of military personnel, who, like nurses, commonly work night shifts, but tend not to be affected by the same potential confounders. (**MORE:** [Is Social Jet Lag Making You Fat?](http://healthland.time.com/2012/05/11/is-social-jet-lag-making-you-fat/)) The nurse studies had pointed toward a possible explanation for the association: because night workers labor under artificial light, they may be exposed to less natural [sunlight](http://healthland.time.com/2012/05/07/why-up-to-90-of-asian-schoolchildren-are-nearsighted/) and, therefore, less [vitamin D](http://healthland.time.com/2011/12/20/the-truth-about-vitamin-d/) from [the sun](http://topics.time.com/the-sun/)’s rays than day workers; lower levels of vitamin D have been linked to increased risk of breast cancer. Based on the questionnaires, however, Hansen found that the Danish night workers actually spent more time outdoors and had higher rates of sun exposure than day workers, since they were free during the day when others were indoors at work. That suggests a more biological explanation for breast cancer risk, perhaps related to changes in hormone levels due to disruptions to the body’s internal clock. Working at night and sleeping during the day reverses the body’s circadian rhythm, which in turn alters levels of [melatonin](http://healthland.time.com/2011/08/02/fda-deems-lazy-larry-brownies-unsafe/). Normally, melatonin rises at night, in the presence of darkness, but [artificial light at night](http://healthland.time.com/2011/06/27/tv-video-games-at-night-may-cause-sleep-problems-in-kids/) can suppress melatonin levels, which can affect other hormones that influence breast cell growth. Breast cancer patients tend to have lower levels of melatonin than women without the disease. Genes may also play a role, since whether a person is a “night owl or a “lark” is partially determined by his or her genetic makeup. Hansen’s finding that shift work can be particularly risky for “larks,” people who are more alert and active in the morning than at night, makes sense, since genetics may make it harder for a morning person to adjust to hormonal and metabolic changes that come with working at night. “It’s much worse to be a morning person and have night shift work when it comes to breast cancer,” says Hansen. (**MORE:** [Night Shift: 10 Surprising Businesses You Can Patronize at 3 a.m.](http://moneyland.time.com/2012/01/30/10-unusual-places-that-are-open-all-night/#around-the-clock)) Despite the health risks, night shift work may be unavoidable for many women. The good news is that Hansen’s study suggests that for most women, one or two shifts a week probably won’t have a significant effect on breast health. _Alice Park is a writer at TIME. Find her on Twitter at_ [_@aliceparkny_](http://twitter.com/#%21/aliceparkny)_. You can also continue the discussion on TIME’s_ [_Facebook page_](http://www.facebook.com/time) _and on Twitter at_ [_@TIME_](http://twitter.com/#%21/TIME)_._ ```json [{"@context":"https://schema.org","@type":"NewsArticle","@id":"https://time.com/archive/7138436/working-the-night-shift-may-boost-breast-cancer-risk/","mainEntityOfPage":{"@type":"WebPage","@id":"https://time.com/archive/7138436/working-the-night-shift-may-boost-breast-cancer-risk/"},"headline":"Working the Night Shift May Boost Breast Cancer Risk","datePublished":"2012-05-29T19:52:21.000Z","dateModified":"2026-02-25T05:38:13.265Z","description":"Working the night shift can throw off your body clock, leaving you feeling tired and sleep-deprived. 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