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Shiftwork and Breast Cancer: The Melatonin Connection

© Stephen Weistling

I almost missed the announcement on the radio while I was driving through town. Sandwiched between the war on terrorism and the local weather report, the bits and pieces I caught indicated that someone found a link between working nights and breast cancer. For women who are shiftworkers, this certainly wouldn’t be good news.

After a couple of days searching the Internet, I was able to find more details, and uncovered the basis for the news bulletin.

It starts with the hormone melatonin. At the end of a normal day, we sense the reduction of natural light levels through our eyesight. This triggers a reaction in the pineal gland to begin the secretion of melatonin. As melatonin levels rise in our bodies, we tend to get sleepy.

Furthermore, as melatonin levels rise in women, the production of estrogen drops. (Estrogen is the female sex hormone.) It is believed that the production of estrogen stimulates the growth of breast tissue including some breast cancers.

So to recap, more light means less melatonin, more estrogen, and greater risk of cancer.

In experiments in the 1970’s and 1980’s, pineal glands were removed from rats, which caused a plethora of breast cancer tumors.

And in 1991, researchers exposed rats at night to dim light, which suppressed melatonin production. When malignant tumors were transplanted into their bodies, the tumors grew significantly more rapidly than animals kept in total darkness.

The next step was to test sightless women, who according to the theory should have fewer instances of breast cancer.

The results of the experiment supported the theory. Totally blind women had 60% fewer breast cancers than women of normal sight, with step-like progressions of cancer with women at various stages of partial sight.

Then it was time to investigate women who worked nights. Women with nighttime occupations are exposed to greater durations of light. (Night workers often sleep less during the day, and most do not sleep in total darkness)

In a Seattle study, researchers interviewed approximately 1600 women, half of whom worked nights. They asked them about their work schedules, sleep habits, the amount of light in their bedrooms, past pregnancies, the use of hormones and birth control pills, smoking, and alcohol use. They also asked about family histories of breast cancers and other factors.

In the findings, which prompted the current press release, women who worked night shifts were 60% at greater risk for breast cancer than women who did not. And the greater the number of nights worked, the greater the incidences of breast cancer.

The copyright of the article Shiftwork and Breast Cancer: The Melatonin Connection in Shift Work is owned by Stephen Weistling. Permission to republish Shiftwork and Breast Cancer: The Melatonin Connection in print or online must be granted by the author in writing.