Women who work nights and those who are up frequently during the early hours of the morning may be at increased risk for developing breast cancer.

The findings, combined with those of other studies in humans and in animals, suggest that nighttime exposure to light may elevate breast cancer risk by suppressing production of melatonin, a brain hormone that is made during darkness and that normally peaks at night.

Nighttime light exposure can come from a variety of sources, including working the night shift, having insomnia or even being exposed to light in the bedroom during sleep.

Breast cancer risk was higher among women who often did not sleep during the period of the night when levels of the sleep-regulating hormone melatonin are typically at their highest, the study indicates. Their breast cancer risk increased 14% for each night of the week that they reported not sleeping during this period.

Working the graveyard shift upped a woman's risk of developing breast cancer by 60%.

The results of this study provide evidence that indicators of exposure to light at night may be associated with the risk of developing breast cancer.

Researchers from Harvard Medical School also examined the relationship between breast cancer and working the night shift in more than 78,000 nurses.

Women who worked between 1 and 29 years on a rotating night-shift schedule had an 8% increase in breast cancer risk, while women who worked 30 or more years on the night shift showed a 36% increase in their risk of developing breast cancer.

Women who work on rotating night shifts with at least 3 nights per month, in addition to days and evenings in that month, appear to have a moderately increased risk of breast cancer after extended periods of working rotating night shifts.

Evidence is accumulating for an association between exposure to light at night and breast cancer risk.

One possible explanation for this relationship has to do with melatonin production. Melatonin is produced by the brain's pineal gland. Because it is released into the bloodstream at night, the hormone is thought to play a role in regulating sleep.

When normal melatonin cycles are interrupted in women, the production of estrogen from the ovaries is higher and this may contribute to their breast cancer risk.

The new studies add to a large body of research that has raised concerns about the long-term health effects of work schedules that involve irregular hours and frequent changes between day and night duty -- schedules that have become increasingly common.

Night-shift work is associated with an increased risk for heart disease, digestive tract disorders and pregnancy complications. Researchers are planning studies to explore whether it affects the risk for prostate cancer or other tumors.

Exposure to light at night, by suppressing melatonin production, may increase women's levels of the sex hormone estrogen, which stimulates growth of breast tissue, including some breast cancers.

Blind women, whose melatonin production does not drop upon exposure to light, have lower breast cancer rates than sighted women. Laboratory studies and experiments in animals suggest that melatonin may directly inhibit the growth of breast cancer cells.

Journal of the National Cancer Institute October 17, 2001;93:1513-1515, 1557-1568

Dr. Mercola's Comments:

Clearly, when one violates the normal circadian rhythyms with the use of artificial lights there is a huge price to pay. One of them will be an increased risk of cancer. This is most likely mediated through alterations in melatonin levels. It would be nice if popping a melatonin supplement would help, but this does not appear to be the case. Additionally, at the end of October we will again have to suffer the ritual of one of the worst government decisions with respect to our health. Daylight savings time. This is one absolute disaster. The solution is NOT to turn the clock back but start school later and let kids sleep more as the article above describes. However, this is only a partial solution. The full solution involves recognizing that ideally we should be in bed shortly after sunset. In winter, we all need about nine hours of sleep. This is the average amount of sleep that we had about 100 years ago in the winter, prior to the widespread adoption of electric lights. But now we are down to seven hours of sleep and counting down. This is just not compatible with ideal health. This is one of the main reasons why so many get sick with colds, coughs and flus during the winter. One of the best books I ever read in this area is Light's Out. Related Articles: Increased Breast Cancer Risk with Sleep Pattern Disruptions Decrease Your Sleep and Increase Your Risk for Diabetes Guide to a Good Night's Sleep Sleep In TOTAL Darkness to Decrease Cancer Risk Lack of Sleep Affects Hormone Levels