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Two studies have shown that sleep interruption is associated with an increased risk of breast cancer. The studies specifically looked at women who worked night or graveyard shifts.

The first study was conducted at the Fred Hutchinson Cancer Research Center in Seattle. Researchers studied about 800 women who were diagnosed with breast cancer and 800 healthy women. All subjects were queried on their risk factors for breast cancer such as their sleep patterns, the lighting in their bedroom and their work history.

Graveyard shift work seemed to increase the risk of breast cancer by up to 60 percent. Experts said that women who reported not sleeping between 1 a.m. and 2 a.m. had a 14 percent increase in breast cancer risk for each night per week. They also found that participants with the brightest bedrooms had an increased risk of breast cancer.

The second study was conducted the Brigham and Women's Hospital and Harvard Medical School. In this study, researchers analyzed data from more than 78,000 women who took part in the Nurses' Health Study. The women were asked questions about their shift work, interrupted sleep patterns and bedroom lighting.

The Journal of the National Cancer Institute (2001;93:1557-1562 and 2001;93:1563-1568) reported that there was an 8 percent increase in breast cancer risk among the nurses who worked rotating night shifts for up to 29 years. They also reported a 36 percent increase in breast cancer risk in women who had worked night shifts for more than 30 years.