Exposure to frequent artificial light at night appears to increase the breast cancer risk among women in their homes but apparently not among those who work late shifts, according to an analysis by a Long Island, N.Y., researcher.

Dr. Cristina Leske, a professor of preventive medicine at Stony Brook University, questioned 576 Long Island women who developed breast cancer and 585 who never had the disease to determine whether exposure to artificial light plays a role.

The notion is not farfetched, according to Leske and other researchers who have been chasing the hypothesis that exposure to light at night is a breast cancer risk factor. They contend that hormones function in a circadian rhythm, based on the body’s internal 24-hour clock.

At night, the hormone melatonin, the so-called sleep hormone, streams into the blood and flows throughout the night, suppressing cancer cells, scientists say. Melatonin is switched off by the presence of light and then estrogen switches on. Estrogen, the female hormone produced by the ovaries, is a well-known cancer promoter.

Leske found in a subset of women who had developed breast cancer that they tended to wake up frequently during the night and turned on lights during what should have been hours set aside for sleeping. In her study, the positive association with breast cancer was 65 percent. Among overnight shift workers, Leske found a 45 percent lower breast cancer risk.

Her epidemiologic study is reported in the current issue of the American Journal of Epidemiology.