MIAMI -- It's 11 p.m. at Jackson Memorial Hospital and Shawnette Patterson's workday is just beginning.

Patterson works overnight as a pediatric transplant nurse. A new study shows that the irregular shift could jeopardize her health.

"Women who worked night shifts or were exposed to light in the nighttime hours when most people are asleep had an increased risk of breast cancer," said University of Miami researcher Dr. Catherine Welsh.

Patterson and others who work overnight, under constant exposure to light, don't get something our bodies need -- darkness.

In the dark, a small gland deep in the brain produces melatonin. Exposing your eyes to light turns off production of melatonin, which is a hormone that appears to offer protection against cancer cells.

"That was very startling to me," Patterson said. "Working on the night shift and coming home in the day time with bright light, it's something to really take into account for me actually."

Sleeping with the TV or a light on and working at your computer late at night can also turn off melatonin.

In a recent study, researchers implanted breast cancer cells in rats, and then gave them blood from subjects exposed to either darkness or light at night.

"They found that blood from subjects who had been exposed to bright light at night, when it was infused into the breast cancer, caused the cancer to grow at a faster rate compared to tumors that were exposed to blood from people who had remained in the dark," Welsh said.

Nurse Sonja Johnson Sheppard used to work the night shift at Memorial West until she was diagnosed with breast cancer two years ago. Sheppard has no family history of breast cancer and thinks prolonged exposure to light at night could be a factor.

"When I was diagnosed I really never thought about it. I never put the two together, the correlation," she said. "But I have since had three other coworkers of mine who work nights, who ended up coming down with the same thing so we became a bit suspicious."

Those four cases are being investigated. Meanwhile, Sheppard is undergoing chemotherapy at Memorial.

The medical director at the cancer institute there said not sleeping in darkness can cause disorder in our bodies, and cells are like small machines that need order to function properly.

"This should make people think, 'How can I reduce the stress and the disorder on my body?'" Dr. Atif Hussein said.

Cancer experts said taking melatonin supplements, which are sold over the counter, is probably not the answer.

"Because we really don't know how it works and whether in fact it reduces the risk of breast cancer," Hussein said.

Welsh feels studies are needed to answer questions about melatonin supplements.

"What is the safety of this agent? What is the long-term safety? Does it interact with other hormones in the body or can it possibly suppress your own melatonin production?" she said.

To increase melatonin naturally, doctors said make sure the room you sleep in is as dark as possible.