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Light at Night May Be Linked to Cancer

Disrupting the Body's Internal "Clock" Could Have Impact
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WebMD Health News

Sept. 9, 2004 -- Chronic exposure to artificial light at night can take its toll on the body, throwing off the internal "clock" that governs the rhythm of life. Those disruptions may do more than make people feel tired; they may also lead to higher cancer risk, say researchers.

British and U.S. researchers discussed the possibility in London at a meeting of childhood leukemia specialists.

The conference focused on childhood leukemia, which has risen by more than 50% in kids under age 5 during the second half of the last century. In the last 100 years, artificial light has allowed people to stay active long after the sun sets.

There is no evidence linking light at night specifically to childhood leukemia, the causes of which are not fully understood.

This area of research is still very preliminary, but it's not the first time that light at night has been seen as a possible cancer risk.

Studies have shown that night-shift workers, who are chronically exposed to bright lights at night, have higher risks of breast cancer and colorectal cancer.

Disrupting the body's circadian rhythms accelerates cancer progression in rodents and humans, writes Russell Foster, visual neuroscience professor at London's Imperial College, for a presentation at the conference. The circadian rhythm is the body's internal 24-hour clock that regulates various biologic processes.

Exposure to artificial light at night suppresses the body's production of the hormone melatonin, which is naturally produced at night.

Melatonin's effects on the body aren't completely understood, says Foster. But in some animal experiments and laboratory studies, melatonin has shown the ability to protect against the development of cancers, prompting some researchers to suggest that it might do the same thing in humans.

If that proves true, low levels of melatonin could reduce the body's cancer-fighting abilities.

Foster also notes a study done by other researchers of 118 children who were getting chemotherapy for acute lymphoblastic leukemia. "The risk of relapse was 2.5 times higher in children who received chemotherapy in the morning than in those receiving the same treatment in the evening," writes Foster.

More studies are needed. Meanwhile, all the light bulbs in the world don't free us of our bodies' rhythms.

"Biological clocks drive or alter our sleep patterns, alertness, mood, physical strength, blood pressure, and every other aspect of our physiology and behavior," writes Foster.

"Until we turned our nights into days, and began to travel in aircraft across multiple time zones, we were largely unaware of these internal clocks. Yet the striking impairment of our abilities at 4:00 in the morning soon reminds us that we are slaves to our biology."

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