By Linda Carroll

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NEW YORK (Reuters Health) - An extra hour of bright light can lift the levels of a pituitary hormone that enhances testosterone in men, a new study shows.

Researchers found that men who were exposed to an hour of bright light before sunrise experienced an increase in luteinizing hormone (LH), a hormone produced in the brain's pituitary gland.

LH influences reproductive hormones in both men and women. Increases in LH in men drive up testosterone levels, while the hormone triggers ovulation in women.

The new findings, published in the journal Neuroscience Letters, show that testosterone could be enhanced simply by extra hours of bright light, said study author Dr. Daniel F. Kripke, a professor of psychiatry at the University of California at San Diego.

"There are many possible beneficial effects of increased testosterone," Kripke told Reuters Health. "Studies have shown that it may help with depression. It can improve libido. And there may be a muscle-building and strengthening effect."

Light also impacts women's reproductive cycles, Kripke said. In an earlier study, his group showed that when women with long and irregular menstrual cycles were exposed to bright light, the cycles regularized.

"We had some indication then that it was working through luteinizing hormone," the researcher said.

For the new study, Kripke and his colleagues studied 11 healthy young men who were between the ages of 19 and 30, dividing participants into two groups.

In the first part of the study, both groups were awakened at 5:00 am and exposed to either a bright light of approximately 150 watts or a low-wattage red light.

During the second part, the groups were switched, so that the men who were exposed to bright light in the first part were then exposed to low light, and vice versa.

The men's LH levels were tested before and after exposure to the lights.

The researchers found that the men showed a 69-percent increase in LH levels after exposure to bright light.

In the report, they note that sexual problems such as low libido are known to be part of depression, as well as a side effect of antidepressant medications.

"It is theoretically possible," they write, "that light exposure, which increases LH secretion, will alleviate sexual dysfunctions in depressed patients."

They suggest that studies like the current one be repeated in people with depression.

SOURCE: Neuroscience Letters 2003;341:25-28.

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