The importance of the finding isn't yet clear. But it's possible that light therapy could one day be used to control ovulation in women or treat people who take antidepressants and find themselves with low sex drives, the researchers say.

"It's a very promising lead," says study co-author Dr. Daniel Kripke, a professor of psychiatry at the University of California at San Diego. Light therapy is natural and could be a safe and effective way to "accomplish some important health goals," he says.

Researchers have known for decades that exposure to light affects the way animals live. Changes in the light from the sun, for example, automatically set off hibernation in some mammals. Seasonal changes in light also control reproduction in rats and mice so they only mate during warmer months, Kripke says.

Researchers are still working to understand how exposure to light affects humans. Kripke and colleagues discovered two decades ago that light therapy -- shining powerful lamps at people's eyes -- affects mood. Light therapy has become a common treatment for seasonal affective disorder, a type of depression that strikes when days grow shorter.

In his new study, Kripke enlisted 11 healthy male volunteers, aged 19 to 30, to test whether light affects the body levels of luteinizing hormone, which is produced by the pituitary gland and assists in the production of other hormones, such as testosterone, in men. The men woke at 5 a.m. for five days and spent an hour in front of a light box giving off 1,000 lux, or much more brightness than typical indoor lighting. Later, they spent five days in front of a light box that only gave out 10 lux.

The findings of the study, supported by the National Institutes of Health, appear in the April 24 issue of the journal Neuroscience Letters.

Researchers found the body levels of luteinizing hormone grew by 69.5 percent in the men while they were exposed to the high levels of light.

The researchers didn't look at women because the rapidly cycling hormones in their bodies would make it difficult to study the effect of light, Kripke says. However, luteinizing hormone does affect ovulation, he adds, and "we think light is potentially a very promising treatment for women who have ovulatory problems or long and irregular menstrual cycles."

Light therapy could also boost testosterone in men, potentially increasing sexual potency and muscle mass, he says. Researchers, however, didn't monitor testosterone levels in the men.

The researchers hope to test light therapy on people with low sex drives and on postmenopausal women.

A hormone expert cautioned that plenty of research is still needed. The newly released study was relatively small, and it's not clear the changes in the level of the hormone are significant enough to actually cause changes in the body, says Dr. Ronald Swerdloff, chief of the division of endocrinology at Harbor UCLA Medical Center, part of the University of California at Los Angeles School of Medicine.

"I don't think it's clear where this is going to take us," he says.

More information

Learn more about light therapy from the Society for Light Treatment and Biological Rhythms or the University of Washington, which offers a fact sheet about its use to treat seasonal affective disorder and other conditions.