Scientists at the University of Siena in Italy found that regular, early-morning use of a light box - similar to those used to combat Seasonal Affective Disorder or SAD - helped men increase testosterone and improved their sex lives.

Andrea Fagiolini, a professor who led the study and presented it at the European College of Neuropsychopharmacology conference in Vienna on Monday, said the treatment may prove useful during the Northern hemisphere's darker winter months.

"The increased levels of testosterone explain the greater reported sexual satisfaction," he said. "In the Northern hemisphere, the body's testosterone production naturally declines from November through April, and then rises steadily through the spring and summer with a peak in October."

Low sexual desire can affect significant numbers of men after the age of 40, with studies finding that up to 25 percent of men report problems.

Fagiolini's team recruited 38 men diagnosed with either hypoactive sexual desire disorder or sexual arousal disorder - both characterised by a lack of interest in sex.

After taking baseline readings, they divided the men into two groups and gave one regular treatment with a bright light box, while the control, or placebo, group was treated with a light box adapted to give out significantly less light.

"We found fairly significant differences," Fagiolini said.

"Before treatment, both groups averaged a sexual satisfaction score of around 2 out of 10. But after treatment, the group exposed to the bright light was scoring sexual satisfaction scores of around 6.3. In contrast, the control group only showed an average score of around 2.7 after treatment."