Exposure to bright light boosts levels of the male hormone testosterone and increases sexual satisfaction, the research showed.
Low sexual desire affects significant numbers of men after the age of 40. According to some studies, up to a quarter of men report problems depending on age and other factors.
Because sexual interest varies with the seasons, scientists have suspected that it might be influenced by ambient light levels.
To test the theory a team of Italian researchers from the University of Siena recruited 38 men diagnosed with low libidos and treated half of them with daily doses of bright light.
Early each morning for two weeks the men spent half an hour in a room with a light box - a panel designed to emit bright white light.
At the end of the trial tests showed that their testosterone levels had risen significantly. They also reported higher levels of self-rated sexual satisfaction.
Similar effects were not seen in the other group of men whose light box was adapted to emit a low non-therapeutic level of light.
Lead researcher Professor Andrea Fagiolini said: "Before treatment, both groups averaged a sexual satisfaction score of around two 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.
While average blood levels of testosterone in the "control" group remained at around 2.3 nanograms per millilitre (ng/ml) before and after the study, those of the men receiving active light treatment rose from 2.1 ng/ml to 3.6 ng/ml.
"The increased levels of testosterone explain the greater reported sexual satisfaction," said Professor Fagiolini. "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. You see the effect of this in reproductive rates, with the month of June showing the highest rate of conception."
The findings were presented at the annual meeting of the European College of Neuropsychopharmacology in Vienna.
