For many, the art of seduction starts with dimming the lights. But a new study suggests that bright light may help men with flagging sex drives to shine in bed.
Exposure to bright light - similar to daylight - boosts levels of the male hormone testosterone and can triple sexual satisfaction, the new research found.
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: "We found fairly significant differences between those who received the active light treatment, and the controls.
"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 - a more than three-fold increase on the scale we used. 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 use of the light box really mimics what nature does."
He added: "We believe that there may be several explanations to explain the underlying mechanism. For instance, light therapy inhibits the pineal gland in the centre of the brain and this may allow the production of more testosterone, and there are probably other hormonal effects.
The findings were presented at the annual meeting of the European College of Neuropsychopharmacology (ECNP) in Vienna.
Professor Fagiolini said the research was at too early a stage to recommend light therapy as a clinical treatment for low libidos.
"We're not yet at the stage where we can recommend this as a clinical treatment .. however if this treatment can be shown to work in a larger study, then light therapy may offer a way forward," he said. "It's a small study, so for the moment we need to treat it with appropriate caution."
Currently treatment for low libidos includes testosterone injections, antidepressants, and other medications. Scientists said light therapy may offer the benefits of medication, but with fewer side effects.
Professor Eduard Vieta, chairman of the Department of Psychiatry and Psychology at the University of Barcelona Hospital Clinic said: "Light therapy has been used successfully in the past to treat some forms of depression and this study suggests now that it may also work to treat low sexual desire in men. The mechanism of action appears to be related to the increase of testosterone levels.
"Before this kind of treatment, which is likely to be better tolerated than pharmacological therapy, gets ready for its routine use, there are many steps to be implemented, including replication of the results in a larger, independent study, and verifying whether the results are long-lasting and not just short-term".
Previous studies have found lack of sleep can reduce the libido.