Lead author James Roney, a professor in the Department of Psychological and Brain Sciences, said that they have found two hormonal signals that had opposite effects on sexual motivation.
If you are feeling frisky, then chances are greater your estrogen level-and, perhaps, fertility-are hitting their monthly peak, according to the researchers.
If not, then you're more likely experiencing a profusion of desire-deadening progesterone, and the less fertile time in your cycle.
When hormone levels and sexual desire were factored against the menstrual cycles of test subjects-in this case, undergraduate students-the researchers saw a measurable increase in progesterone levels at the same time the subjects noted decreases in sexual motivation.
Progesterone, the researchers said, is mediating this drop in desire from the fertile window to the luteal phase-the second half of the menstrual cycle.
"Progesterone acting as a potential stop signal within cycles is a novel finding in humans," noted Roney.
The researchers' findings have potential implications on the treatment of low sexual desire and how hormone replacement trials are done.
Roney noted that his findings don't present a full model, and he'd like to replicate his results with women of different age groups.
Eventually, Roney continued, the goal would be to have a better model of the signals in a natural cycle that might then inform medical research.
Another interesting finding, according to Roney, was the impact-or lack thereof-of testosterone on the women's sexual motivation.
"There's a common belief in the medical literature that testosterone is the main regulator of women's libido. Doctors tend to believe that, though the evidence isn't that strong in humans. In the natural cycles, we weren't finding effects of testosterone. It wasn't significantly predicting outcomes," he explained.
Roney doesn't deny that testosterone does seem to have a positive effect in hormone replacement therapy, but suggests the effects may be pharmacological.
"Testosterone has those effects if you inject it externally in women who are menopausal, and there are a lot of reasons that might be the case," he said.
"For example, testosterone can be converted to estrogen through a particular enzyme. If you inject menopausal women with testosterone, it might be acting as a device that's delivering estrogen to the target cells. So the fact that it works doesn't necessarily mean it's an important signal in the natural cycle."
Their findings appeared in the current issue of the journal Hormones and Behavior.