Permanently depressed nervous system could be to blame for severe cases of premenstrual syndrome (PMS), Japanese researchers say.
They found that PMS was tied to decreased activity in the autonomic nervous system - which controls the body's equilibrium - each month.
The effect was most pronounced in women with the most severe, and potentially debilitating, PMS symptoms.
The study, by Osaka's International Buddhist University, appears in the journal BioPsychoSocial Medicine.
PMS may include a whole range of symptoms - irritability, moodiness, depression, fatigue, abdominal bloating, breast swelling, general aches and decreased concentration.
While for some women PMS is a minor monthly annoyance, for others, more severe symptoms seriously disrupt their lives.
It is known that the condition is linked to hyper-sensitivity to the hormone progesterone, which is released by the body after ovulation.
However despite the number of women affected, science has yet to offer a full explanation or universal treatment.
The Japanese team measured heart rate variability and hormone levels in 62 women, and also used questionnaires to evaluate physical, emotional and behavioural symptoms.
They found that women troubled by PMS showed decreased nervous activity in the late luteal phase, which precedes menstruation.
Those with the most marked symptoms - known as premenstrual dysphoric disorder (PMDD) - had lower rates of nerve activity than the other groups during the entire menstrual cycle.
Researcher Dr Tamaki Matsumoto said the underlying biological mechanism of PMS remained unclear.
But he said the latest findings suggested that women with a lower autonomic function might be vulnerable to symptoms.
Professor Shaughn O'Brien, an expert in obstetrics and gynaecology at Keele University Medical School, said the findings were interesting - but more work was needed before it could lead to a real breakthrough in diagnosis or treatment of PMS or PMDD.
He said: "If the newly published work did prove to be clinically useful it has the potential at least to provide a relatively non-invasive method to distinguish women with PMS from those who have a different non-hormonal types of mood disorder."
Professor O'Brien said the Royal College of Obstetricians and Gynaecologists was due to issue guidance on how to treat PMS early in the new year.
"It is a major problem. Nearly all women get some premenstrual symptoms," he said.
"But there is an extreme version which has a major impact on people's work, family and social lives."
At present severe symptoms can be treated by preventing ovulation using hormones such as certain oral contraceptive pills or by using SSRI medications which act differently in PMS from the anti-depressant effects.
However, although both are specifically licensed for this use in the US, they are not in Europe.
Only in extreme cases the would ovulation be stopped by surgical removal of the ovaries and then usually only if a woman was already having her uterus removed for another gynaecological condition.