A third of all women have experienced post-sex blues at some point, a study shows, but researchers still don't understand why.

The period immediately after sex normally results in feelings of well-being, and mental and physical relaxation. But the study of more than 200 young women reveals many have experienced the reverse, including feelings of melancholy, anxiety and tearfulness.

Anti-climax ... sex can bring on feelings of sadness and anxiety in many women.

Queensland University of Technology Associate Professor Robert Schweitzer, who carried out the research, said 32.9 per cent of respondents had experienced the phenomenon at some point. That was despite the sex being otherwise satisfactory.

Professor Schweitzer said the cause of such negative feelings was unknown - but it was clear women wanted more information about the phenomenon. ''Research on the prevalence and causes of post-coital dysphoria has been virtually silent but internet searches reveal information on the subject is widely sought,'' he said.