In the study, Jen-Chuen Hsieh, a neuroscientist at National Yang-Ming University in Taipei, Taiwan, and colleagues, found that the region of the brain used for coping with stress flips to the opposite side of the brain during a woman's period - from an area linked to negative emotion to one that usually deals with cheerier thoughts.
Such a change could help women deal with the hormonal maelstrom going on in their bodies without causing huge behavioral shifts. Estrogen levels, in particular, plummet around menstruation.
The researchers studied 14 women using a magnetoencephalograph - a machine that measures magnetic waves created by brain activity.
All their subjects were right-handed, to ensure that the left-right orientation of their brains matched.
When the women were shown frightening images, they normally triggered activity in the right half of the women's brains. This side of the brain tends to process negative feelings, such as anxiety.
However, during the women's menstrual periods, the images activated areas in the left half of their brains, which handles positive emotions.
The switch in brain dominance during menstruation could help women cope with stress linked to hormone changes.
Throughout their periods, participants' average scores on a test of anxiety hardly changed.
"I was really pleased to read this," New Scientist quoted Jill Goldstein, a neuroscientist at Harvard Medical School in Boston, as saying.
The study is published in the journal Hormones and Behavior.