For many people around the world, the idea that women’s menstrual cycles are governed by the Moon has deep roots. In fact, Charles Darwin theorised that this lunar link came from our species’ time as seashore-dwelling creatures who lived in sync with the tides.
But a new study, published in journal Science Advances, reveals that these cycles likely don’t follow the 29.5 day cycles of the Moon – but are instead governed by your body’s internal clocks.
What’s more, they found that any lunar link that is present varies by continent. The scientists observed that in Europe, for example, menstrual cycles tend to begin at the waxing crescent – but for North American women it often begins at the full Moon.
Menstrual cycles usually have a monthly pattern, but the mechanisms that decide this pattern are mysterious. To better understand how they work, the researchers analysed the data of almost 27,000 menstrual cycles from over 3,000 European and North American women. Specifically, they tracked the first day of menstruation in each of these cycles.
They found that there was only a weak correlation between menstrual and lunar cycles, contrary to some popular belief. The fact that the correlation varied across continents supported their theory; this is an indication, they say, that any lunar correlation is actually likely driven more by lifestyle factors (such as sleep-wake cycles) than the Moon.
However, the authors do point out that in communities where women are more exposed to moonlight, the influence of the Moon on menstrual cycles may be stronger.
What’s more likely though, the paper says, is that the body’s internal clock, known as the ‘circadian clock’, maintains the rhythm of women’s ovaries.
That’s because the data revealed something the researchers called ‘phase jumps’, which is when a woman’s internal clock becomes out of sync with regular cycle lengths so self-corrects by jumping ahead to the next stable state.
“If the cycle lengthens, for any reason, this clock-based process adapts to quickly shorten it,” paper author Dr Claude Gronfier told BBC Science Focus.
Phase jumps are signs of a phenomenon known as ‘relative co-ordination’, a typical event in circadian clocks. One example of this, the scientists say, is the out-of-sync feeling that happens after you’ve travelled across time zones.
In other areas of human health including cancer, sleep disorders, and depression, scientists have taken chronobiological approaches (meaning relating to the body’s biological clock) like light therapy to find successful treatments. This new research opens up possibilities for similar and new medical treatments for ovulation disorders – which could even help to improve fertility.
“There is a lot of work ahead of us, and we hope that our colleagues embark with us of what could be a future area of circadian medicine,” Gronfier said.
Read more: