People from many different cultures have long associated the human menstrual cycle with the phases of the Moon. Yet as uncanny as the similarities in average cycle time and the month might be, there's been little evidence of a link.

Now a research team from France and the US has found menstrual cycle rhythms are likely governed by the body's internal clock, rather than being a sum of processes intrinsic to the cycle itself.

What's more, there is a weak but significant association with the Moon's orbital period, hinting at a more fundamental biology once reliant on the timing of the tides.

It makes sense that so many cultures associate lunar and menstrual cycles; the Moon's phases repeat every 29.5 days, give or take about seven hours, while menstrual cycles have a mean length of 29.3 days, albeit with much variation between individuals.

Skeptics argue this perceived synchronicity is a mere coincidence, convenient to timekeeping, or perhaps just magical thinking. Though he never referred to menstruation specifically, in 1871 Charles Darwin proposed that lunar links may have an evolutionary origin: "In the lunar or weekly recurrent periods of some of our functions we apparently still retain traces of our primordial birthplace, a shore washed by the tides."

There's been evidence this is the case for species that rely on high tides for reproduction, like some fish and bivalves.

The researchers examined data on almost 27,000 menstrual cycles representing 2,303 European women and 721 North American women. Their analysis revealed something called 'phase jumps', in which menstrual phases 'jump' ahead to match up to a body clock external to the process.

"If the cycle lengthens, for any reason, this clock-based process adapts to quickly shorten it," neuroscientist Claude Gronfier, from The University of Lyon in France, told BBC Science Focus.

If the Moon did have any role in the length of human menstrual cycles, either as an evolutionary relic or in an ongoing influence, it would be more of a backseat driver with the body's own internal clock firmly at the wheel, the new study suggests.

The human body's 'clock' has a period of just over 24 hours, regardless of external factors such as the amount of time exposed to sunlight. Although it varies slightly between individuals, this circadian rhythm is highly stable for each person.

In the absence of sunlight – which resets the body clock to the 24-hour day – it can drift out of sync, as anybody with jetlag can attest to.

The researchers suggested that if an internal clock was involved in the menstrual cycle, then cycle lengths would be similarly stable within individuals, and have a narrow spread across a population.

While the study suggests this is the main mechanism behind menstrual timing, they did find a weak but statistically significant relationship between menstrual and lunar cycles, which varied depending on geography.

"The menstrual cycle began more often at the waxing crescent in Europe, whereas it was at the full moon in North America," the authors write.

While they don't have a clear explanation for this difference, they suggest it might be due to lifestyle differences (like sleep-wake cycles) between people from these continents.

"There is a lot of work ahead of us, and we hope that our colleagues embark with us on what could be a future area of circadian medicine," Gronfier told BBC Science Focus.

The team thinks their results could lead to potential fertility treatments, and says learning more about how genes regulate the menstrual cycle is key to understanding its chronobiology. A previous study found a specific gene variant linked to a hormone that affects cycle length.

However, this doesn't explain why cycles can phase jump based on previous cycles' duration, or how the body's internal clock could affect cycle length. More research with larger groups is needed to confirm these new findings and understand the underlying mechanisms.

"If the existence of an internal clock that controls the menstrual cycle is confirmed in further studies, then the medical treatment of ovulation disorders could use the chronobiological approaches," the authors write.

This research was published in Science Advances.