It makes sense to believe that the lunar cycle influences the menstrual cycle. The gravitational influence of the moon's alignment with Earth causes the large bodies of water to move with the tides - and what are we but big ol' bags of water?
Both tides and the light of the lunar cycle have an effect on animals - for example, coral spawns on a full moon.
But there isn't actually any strong evidence that the moon is influencing our periods.
Research linking the menstrual cycle to the lunar cycle doesn't show any clear consensus as to which phase of the moon is responsible for menstruation: some show menstruation starting at around the full moon, others at around the new moon.
This means research cannot demonstrate a mechanism for *how* the moon is influencing our cycles - while there is arguably a benefit for either being fertile at a new moon (complete darkness) or a full moon (most light), it can't be both on a 29 day cycle!
It's worth noting at this point that neither the lunar cycle nor the menstrual cycle are exactly 29 days: the lunar cycle is in fact 29.5 days long, and the menstrual cycle can be anything between 21 and 40 days, and varies both within the same person and between people.
The huge variance in the length of the menstrual cycle suggests that the moon's influence, if it exists at all, is pretty minimal.
Attached: 1 image OLD THREAD REPOST This is the Ishango Bone. This carved bone is about 22,000 years old and is covered with columns of markings. The bone has puzzled researchers since its discovery in the Democratic Republic of Congo. What do the markings mean? Image credit: the Ishango Bone on display at the Royal Belgian Institute of Natural Sciences credit: Joey Kentin (CC-BY-SA-4.0)
It's also unclear as to whether people in the past consistently saw links between the menstrual cycle and the lunar cycle. For example, many traditions have a female moon deity... but many others, such as Japanese, Egyptian, Norse and Irish have a male moon deity.
Pliny, in his altogether entirely wrong explanation of periods, didn't mention anything about phases of the moon (though did say that period sex in an eclipse was risky) https://masto.ai/@vagina_museum/109869305327266405
In the first century, Pliny the Elder wrote an encyclopaedia. Natural History contained topics ranging from maths, art, anthropology, zoology and everything in between. Oh, and a section on menstruation. Here's a thread of everything he had to say about periods and the supernatural qualities ascribed to menstrual blood (spoiler: it is 100% incorrect)
Is it true that menstrual cycles influence one another and sync up among close friends, housemates and partners? Is menstrual synchrony real? Let's take a look at the evidence...
It's an interesting though that the split could be an evolution to temper population & resource swings
Similar but not quite like the 13 & 17yr cicadas with offset cycles to avoid resource competition