I think the reason some people might believe this claim is because we’re taught in school that the moon’s gravity causes the tides. I think the reasoning goes, “well if the moon’s gravity can affect the tides, surely it can affect smaller things too”
Maybe your friend is thinking about menstrual cycles? Of course they aren’t really influenced by the moon but they (often) have a period of 1 month, like the moon phases.
Yeah, archaeologists have found evidence of women timing them with groups of 28 scratches on stone- perhaps trying to conceive?
Or avoid conception
Or at least know when not to wear white pants
Iirc, this is actually true. There has been some studies that suggest that the moon phase does influence menstrual cycles. I don’t fully remember what the researchers concluded, but I remember vaguely that it was something like, not seeing the moon (if the sky is overcast, for instance) doesn’t affect the menstrual cycle, but looking at the moon will “reset” the cycle in some way.
Well, now you made me find this paper: https://www.science.org/doi/10.1126/sciadv.abe1358?intcmp=trendmd-adv
OP’s friend might have read something based on it too. This is really weird. I guess more reading is necessary to know if this paper was already debunked or not