The lunar cycle is about 29 days.

And so is the average menstrual cycle, ish. Coincidence?

People have been making the connection between the moon’s phases and the  menstrual cycle for centuries. And as it turns out, those  people had more solid rationale for connecting periods with  the moon than you’d think. But the weirdest part is that in terms  of stuff controlling human periods, we may have been looking at the  wrong thing in the sky all along. [♪ INTRO] Let’s start with a flashback to health class

and go over what a menstrual cycle is.

The menstrual cycle is the  process that gets your body ready to possibly become pregnant, and in humans, the whole thing is about a month long. The cycle starts on day 1 of your period, when the uterine lining starts to shed. This is the menses phase, which generally lasts three to seven days and  overlaps with the follicular phase.

The follicular phase goes  from the start of your period until one of your ovaries pops out a new egg cell, usually about ten to fourteen days after the beginning of your last period. This marks the beginning of the luteal phase, which is when that little  egg cell you made travels down the ovarian tube, AKA the fallopian tube, and into the uterus. And if  that egg wasn’t fertilized, then a change in hormone levels  will trigger your next period, and the whole thing starts over again.

Since this all happens more or less monthly, maybe it’s not surprising that  it’s been coupled to the other mostly-monthly cycle out  in nature, the lunar month. That’s about the amount of time it takes to go from one full moon to the next, which is 29 and a half days. And the idea that these two cycles are linked is so old that our word for both of them comes from the Indo-European root word  mē, which means "to measure." The Greek philosopher Aristotle  believed that the menses occurred during the waning of the moon, because that time of the month  is colder and more humid, which … Okay, I guess But this connection isn’t just a European thing.

Versions of this also show  up in the belief systems of indigenous groups from Tanzania,  the Central African Republic, Mexico, the US and Canada … You get the idea. And if all these people around the  world have made that connection, could they really all be wrong? Yes, almost definitely.

There’s been a lot of research into whether the moon really does have a say over when aunt Flo comes to visit,  and the outlook isn’t great. For starters, the average cycle length  actually varies person to person. Because the clinically “normal” range can be anywhere from 24-38 days.

And cycle length can also  vary for an individual, too. A 2006 paper found that  about 42% of people’s cycles varied by more than 7 days in a single year. Okay, so maybe not everyone  is synced up to the moon, but, like, is anyone?

To answer that, we need to look at  all the people with four-ish week cycles and see if their cycles line  up with any phases of the moon. And yeah, people have done  a lot of studies about this. Thanks to Trainwell for  supporting this SciShow video!

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To get 14 days free with  your own personal trainer, go to https://go.trainwell.net/SciShow or click the link down in the description. A few of the really old  studies from the last century found tentative correlations. But more recent, longer studies with more  participants, really don’t.

One large-scale analysis in  2016 by a period-tracking app called Clue found that  across the 1.5 million users and 7.5 million menstrual cycles they looked at, there was no correlation at all between lunar cycle and period starting. So we have a pretty huge dataset  to show that people’s periods don’t line up with any  particular phases of the moon, and they happen at pretty  much any point in the month. Between that and the amount  of variation in cycle length, there’s really not any evidence  backing up any sort of connection between the moon and human reproduction.

Sorry to burst anyone’s bubble. But weirdly enough, there are some animals whose reproductive cycles do line up with moon. For instance, some studies have  shown that sea urchins’ gonads get larger under the full  moon, but they’re basically nonexistent under the new moon.

And this matters because we  like to eat those gonads, which you’ve probably seen on a sushi menu as uni. And if you didn’t know what  that was until just now…. Well, I hope I didn’t ruin it for you, because they’re still delicious.

Anyway, sea urchin fishers would describe the urchins as being “full”  at the full moon and “empty” at the new moon, and would  specifically only gather them when they knew they’d be full. And it’s not just urchins. Some corals spawn en masse on  a particular night of the year, a few nights after the full moon.

We think they can time it so well based on the amount of moonlight they’re exposed to, thanks to specialized photosensitive  molecules in their tissue. Spawning all at once like  this means they’ve got better odds of sperm and egg joining  up, so it works out well that they have nature’s biggest  nightlight telling them all when to go. Some marine zooplankton also time their mating by the moon’s phases, mating at  both the new and full moon.

Now you may have noticed a trend here. These are all marine animals. Researchers think that these  animals might be staying synced up to the lunar calendar by using the changing tides, or maybe even by reacting to the  brighter moonlight under a full moon.

And since humans have lived  ocean-side for a very long time, it’s not a huge leap to think that  they’d have been paying attention to what they saw going on in the water, especially when it has to do with  what goes on their dinner plates. So part of this moon-periods  connection might come from people’s observations of reproduction in the natural world, especially when those people were  depending on marine food sources, and therefore paying extra  attention to these cycles. And while the evidence is pretty clear that the moon doesn’t influence human menstruation, there’s growing evidence that  the Sun actually plays a role.

A study of periods in a Russian city showed that the amount of sunlight a person sees could affect how long their period will be. Getting exposed to more sunlight just before they ovulated began meant  they’d have a shorter period, relative to when they’re exposed to less light. More sunshine led to people’s menstrual cycles being about a day shorter during summertime.

And the correlation was specific to sunlight, not temperature or other weather variables. Other studies have suggested  that increased exposure to artificial light could  shorten menstrual cycles too. So artificial light appears to  suppress how much melatonin we make, and it turns out that melatonin could be involved in regulating some  period-related hormones.

And essentially, the suggestion is that more light means less melatonin which means shorter periods. So there may well be a connection  between what’s happening in the sky and what happens  with people’s menstrual cycles, it’s just not the one we’ve all  been fixated on for centuries. And even though our ancestors  were totally wrong when they tried to connect the moon’s  phases to their time of the month, now you can see there was a bit more logic behind that connection than you’d think.

I hope we’ve helped to shed a  little moonlight on the topic. [♪ OUTRO]