Your Menstrual Cycle is Not Caused By the Moon

Menstrual Cycle

If you like seeing where you’re going during a stroll on a beach far from streetlights, you can thank the Moon for that. If the ocean makes its way up the beach and cools your feet during the high tide, you can thank the Moon for that too, and also for the low tide that enlarges the beach, allowing you to lay out your towel, or to run around flying a kite. In times past, it also made sense to thank Earth’s only natural satellite for the extra hours of harvesting time during an especially large full Moon. You can thank the Moon also for being the basis of the calendars in certain traditions. The New Year in the Jewish tradition that’s being celebrated right now, for instance, is timed with a calendar that depends on the lunar cycles. So you can thank the Moon for that honey-cake, apples, and brisket, if you’re Jewish, and otherwise, for the suspension of alternate side-of-the-street parking in New York for two days this week.

But don’t thank the Moon for your menstrual cycle and the ovulation and monthly period that are attached to that cycle. It’s true that the synodic month (the period of the Moon’s phases as viewed from Earth) and the average human menstrual cycle have similar lengths –about 29.5 days and 29 days, respectively. But science shows that the similarity is, in all likelihood, coincidental.

For millennia, the similar cycle lengths have linked the Moon and menstruation in the human mind. Consequently, the two phenomena are linked linguistically. The words menstruation and menses both derive from the Latin word for month (mensis), which relates to the Greek word for Moon (mene). The linguistic connection exists because, for most of recorded history, humans assumed a causal connection from the Moon to the menstrual cycle. In early times, the cause was thought to be metaphysical, involving a lunar goddess. In modern times, this thinking continues in various new age spiritual beliefs, but it also has been replaced with ideas that our closest celestial neighbor affects biology through mechanisms involving gravity, light, or a combination of both.

But just because someone utilizes terminology and concepts from modern science to form an hypothesis (an idea of how something may work) does not mean that the hypothesis is correct. The hypothesis must be tested. To pass the test, the hypothesis must be plausible, and the testing must be designed to account for any factors that can confuse the result, making the hypothesis appear correct, when it actually isn’t. In science, testing is designed to disprove an hypothesis. If it’s unable to disprove the hypothesis, and if repeated testing comes out the same, at that point the hypothesis is accepted –at least until somebody can find a way to disprove it—then it is elevated to what in science is called a theory, which means that we are very sure that this is how the phenomenon works. When an idea reaches theory status in science, our level of certainty is high enough for us to operate under the assumption that the theory represents reality.

The term pseudoscience refers to a collection of beliefs, ideas, and practices that people mistake for science. In pseudoscience, proponents of an idea often misuse the term theory. Typically, they apply it to speculations that form the basis of their belief systems. Pseudoscientists apply scientific terminology to make their beliefs sound scientific. That’s what’s happening in the case of the menstrual cycle, where modern day Moon worshippers and astrologers not only misuse the word “theory”, but also may tell you that the Moon affects human physiology by way of gravity and light.

Let’s unpack these ideas, first considering light, since the possibility that moonlight could modulate the cycle of menstruation and ovulation is plausible. That’s because the amount of light that hits the retina of your eye, and the wavelength of that light (a physical property that determines the light’s color and energy level), affects a gland in your brain called the pineal gland. When you’re in darkness, your pineal gland secretes a hormone called melatonin, which makes you sleepy, but sunlight –or artificial light on the blue end of the light spectrum– inhibits the production of melatonin. That’s why light wakes you up. The amount of sunlight changes seasonally. If you were sleeping outside, far from the city lights and streetlights, your melatonin level in your blood at night would be higher on moonless nights than on nights with a full moon. And you’d experience intermediate melatonin levels during the intermediate lunar phases, when the Moon is not full and not showing in the sky for the entire night. Different hormones in the body affect one another, directly or indirectly, and that’s the basis of the idea that the cycle of the Moon’s phases underlies the menstrual cycle in humans. The problem with this idea, however, is that it’s exquisitely difficult to test the hypothesis, because there is so much artificial lighting outside, and most people sleep inside, at least in areas of the world where you might conduct such research. Thus far, there is no good evidence –no results from scientific experiments with good methodology and good statistical analysis– to say that it actually happens.

As for the gravity idea, it hasn’t been tested at all, but it’s not even plausible, because the amount of gravitational change as the Moon’s position with respect to one’s location on Earth changes through the month is fairly small on an individual person. Yes, the Moon causes ocean tides. Huge amounts of water shift throughout the day as the Earth rotates, thereby changing the local gravitational environment, but what you’ll hear from lunar cycle menstrual advocates is that the gravitational change must be pulling and pushing follicles in the ovaries. This idea is complete rubbish. We know from gravitational biology experiments conducted in spaceflight that small gravitational changes don’t exert such an effect on cells and tissues. Nor do menstrual cycles change when women go into space, or to Earth’s poles, in ways that the gravity hypothesis of the lunar cycle underlying the menstrual cycle would predict that the cycles should change.

Four years ago, there was a study of women showing no synchrony between the menstrual and lunar cycles, whereas an earlier study –published in 1980 and cited frequently as evidence for a connection– actually showed only a very weak synchronization effect. Furthermore, while the average length of a human menstrual cycle is about 29 days, there is a very wide range. Many women have much shorter cycles while others have much longer cycles, and finally we should mention that the 29 day average cycle length is not the case in many of our fellow primates. In chimpanzees, for instance, our closest living relatives, the menstrual cycle length is 35 days. And as for non-primate mammals, they have what’s called an estrous cycle, which is quite a bit different, and certainly not related to the Moon.

Taking all of this together, it appears that the belief that Moon underlies the female cycle is an urban myth.

David Warmflash
Dr. David Warmflash is a science communicator and physician with a research background in astrobiology and space medicine. He has completed research fellowships at NASA Johnson Space Center, the University of Pennsylvania, and Brandeis University. Since 2002, he has been collaborating with The Planetary Society on experiments helping us to understand the effects of deep space radiation on life forms, and since 2011 has worked nearly full time in medical writing and science journalism. His focus area includes the emergence of new biotechnologies and their impact on biomedicine, public health, and society.

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