By Ellen Phiddian

Copyright abc

Ever noticed you’re on your period during a full moon — or a new moon?

Throughout history, the similarity between women’s menstrual cycles and the lunar cycle has led people to wonder if the two are linked.

A new study has suggested they are — or at least, they used to be — but independent experts are not convinced.

While health and lifestyle factors influence the timing of periods, there remains little evidence they align with any other cycles, such as lunar cycles or menstrual cycles of housemates.

With a super full moon on the horizon this week, let’s take a closer look at the new study, and unpack the evidence.

What did the study find?

The study, published in Science Advances, reported on a collection of records kept by women, noting down when their periods had started.

The researchers, led by Julius-Maximilians University of Würzburg neurobiologist Charlotte Helfrich-Förster, analysed records kept by 176 women, mostly from European countries, and none of whom had used contraceptives.

The team compared the menstrual records against various lunar cycles, including what phase the Moon is in, when it is closest to Earth, and when eclipses are expected.

The researchers also examined Google Trends data for searches related to menstruation internationally.

According to the researchers, they found several correlations between the full or new moon phases and menstrual cycles in data prior to 2010, but the cycles aligned less frequently after this.

The team suggested that artificial light might be responsible for this weakening link between the Moon and menstruation, with people increasingly exposed to LED lights and the glare of smartphone screens at night over the past 15 years.

The researchers admitted their findings were only correlations and they were unable to establish a causal link.

But Professor Helfrich-Förster did not believe the correlations happened by chance.

“We cannot rule out the possibility that our observations are coincidences. However, I think this is highly unlikely,” she said.

Other scientists, however, said the study had many limitations, and can’t rule out that the results happened by chance.

Gino Pecoraro, an obstetrician and gynaecologist and adjunct researcher at the University of Queensland, pointed out the study was small and relied on self-reported data.

“The study is hampered by small numbers and difficulties in objective measurements,” Dr Pecoraro said.

Emmallee Ford, a researcher in reproductive health at the University of Newcastle, said the researchers pulled together disparate evidence and looked for multiple connections between various astronomical and menstrual cycles.

And periods can be unpredictable.

How long is a menstrual cycle?

Menstrual cycles usually last for about four weeks, with anything from 21 to 35 days considered normal.

“Sometimes, they don’t even sync with our own bodies, and they’re irregular our whole lives,” Dr Ford said.

Periods are triggered by changes in the levels of sex hormones oestrogen and progesterone when an egg released into the uterus has gone unfertilised.

Health, lifestyle, medications and age can all prompt big changes in one’s menstrual cycle too.

Is there any connection between menstruation and the Moon?

Dr Ford said studies from the 1980s and earlier often suggested a connection between menstrual cycles and the lunar cycle, which takes 29.5 days.

But in recent years, with the advent of period-tracking apps and much larger troves of data, this connection has vanished.

Period tracking app Clue, for instance, published a non-peer-reviewed report in 2017 finding no correlation between periods and moon phases, based on data from 1.5 million users of the app.

The researchers in the new study speculated that gravitational interactions between the Earth, Moon and Sun could explain some of the menstrual syncing, pointing out menstrual cycles and Google searches in some countries seemed to follow lunar cycles in January when the Earth is closest to the Sun each year.

Jonti Horner, an astrophysicist at the University of Southern Queensland, said this idea “really doesn’t make sense” from an astronomical perspective.

While the Earth is closest to the Sun in early January — a point known as perihelion — Professor Horner said the variation in distance was small, prompting only a small effect on gravity.

Also, he said, any gravitational effect should be seen in December and surrounding months as well as in January, because the Earth stayed closer to the Sun for a long time.

“We’re at a distance very similar to perihelion distance for a lot longer,” Professor Horner said.

It is also unlikely that moonlight could affect menstrual cycles, according to Sean Cain, a researcher at Flinders University.

This is because moonlight is very dim compared to sunlight and other light sources.

“Unless it’s a full moon, you’ve got a perfectly clear night, and you’re staring at it, it’s unlikely to have an effect.”

Can artificial light affect the timing of periods?

Yes, it potentially can.

But the researchers did not directly collect data on light exposure, making it difficult to investigate the light-menstruation relationship more closely.

Although moon cycles or moonlight are unlikely to influence the timing of menstruation, light has been shown to affect hormones and health in other situations, Dr Pecoraro said.

“There is some data that light does affect functioning of the brain, and the brain does affect everything in the body,” he said.

“We know that with increasing sunlight you get more release of serotonin … and in darkness you get melatonin.”

Low amounts of sunlight in winter have been shown to cause conditions like seasonal affective disorder, which can be treated with light therapy.

“We continue to find out more about the interplay between the natural environment and how our bodies work,” Dr Pecoraro said.

Professor Cain said being exposed to bright lights at night can disrupt your “circadian rhythm” — your internal clock, which governs a range of bodily functions, including hormone release.

“Essentially, you go from having nice rhythms where your body knows when it’s day or night to low amplitude rhythms where your body’s a little more confused,” he said.

This means that high levels of artificial light at night could influence menstrual cycles.

“In women who are shift workers and therefore get a lot of light at night, they’ll often lose their menstrual cycles or have more irregular menstrual cycles, and it’s very plausible that that is due to light disrupting the circadian clock.”

Professor Cain said exposure to light at night could affect one’s health in general.

“The best thing is getting loads of natural light in the day, and complete darkness at night.”

Can you sync your menstrual cycles with friends?

Alongside lunar cycles, there’s also a common legend that people who live or spend a lot of time together will have their menstrual cycles align.

But the idea periods can sync up is a myth.

“There’s been a lot of research into menstrual cycles syncing with each other, and I’m really sad to say it’s been also debunked,” Dr Ford said.

“I actually found that hard to believe myself.”

With a period lasting for a few days every month, it’s likely it will overlap with other periods, or other periodic events, from time to time.

But while you might remember the time your period started at the same time as your friend’s or family member’s, you probably haven’t clocked all the times they didn’t match.

“You’re more likely to notice when something does happen as opposed to every single time it doesn’t happen,” Dr Ford said.

She added that “there was nothing wrong with” comparing one’s menstrual cycle to a lunar phase, or other cyclic events.

“But you can’t really make serious decisions or generalisations based on that.”