Recent science has confirmed: Women's menstrual cycles are as long as the lunar cycle.

Throughout history, human daily life has been linked to changes in the sky, such as the moon's different "faces" of light and dark. One of the many myths surrounding the moon is that the phases of the moon influence our daily moods and feelings. In particular, a woman's menstrual cycle is usually 28-35 days , which is quite close to a lunar cycle or a lunar month of about 29.5 days. Is this a coincidence or a regular phenomenon?

According to science writer Luis Villazon - an expert answering questions on BBC Science Focus, a woman's menstrual cycle is only as long as the lunar cycle and does not coincide with a specific lunar phase.

He cites the famous biologist Charles Darwin as saying that the human 28-day menstrual cycle is evidence that our ancestors lived on the coast and needed to pace their steps with the tides. Certainly, the phases of the Moon influence the behavior of many animals. 

Fiddler crabs are more active during the full and new moons because the tides are higher, making their burrows more visible. Nymphs avoid feeding at night during a full moon because the extra light makes them more vulnerable to being eaten by owls. But the human menstrual cycle is only the length of a lunar month, not a specific phase of the moon.

There is some evidence of this coincidence, but the effect is weak, according to a frequently cited study published in the American Journal Of Obstetrics And Gynecology in 1980. In a sample of 312 women, 244 had cycles longer than 29 days or shorter than 27 days, and only 70 percent of the remaining women actually started menstruating within two weeks of the full moon.

(Photo: vatlythienvan.com)

If it is convenient to tie our reproductive cycles to the lunar months, you might expect other animals to do the same. Orangutans and marsupials have 28-day cycles, but our closest relatives—chimpanzees—have 35-day cycles. Non-primate mammals have estrus cycles that operate differently from their menstrual cycles, but none of these species have reproductive cycles that coincide with the phases of the Moon.

However, old myths are now beginning to be scientifically proven to be true.

According to Space, a new study published in the journal Science Advances in late January this year shows that the fairy tales about the moon have partly come true. The researchers said that although all the myths surrounding the connection between the lunar cycle and menstruation are not necessarily true, there may still be a connection between the lunar cycle and the menstrual cycle.

In this study, scientists examined long-term data on the date of menstrual onset over an average of 15 years from the records of 22 women under and over the age of 35. Comparing this data with the fluctuations of the lunar cycle, the scientists found that, in women whose menstrual cycles were longer than 27 days, there was a 'discontinuous coincidence with cycles that influence lunar brightness .' This coincidence gradually disappeared over time as the women aged, and the association decreased with increasing exposure to artificial light.

Menstruation is also affected by changes in the moon's gravitational pull. (Photo: Health Journal)

"Menstrual cycles also corresponded to the tropical month 13.1% of the time in women 35 and younger, and 17.7% of the time in women over 35, suggesting that menstruation is also influenced by changes in the gravitational pull of the moon ," the study concluded.

A tropical month is 27.321 days, or 27 days 7 hours 43 minutes 4.7 seconds. The tropical month is based on the vernal equinox, which is the time it takes for the moon to return to 0 longitude.

A lunar month , or synodic month, is an average of 29.5 days long, the time between two full moons or two new moons, the time it takes for the moon to return to its original position in the sky relative to the sun. A lunar month is longer than a tropical month because as the moon revolves around the Earth, the Earth also revolves around the sun, so the moon needs a few more days to return to its original position in the sky relative to the sun (as seen from Earth).

The moon not only affects our menstrual cycles, but it also impacts our sleep.

Another study, published in Science Advances in January, found a correlation between sleep cycles and menstrual cycles. This study (independent of the above study) was conducted by a team of scientists from three universities: Washington University, Yale University (USA) and the National University of Quilmes (Argentina).

Accordingly, during the days when the moon is about to be full, we tend to go to bed later and sleep a few hours less.

In this study, participants were university students in Seattle, Washington and people living in indigenous communities in northern Argentina, two different environments with diverse access to electricity because artificial lighting can affect participants. 98 people living in three Toba-Qom indigenous communities in Formosa, Argentina and 464 university students in the Seattle area were given sleep-tracking wristwatches.

As a result, the link between sleep and lunar cycles was present in both communities without electricity in Argentina and areas with electricity in the United States, and was more pronounced in the areas without electricity. The nights before a full moon are when people sleep the least and go to bed the latest, and these are also the nights when the sky is brighter because the crescent moon is getting brighter.

"We saw a clear lunar sleep adjustment, with reduced sleep hours and later bedtimes in the days before the full moon," Space quoted lead researcher Horacio de la Iglesia, a professor of biology at the University of Washington. "Although this effect was stronger in communities without electricity, it was also present in communities with electricity, including Washington University students ," Space quoted lead researcher Horacio de la Iglesia, a professor of biology at the University of Washington.

"We hypothesize that the observed patterns are innate adaptations that allowed our ancestors to take advantage of natural nighttime light at a specific time in the lunar cycle ," said study author Leandro Casiraghi, a postdoctoral fellow in the department of biology at the University of Washington.