Everyone knows that the moon affects Earth. The reason seawater moves in high and low tides twice a day is the moon's gravity. Claims that the moon also affects the human body have continued. In particular, because women's menstrual cycles are similar to the moon's cycle of about 29.5 days, people thought the two were related.
A research team at the University of Würzburg in Germany published findings that indirectly support the claim that the moon affects the human body. With the spread of light-emitting diode (LED) lighting and smartphones, women's menstrual cycles no longer follow the moon's rhythm. The study was published in the international journal Science Advances on 24th (local time) last month.
The researchers analyzed women's menstrual cycle data recorded over the past 50-plus years. The subjects were 176 European women who recorded their menstrual cycles for an average of 5 years and 8 months.
The analysis found that before 2010, when smartphones and LED lighting became widespread, menstrual cycles were partly synchronized with the moon's cycle, but after 2010 this phenomenon almost disappeared.
Based on these results, the researchers raised the possibility that humans also have a "lunar clock." A lunar clock refers to a phenomenon in which biorhythms are regulated by changes in the moon's gravity or brightness.
In fact, many marine species time their spawning to the moon's cycle. The researchers said humans, too, were influenced by moonlight in the past, but that artificial lighting, which is far brighter than moonlight, has masked the moon's signals and broken that rhythm.
The human eye is particularly sensitive to the blue light emitted by LED lighting and smartphone screens. This light stimulates the brain at night as if it were daytime, suppressing the secretion of sleep hormones and disrupting circadian rhythms.
Still, Charlotte Förster, a neurobiologist and professor at the University of Würzburg, said, "Lunar clocks are widely found in marine organisms, but in humans they have not yet been clearly proven," and noted, "This study shows a correlation, but it does not confirm causation."
Claude Gronfier, a senior researcher at the Lyon Neuroscience Research Center in France who was not involved in the study, also said, "It is an interesting result, but it needs to be interpreted cautiously, given that the research was conducted on a relatively small group." He added, "Changes in temperature caused by global warming, as well as participants' age or sleep habits, may also have influenced the results," emphasizing the need for further verification.
There have been previous studies suggesting that the moon's cycle affects human behavior. The moon moves between new moon, when it is positioned between the sun and Earth and is not visible from Earth, and full moon, when it is on the opposite side of Earth from the sun. In 2021, researchers at Yale University in the United States reported that people tend to sleep 30 to 90 minutes less around the full moon. This phenomenon was observed even in environments where nights are bright, such as cities.
Scientists believe that changes in the moon's brightness, along with the gravitational pull on Earth that varies with the full and new moon cycle, also affect the human body. In 2018, Thomas Wehr, an American psychiatrist, reported in the international journal Translational Psychiatry that people with bipolar disorder (manic-depressive illness) have a higher rate of switching to mania or depression during full or new moon periods.
References
Science Advances (2025), DOI: https://doi.org/10.1126/sciadv.adw4096
Science Advances (2021), DOI: https://doi.org/10.1126/sciadv.abe0465
Translational Psychiatry (2018), DOI: https://doi.org/10.1038/s41398-018-0203-x