Although most people today are no longer dependent on the light of the moon thanks to artificial light sources, the celestial body apparently influences our natural rhythms. Shortly before the full moon, many people sleep later and for a shorter time. In addition, long-term observations of the female cycle show that it adapts to lunar cycles from time to time – especially in phases in which the gravitational force that the moon exerts on the earth is strongest. The more people are exposed to artificial light at night, the weaker the influence of the moon becomes. However, to some extent, the effects can be seen even in city dwellers.
There are numerous myths and scientifically hardly proven theories about the moon and its influence on our lives. Do we really sleep worse when the moon is full? Are women more fertile when their monthly cycle is in harmony with the moon? Are more children born when the moon is full? Many studies of varying quality have already addressed these questions – with sometimes contradicting results.
Indigenous communities and big city students
A team led by Leandro Casiraghi from the University of Washington in Seattle has now examined members of indigenous communities and students from the city of Seattle to see whether and to what extent their sleep changes during the course of the moon phases. While earlier studies often relied on data from the sleep laboratory or had the test subjects keep a sleep diary themselves, Casiraghi and colleagues used small devices that the test subjects wore on their wrist for several weeks and that precisely recorded their sleep-wake rhythm.
98 members of the Toba-Qom indigenous community from Argentina took part in the study. Some lived in rural areas with no access to electricity, others lived in rural areas but had an electric light source at home, and a third group lived in urban areas with full access to electricity. In addition, the researchers included 464 students from Seattle – a city with high levels of light pollution in which the moonlight is barely noticeable at night.
Sleepless in Seattle
The result: “We see a clear modulation of sleep by the moon, with falling asleep later and shorter sleep duration in the days before a full moon,” says Casiraghi’s colleague Horacio de la Iglesia. “And although the effect is more robust in communities without access to electricity, it is also evident in communities with electricity, including students at the University of Washington.” In the three to five days before the full moon, Toba-Qom members slept for about 20 minutes shorter than at new moon. Groups with less access to electric light were more affected by the changes in moonlight.
To the surprise of the researchers, the same effect was also found in a less pronounced form in students from Seattle: Shortly before the full moon, they went to bed later and slept shorter overall. “We hypothesize that the patterns we observed are an innate adaptation that allowed our ancestors to take advantage of this natural source of evening light that occurred at a specific time during the lunar cycle,” says Casiraghi.
Menstruation according to moon phases?
In another study, researchers working with Charlotte Helfrich-Förster from the University of Würzburg found evidence that the female cycle is also to some extent related to the moon. To do this, they evaluated the menstrual diaries of 22 women, some of whom had documented their cycle over 32 years. “As far as we know, this type of analysis of long-term data has not yet been used,” says Helfrich-Förster.
The research team related the women’s records to the lunar cycles. “From a scientific point of view, the moon has three different cycles that periodically change its luminosity and the force of gravity with which it hits the earth,” explains Helfrich-Förster. On the one hand, this includes the change between full moon and new moon with a periodicity of 29.53 days. On the other hand, the moon is changed with a cycle of 27.32 days, sometimes north and south. And thirdly, due to its elliptical orbit, it is sometimes closer to the earth, sometimes further. This cycle lasts an average of 27.55 days.
Influence of light intensity and gravity
All three cycles influence the intensity of the moonlight as well as the gravitational force exerted by the earth’s satellite. And to a certain extent, according to the data, they also shape the female monthly cycle. However, the synchronization occurs only sporadically: In women under 35, menstruation occurred synchronously with the full or new moon in just under a quarter of the recorded time, in women over 35 only in a tenth of the recorded time. The more women are exposed to artificial light sources at night, the weaker the relationship becomes.
However, according to the researchers, it is not just the light of the moon, but also its gravity that affects the onset of menstruation. In years when the three different lunar cycles formed certain constellations, the proportion of women with a lunar synchronous cycle was highest. However, due to the small database, the informative value of this study is limited.
The influence of gravity could also explain some of the findings of the sleep study: For example, some of the Toba Qom participants showed additional changes in their sleep rhythm when the gravitational force of the moon was particularly strong. “Future research should focus on exactly how the moon affects us,” says Casiraghi. “Does it work through our innate circadian clock? Or through other signals that affect the timing of sleep? There is a lot to understand about this effect. “
Sources: Leandro Casiraghi (University of Washington, Seattle) et al., Science Advances, doi: 10.1126 / sciadv.abe0465; Charlotte Helfrich-Förster (University of Würzburg) et al., Science Advances, doi: 10.1126 / sciadv.abe1358