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Moon Cycles Exert an Influence on Menstruation and Sleep Patterns

The moon may not drive people mad or trigger werewolf transformations, but lunar cycles do, in fact, appear to affect human biology by influencing menstrual and sleep cycles, according to two studies published in the January 29 issue of Science Advances.

An analysis of sleep cycles in rural and urban indigenous Argentinians as well as urban U.S. university students found that people fall asleep later and sleep less on the nights leading up to a full moon, when moonlight fills the night sky after dusk. The findings suggest that human sleep is synchronized with the moon's phases regardless of ethnic or cultural differences — and even in locations where light pollution outshines moonlight.

In a related study, an investigation of long-term menstrual cycle records maintained by 22 women shows that women with cycles lasting longer than 27 days intermittently synchronized with moon phase cycles and the moon's gravitational pull. This synchrony was lost as women aged and when they were exposed to artificial light at night.

The researchers hypothesized that human reproductive behavior may have been synchronous with the moon during ancient times, but that this changed as modern lifestyles emerged and humans increasingly gained exposure to artificial light at night.

"I was surprised and fascinated at the same time, although our study is not at all the first report of an influence of the lunar cycle on humans and in many marine animals reproduction is strongly coupled to the moon cycles," said Charlotte Helfrich-Förster, a professor of neurobiology and genetics at the University of Würzburg in Germany. "Nevertheless, most researchers are skeptical about an influence of the moon on human life."

That Time of the (Lunar) Month

While some older research suggests that women with menstrual cycles that most closely match lunar cycles may have the highest likelihood of becoming pregnant, lunar influence on human reproduction remains a controversial subject. Helfrich-Förster admitted she was skeptical when she first noticed women saying that their periods coincided with the full moon — but her skepticism wavered as she received records of menstrual cycle onsets from different women over the course of several years. When she plotted these records in relation to the full moon, Helfrich-Förster found, to her amazement, that the women's periods did actually occur during the full moon for certain time intervals — but never indefinitely.

"I started to ask more and more women about such recordings," said Helfrich-Förster.

The scientist and her colleagues gathered menstrual cycle data kept by 22 women spanning an average of 15 years, including records from 15 women ages 35 or younger and 17 women over 35. To uncover any times during which the women's menstrual cycles occurred in sync with lunar cycles, the researchers displayed the data as graphs that show time-based relations along with fluctuations in the moon's cycles.

Unlike previous studies of this phenomenon, which tended to investigate large numbers of women over short periods of time, Helfrich-Förster and colleagues analyzed records from a small number of women over long periods of time.

"I think that many of these studies missed a synchronization to the lunar cycle, because there is a high variability between individual women and because all women respond to the lunar cycle only during a relative short time interval in their life, ranging from few months to several years," said Helfrich-Förster.

The researchers found that most women's menstrual cycles aligned with the synodic month (the time it takes for the moon to cycle through all its phases) at certain intervals, with the periods of women age 35 or younger synchronizing with the full or new moon for 23.6% of the recorded time, on average. Women over 35 only showed this synchrony about 9.5% of the time.

Menstrual cycles also aligned with the tropical month (the 27.32 days it takes the moon to pass twice through the same point in its orbit) 13.1% of the time in women 35 years and younger and 17.7% of the time in women over 35, suggesting that menstruation is also affected by shifts in the moon's gravitational pull. Furthermore, the researchers observed greater synchronization between lunar and menstrual cycles during long winter nights, when women experienced prolonged exposure to moonlight.

"It is hard to say what [these findings] mean for our lives, but in the worst case fertility is to some degree dependent of the moon cycle and suffers from our modern lifestyles in large cities," said Helfrich-Förster.

By The Light of the Moon

To investigate how the moon influences sleep, another team of researchers broke away from their own big-city lives and spent several years studying Toba/Qom indigenous communities in Argentina. When community members told stories revealing that social activities ballooned on moonlit nights, the researchers decided to investigate whether the moonlight disrupted their sleep.

"Our hypothesis that during moonlit nights sleep would be inhibited also predicted that the effect would be stronger in communities that had no access to electricity, who would rely more on the moon for a nocturnal source of light than communities that have free access to electricity," said Horacio de la Iglesia, a professor of biology at the University of Washington and an author of the study. "In this sense, these Toba/Qom communities were ideal to test the hypotheses of this study."

While moonlight has been previously shown to affect nocturnal activity in many organisms, the question of whether the moon's cycles affect human sleep and nighttime wakefulness has, as with the question of moon cycles and human reproduction, remained controversial.

"[Previous] studies were mainly based on sleep recordings from laboratory studies that were not designed to longitudinally detect changes in sleep in each study subject," said de la Iglesia. "Instead, they compared sleep parameters from several subjects retrospectively, and detected moon phase differences. Other studies that had been done in the field longitudinally used sleep diaries."

A new study suggests human sleep is synchronized with the moon’s phases even in locations where light pollution outshines moonlight. | Kiyomi Taguchi / University of Washington

Sleep may be synchronized with moon phases even in places with light pollution. | Kiyomi Taguchi / University of Washington

To address this research gap, de la Iglesia and colleagues asked 98 people within three indigenous Argentinian Toba/Qom communities to wear wrist sensors — a more accurate and objective strategy than sleep diaries — to measure their wake/sleep cycles over one to two months. The individuals each lived in an urban community with full access to electricity, a rural community with limited electric light access, or a rural community without electricity.

"We assessed sleep quantitatively and longitudinally within the same participants for a full lunar cycle, sometimes even two full lunar cycles in the same participant," said de la Iglesia. "This strategy allowed us to quantitatively and objectively determine sleep timing throughout lunar phases individually. Because of the inherent variability of sleep studies — every person sleeps slightly more or less, or goes to bed earlier or later than others — this strategy was critical to detect the synchronization between sleep parameters and the moon phases."

Goodnight Moon

Across all communities, sleep patterns were clearly modulated by the moon's cycle, with each person's sleep duration varying by 20 to 90 minutes throughout the cycle. Participants went to sleep the latest and slept the least three to five days before the night of a full moon.

The researchers also observed that groups with less access to electric light were more affected by shifts in moonlight, with people in communities without any electricity sleeping 25 minutes longer on dark nights than on fully moonlit nights. In comparison, people with limited electric light access slept 19 minutes longer, and those with full electricity access slept 11 minutes longer.

Additionally, de la Iglesia and colleagues analyzed sleep recordings from 464 University of Washington undergraduate students, which led to a surprise — the urban students displayed moon-dependent sleep patterns similar to those of the Toba/Qom communities.

"We were extremely surprised to find that the effect, although smaller, was present regardless of the access to electricity, and in fact, even in university students living in Seattle," said de la Iglesia. "We believe that the gravitational pull cycles associated with the lunar month may predispose humans to be particularly sensitive to the effects of light — moonlight or artificial — on the nights close to the full moon light."

"Our future plans are to determine how specific moon phases make humans prone to shorter or delayed sleep," he added. "We have found a very robust phenomenon — that human sleep timing is synchronized with moon phases. Now we want to know how this happens."

[Credit for associated image: György Soponyai/ Flickr]


Shannon Kelleher

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