How Lunar Phases Affect Sleep And Menstrual Cycles

A few days before the full Moon is when we are most exposed to moonlight, and also when we sleep the least, even in cities where we may not note Moon phases at all. Image Credit: University of Washington

The Moon gets plenty of credit and blame for human activities, but on investigation most claims turn out to be mere astrology; good stories lacking reliable evidence. Nevertheless, two newly published studies find statistically significant evidence for lunar effects on two old favorites, sleep and menstruation, albeit partially suppressed by electric lights.

Professor Horacio de la Iglesia of the University of Washington gave sleep-tracking wristwatches to 98 members of Toba-Qom Indigenous communities in Formaosa Argentina, and compared their sleep patterns with the amount of moonlight.

The Toba-Qom were chosen because many live in communities with little or no access to electricity. With more opportunities to do things after the Sun had set, this group went to bed later and slept less as the Moon approached full. Even in urban areas, where street lights outshine the Moon, sleep was similarly reduced, de la Iglesia reports in Science Advances.

Toba-Qom participants slept 46-58 minutes more around the dark of the Moon as when it was nearly full, falling asleep about 20 minutes later.

Horacio de la Iglesia used sleep data from 464 college Seattle-area students collected for another study to check his findings. "Although the effect is more robust in communities without access to electricity, the effect is present in communities with electricity, including undergraduates at the University of Washington," he said in a statement.

In the light of these observations, it's almost strange that efforts to confirm links between the full Moon and crime or car accidents haven't had more success.

Toba-Qom with little or none electricity also showed what de la Iglesia calls a “semilunar” 15 day sleep cycle. Although uncertain of the cause, he thinks it may have to do with gravitational effects, which are stronger when the Sun and Moon’s gravity aligns twice a month.

Even when city lights make us unlikely to notice the Moon, or clouds block it (famously common in Seattle) our sleep patterns still align, going to sleep later and sleeping less when the Moon is nearly full.See the interactive version here. Image Credit: Rebecca Gourley/University of Washington

The same edition of Science Advances carries another paper, with one of the same authors, exploring the Moon influence on menstruation. The fact the typical menstrual cycle so closely matches the time between one full Moon and the next has long been assumed not to be a coincidence.

“In many marine species and some terrestrial species, reproductive behavior is synchronized with a particular phase of the lunar cycle,” the paper notes. Immense coral spawning events represent a particularly dramatic example. Yet it hasn’t been clear if humans have a similar evolutionary benefit, or if period length is just a legacy of our ancestry, long since decoupled from the Moon’s phases.

Past studies have reported that 28 percent of women who menstruate regularly have cycles lasting within a day of the 29.5 day length of the lunar cycle – a minority, but a far greater concentration than at any other number of days. Moreover, 28-29 day periods are associated with a higher chance of conception. However, investigations of whether menstruation is particularly likely to line up with a specific lunar phase have produced conflicting results.

Lead author Professor Charlotte Helfrich-Förster of the University of Würzburg  used just 22 women’s records to explore this issue, far fewer than previous studies, but over a much longer time. Participants recorded their periods for up to 32 years, along with details of their sleeping conditions, such as levels of exposure to artificial light.

Before age 35 participants had periods that synchronised with the Moon 23.6 percent of the time. After 35 this dropped to just 9.5 percent, reflecting a decrease in average period length to 26 days. Synchronization was much more common for those living in rural areas and going to bed early than those more exposed to artificial light. The most common phase to match with was shortly before full, when evening light is greatest.

Helfrich-Förster also found  a weak correlation between gravitational peaks and menstrual cycles, matching de la Iglesia’s theory light is not the Moon’s only way of influencing us.


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