Researchers Find That The Full Moon Affects Sleep (But They Aren't Sure Why)

mbg Senior Sustainability Editor By Emma Loewe
mbg Senior Sustainability Editor
Emma Loewe is the Senior Sustainability Editor at mindbodygreen and the author of "The Spirit Almanac: A Modern Guide To Ancient Self Care."
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Historically, claims that moon cycles somehow affect human behavior have been largely anecdotal, without rigorous research to back them up. But a new study published today in Science Advances presents scientific evidence that the full moon does indeed mess with people's sleep, for reasons that remain largely unknown.

Studying celestial sleep patterns in rural and urban Argentina.

Under the leadership of Horacio de la Iglesia, Ph.D., a professor of biology at the University of Washington (U.W.), the research team began by studying three Toba-Qom Indigenous communities in northern Argentina. Each one had a different level of access to electricity and modern amenities: One was based in an urban area, one had limited sources of lighting and electricity, and one was completely off the grid.

The research team figured that individuals in these last two groups would be the ones whose sleep was affected on full moon evenings.

"Our hypothesis was that if we did find an effect of the moon on sleep, it would only be present in these communities that had no access to electric light, or very limited access, because they would be the ones taking advantage of the light of the moon," Leandro Casiraghi, the study's lead author, tells mbg via Zoom.

The urban group was meant to act as a sort of "cultural control" and demonstrate that the further removed we become from natural cycles, the less they affect us.

After equipping participants with wrist monitors (think highly sophisticated Fitbits) and monitoring their sleep over the course of one to two moon cycles, the researchers did indeed find that those who lacked electricity went to bed later in the evening and slept for a shorter period of time in the three to five days leading up to the full moon. But here's the kicker: The same pattern emerged among urban dwellers, too, challenging their original theory.

"That was pretty surprising," Casiraghi recalls. "We looked at [the data] like 10 times before we said 'OK this is actually happening.'"

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Something about the moon seems to affect sleep — but it's not its light.

Confident in their findings on indigenous populations, the research team then turned to existing data they had collected on the sleep patterns of 464 U.W. students living in Seattle.

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Sure enough, this population experienced similar shifts in sleep. While their sleep quality wasn't necessarily worse around the full moon (that's quantified by factors like time spent in REM sleep, but this study only looked at sleep timing and duration), they went to bed later and spent less time asleep in the nights leading up to it.

This told researchers a few things: For starters, sleep delays from the full moon were way more widespread than they initially anticipated. And on top of that, they probably weren't the result of moonlight but something more unseen.

"Your computer is probably brighter than the moon," Casiraghi says, "so it doesn't explain why Seattle students would be affected." In other words, since the lighting and technology of modern life totally disconnect most people from moonlight, there must be another mechanism at play.

Moving forward, the team hopes to shine some light on what it could be. One theory is that the moon's gravitational pull, responsible for the planet's tides, can somehow cause our sleep patterns to rise and fall too.

This natural cycle is difficult to manipulate and measure in a lab setting, though, so don't expect confirmation anytime soon. This is one celestial phenomenon that will remain a mystery a little longer. In the meantime, maybe hit the hay a little early when the moon outside your window is shining bright, just in case.

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