Sleep and circadian rhythms have long been associated with the powerful effects of the sun cycle. But in recent years, a growing number of studies have suggested that another familiar celestial body might also be impacting your ability to get a restful night’s sleep: the moon.

A paper published in the journal Science Advances found that people tend to have a harder time sleeping in the days leading up to a full moon. Researchers reported that sleep patterns among the study’s 98 participants appeared to fluctuate over the course of the 29½-day lunar cycle, with the latest bedtimes and least amount of rest occurring on nights three to five days before the moon reaches its brightest phase. They found a similar pattern in sleep data from another group of more than 460 people.

Ahead of the full moon, it took people, on average, 30 minutes longer to fall asleep and they slept for 50 minutes less, said Leandro Casiraghi, the study’s lead author and a postdoctoral researcher in the Department of Biology at the University of Washington.

“What we did is we came up with a set of data that shockingly proves that this is real, that there’s an actual effect of the moon on our sleep,” Casiraghi said.

Previous studies examining the moon’s effect on sleep have produced contradictory results. Some research has found minimal or no association between the lunar cycle and sleep, while other studies have demonstrated correlations in controlled settings. The findings of the Jan. 27 paper support existing observations that there is a link, Casiraghi said. But, he noted that the work he and his fellow scientists did is distinct from past research by a critical difference in methodology.

“This was real life,” he said, referring to the part of the study that actively monitored participants over lunar cycles. Other studies have primarily focused on retrospective analyses of data from people in sleep laboratories who were being evaluated for different research purposes.

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The study involved analyzing the sleep patterns of three Toba Indigenous communities, also known as the Qom people, in northeast Argentina: one rural with no electricity access, a second with limited access and a third in an urban setting with full access.

Horacio de la Iglesia, one of the study’s co-authors and a professor of biology at the UW, said the communities were “ideal” to study because “they are all ethnically and socioculturally homogeneous, so it has become an outstanding opportunity to address questions about sleep under different levels of urbanization without other confounding effects.”

To track sleep, participants were outfitted with wrist monitors that logged activity, and information was gathered over a period of one to two lunar cycles.

At first, the researchers hypothesized that sleep would probably be most affected on the night of the full moon “because you walk out and you see this amazing light,” said de la Iglesia. Exposure to light, though usually in more intense amounts than what the moon generates, is known to have a negative effect on sleep.

“We were after that finding, and we found that that was not the case,” de la Iglesia said.

Instead, the data revealed the unusual pattern of decreased sleep quality on nights leading up to a full moon, a trend that was observed across the three groups.

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“When you find what you expected, typically you say, ‘Well, is this really true?’ ” he said. “But when you find something that you did not expect, then you say, ‘Well, this is a real phenomenon.’ “

And the surprises kept coming, Casiraghi said.

Though it was not part of the study’s original plan, the scientists also evaluated sleep data from 464 Seattle-area college students that had been collected for other research. The same trend was observed in that population, Casiraghi said.

“The moment I was, like, just completely in awe was when we [looked] at the data from the students,” he said. “This beautiful lunar rhythm emerged with the exact same shape and phase as our Toba-Qom subjects … I had to take a couple of days just to do it five times in a row just to know that I was doing the right thing.”

What the data didn’t show, though, was a clear answer to a critical question: Why does this happen?

“The main limitation is that we cannot establish causality,” de la Iglesia said. “We have no idea how the moon is doing this to us.”

That didn’t stop the researchers from offering some theories. One suggestion is that the availability of moonlight changes as the lunar cycle progresses.

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“It turns out that the nights that precede the full moon are the nights that have more moonlight availability on the first half of the night,” de la Iglesia said. The waxing moon not only becomes brighter as it gets closer to a full moon, it also typically rises in the late afternoon or early evening, potentially exposing people to more light. “So if you are awake, you will be kept awake by this availability of moonlight during the evening.”

Observations about when the moon rises ahead of a full moon and the extra light that may become available could “partly explain” the differences in sleep, said Michael Smith, a postdoctoral researcher at the University of Pennsylvania School of Medicine who published a paper in 2014 on moon phases and sleep.

But that theory might not work as well when applied to people living in urban environments who are exposed to artificial light at night, which is often more intense than moonlight, said Mark Wu, a professor of neurology, medicine and neuroscience at Johns Hopkins University who did not work on the study. At most, moonlight produces about 0.1 lux, which is “very low,” Wu said, noting that the specific photoreceptor in the human eye that is believed to have a “special, privileged pathway to the circadian system in the brain” detects much higher intensities of light.

“If there’s no light at all, then [moonlight] can be meaningful,” he said. “But with modern lighting, it’s essentially irrelevant.”

The findings from the urban populations prompted researchers to include an “underlying hypothesis” in the paper, suggesting that the sleep pattern might be linked to changes in the moon’s gravitational pull.

It’s possible, de la Iglesia said, that gravitational pull from the moon might make people more sensitive to the availability of light in the evening, whether it’s artificial or natural moonlight. But Smith noted that “gravity is actually a pretty weak force overall.”

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“Although I’m open to the idea that it could at least partly explain it, I would definitely want to see more evidence,” Smith said.

Casiraghi said the researchers plan to “pursue these avenues on these questions, trying to figure out what’s the force driving these changes.”

Still, de la Iglesia said the study’s findings suggest that the moon’s effect on sleep is “so robust that even if we don’t know the mechanism, we can still capitalize on the finding.”

For people who suffer from insomnia and have trouble falling asleep, de la Iglesia said knowing that your sleep may be worse in the days before a full moon could help you figure out which nights to pay more attention to good sleep hygiene.

Sleep experts often recommend reducing exposure to light at night, especially blue light, which arouses the brain, causing delays in sleep onset, and can shorten sleep.

Perhaps beyond practical applications, Casiraghi said, the study’s findings may serve as a reminder of nature’s power.

There is already good evidence that trying to “fight against environmental cycles and trying to counterpose your will to sleep at a different time against the natural cues is actually very bad for your health,” he said. “We now have more evidence that you can’t just get rid of environmental cues.”