New study links our built-in sleep patterns to phases of the moon — even in Seattle

A newly published study adds to the long-debated evidence that humans are hard-wired to sleep less when the moon is full or the lights are on, probably due to the ancestral quirks of circadian rhythm.

The pattern has been documented in a variety of indigenous communities in Argentina — and at the University of Washington in Seattle, where bright lights and cloudy weather tend to dull even the full moon’s glare.

“We see a clear lunar modulation of sleep, with sleep decreasing and a later onset of sleep in the days preceding a full moon,” senior study author Horacio de la Iglesia, a UW biology professor, said in a news release. “And 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.”

The research was published today in the open-access journal Science Advances.

This study isn’t the first to report a correlation between lunar phases and sleep cycles. But it does make use of cutting-edge technology, in the form of wrist monitors, to track the sleep patterns of hundreds of experimental subjects reliably under natural conditions. In contrast, most of the earlier studies relied on user-reported sleep diaries, or monitored subjects in controlled lab environments.

During a series of one- to two-month-long campaigns in 2016, 2017 and 2018, de la Iglesia and his colleagues from UW, Yale and Argentina’s National University of Quilmes collected data on 98 members of northern Argentina’s Toba-Qom indigenous communities.

The experimental subjects lived in three carefully chosen settings. One group resided in a rural setting with no access to electric light. Another group had only limited access to electricity, and the third group lived on the outskirts of a town with streetlights as well as 24/7 access to electric light at home.

All three groups showed a variation in sleep patterns that traced the moon’s 29.5-day cycle of brightness. Bedtimes varied by up to 30 minutes, and the total amount of nightly sleep swung up and down by an average of 46 to 58 minutes over the course of the moon’s phases, depending on the setting.

The variations were less dramatic for the urban residents who were exposed to more artificial light. And throughout the moon’s phases, the town dwellers tended to go to bed later and sleep less than the rural residents.

A separate sleep-monitoring study involved 464 UW students in the Seattle area. Those monitoring sessions were conducted for up to three weeks at a time, during a period between 2015 and 2018 — and they traced a similar pattern.

The three to five nights leading up to the full moon tended to be the most sleepless, in Seattle as well as Argentina.

So what’s behind the pattern? The researchers note that the evenings just before the full moon typically provide more natural light after dusk, with a bright celestial orb hanging high in the eastern sky. In contrast, a moon that’s just past full doesn’t rise above the horizon until later in the evening.

“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,” said study lead author Leandro Casiraghi, a postdoctoral researcher at UW.

De la Iglesia said artificial light tends to disrupt our innate circadian clocks, especially in the evening.

“It makes us go to sleep later in the evening; it makes us sleep less,” he said. “But generally we don’t use artificial light to ‘advance’ the morning, at least not willingly. Those are the same patterns we observed here with the phases of the moon.”

Although the moon’s brightness would explain most of the variation in sleep patterns, the researchers saw hints of a secondary cycle for rural communities that peaked during the new-moon and full-moon phases. This 15-day cycle may be due to other factors, such as the moon’s slight gravitational pull, but that’s a subject for further study.

It may seem strange that lunar phases would affect college students in Seattle’s urban environment, where many folks don’t typically take note of the moon or its glow. For that reason, Casiraghi thinks the biological mechanisms behind shifting sleep patterns deserve further study as well.

“Is it acting through our innate circadian clock, or other signals that affect the timing of sleep?” he asked. “There is a lot to understand about this effect.”

And it’s not just sleep: Yet another study in Science Advances, based on a decades-long study of 22 German women, found intermittent synchronization between moon cycles and menstrual cycles.

“With age, and upon exposure to artificial nocturnal light, menstrual cycles shortened and lost this synchrony,” the researchers reported. “We hypothesize that in ancient times, human reproductive behavior was synchronous with the moon but that our modern lifestyles have changed reproductive physiology and behavior.”

Over the centuries, the cycles of the moon have been linked to everything from love and madness to traffic accidents. These newly published studies just might make such linkages look a little less loony.

Update for 3:15 p.m. PT Jan. 28: Chris Chabot, a neurobiologist at Plymouth State University in New Hampshire who specializes in animal behavior and biological rhythms, said the sleep study delivered “pretty compelling results” using a new monitoring method. “It’s a great addition to the literature,” he told me.

Chabot noted that “people are pre-conditioned to accept that there’s an effect of the moon [on human behavior], but maybe not necessarily for the right reasons.”

He acknowledged that the moon’s effect on the sleep patterns of students in Seattle is a bit of a puzzler. “I spent four years in Eugene [in Oregon], so I’m well aware of the Northwest,” Chabot said. “I love it, but you know, it’s a question mark as to how much light actually filters through on any given night. And honestly, there’s a lot of artificial light out there.”

That’s why he’s intrigued by alternate hypotheses about the moon’s influence. “I find it intriguing that they’re talking about gravitational effects, in this paper and the menstrual-cycle paper as well,” Chabot said. “I’m not completely convinced about that, but I like it as a potentially testable hypothesis.”

In addition to Casiraghi and de la Iglesia, the authors of “Moonstruck Sleep: Synchronization of Human Sleep With the Moon Cycle Under Field Conditions” include Ignacio Spiousas, Gideon Dunster, Kaitlyn McGlothlen, Eduardo Fernández-Duque and Claudia Valeggia.

The authors of the other study, “Women Temporarily Synchronize Their Menstrual Cycles With the Luminance and Gravimetric Cycles of the Moon,” include Charlotte Helfrich-Förster of the Julius-Maximilians University of Würzburg and Spiousas as well as S. Monecke, T. Hovestadt, O. Mitesser and T.A. Wehr.

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