Summer in the Pacific Northwest brings gloriously late sunsets—and frustratingly few hours of total darkness for us to get optimal shut-eye. As research repeatedly shows, excess light wreaks havoc on sleep schedules.

But something that comes up once the sun goes down may also be to blame for our morning grog: the moon. A breakthrough study with Seattle ties, published earlier this year in the journal Science Advances, found a relationship between sleep and the lunar calendar. Specifically, in the several days leading up to a full moon, study participants slept 50 minutes less and dozed off 30 minutes later than normal on average. “That was a big surprise,” says Horacio de la Iglesia, one of several University of Washington researchers who co-authored the paper. 

Prior studies on the connection between moon phases and sleep had been inconclusive or controversial, using multiple participants to cover one lunar cycle in certain cases. “Some people didn’t believe it, some people believed it,” says de la Iglesia. 

So the UW-led group took a long-term approach to their study. Initially they measured sleep habits over at least one lunar cycle in three different Toba, or Qom, Indigenous communities in Argentina. Participants wore Fitbit-like wrist monitors to track their sleep in areas with different levels of electric light. De la Iglesia and company expected to find that the rural groups would sleep the least on nights with a full moon, which is the most luminous of the phases; the glow would keep them up. Instead tracker data revealed that the three to five nights before a full moon were the worst for getting some rest in those communities. Even stranger? A light-polluted urban area saw a similar effect. 

Sensing a real discovery, the researchers retroactively examined the sleep patterns of another group. A very different group: 463 UW students in Seattle. And yet, they found the same thing. These millennials and zoomers reported less sleep during the waxing phases of the moon, even with an abundance of artificial light forms nearby. 

The researchers can’t say for sure why this happened, but they have a theory. Whereas a full moon comes up later in the evening, when people are already asleep, waxing phases rise in the sky earlier, before bedtime. “So that’s highly stimulating,” says de la Iglesia. 

Still, moonshine wouldn’t seem to affect a Seattle dorm as much as a stray strobe (or a noisy roommate). De la Iglesia suggests gravitational pull or some other moon-related effect might also be in play. 

No matter the true cause, insomniacs can perhaps use these findings to ward off sleep struggles. “Knowing that those nights are particularly bad in terms of trying to fall asleep,” says de la Iglesia, “you could be a little bit more stringent with whatever practices you have in order to improve your sleep start, like decreasing light intensity, not using screens, not using highly stimulating social media.” 

Not the worst advice for any stage of the moon cycle, really.

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