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Sleepless night, the moon is bright. People sleep less soundly when there's a full moon, researchers discovered when they analyzed data from a past sleep study.

Henri Rousseau (1897), Museum of Modern Art, New York; Corbis Images

Yes, You Can Blame the Moon for a Bad Night’s Sleep

If you were tossing and turning and howling at your pillow this week, you’re not necessarily a lunatic, at least in the strictest sense of the word. The recent full moon might be to blame for your poor sleep. In the days close to a full moon, people take longer to doze off, sleep less deeply, and sleep for a shorter time, even if the moon isn’t shining in their window, a new study has found.

“A lot of people are going to say, ‘Yeah, I knew this already. I never sleep well during a full moon.’ But this is the first data that really confirms it,” says biologist Christian Cajochen of the University of Basel in Switzerland, lead author of the new work. “There had been numerous studies before, but many were very inconclusive.”

Anecdotal evidence has long suggested that people’s sleep patterns, moods, and even aggression is linked to moon cycles. But past studies of potential lunar effects have been tainted by statistical weaknesses, biases, or inconsistent methods, Cajochen says.

Between 2000 and 2003, he and his colleagues had collected detailed data on the sleep patterns of 33 healthy volunteers for an unrelated study on the effects of aging on sleep. Using electroencephalograms (EEG) that measure brain activity, they recorded how deep and how long each participant’s nightly sleep was in a controlled, laboratory setting. Years after the initial experiment, the scientists were drinking in a pub—during a full moon—and came up with the idea of going back to the data to test for correlations with moon cycles.

“What’s nicest about this study is that it uses data that wasn’t originally intended for this purpose, so you know there couldn’t be any bias and that makes it quite convincing,” says neuroscientist Kristin Tessmar-Raible of the Max F. Perutz Laboratories in Vienna who was not involved in the new work.

When the researchers investigated how sleep patterns changed during moon cycles, they found a striking association between poor sleep and lunar cycles. In the few days before and after a full moon, people took an average of 5 extra minutes to fall asleep, slept 20 minutes less per night, and had 30% less deep sleep, as measured by the EEG. Moreover, the volunteers recorded poorer sleep on a survey around the full moon, the scientists report online today in Current Biology.

“This paper showed that it’s possible to detect a correlation between the human sleep cycle and lunar phases, which strongly suggests to me that there is some kind of synchronization,” Tessmar-Raible says. “And the question now is what is the mechanism behind this?”

Because the subjects couldn’t see the moon, increased light levels aren’t producing the effect, at least not entirely. It’s more likely influenced only in a small part by light or other external factors, and maintained through internal hormones, like people’s 24-hour sleep-wake cycles, which persist even in the absence of light or darkness, Cajochen speculates. “In terms of the lunar cycle, light could be important to synchronize this biological clock with environmental stimuli,” Cajochen says. “But the clock itself then continues on independent of light.”

To test that possibility, he says, scientists could set up additional controlled experiments that measure how physiology and brain activity varies over the 29.5-day lunar cycle. Studies on animals with mating or migration patterns that revolve around lunar cycles could also illuminate the underlying biological drivers as well as the evolutionary benefit of having a moon-synced clock. Whatever the mechanism, inconsistent sleep around the full moon could be partially responsible for the origin of the word lunatic, which derives from the Latin for “moonstruck.”