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A full moon is often blamed for a whole range of human problems, from a lack of sleep to seizures. Some even say there are more crimes on a full moon night and more accidents. But there is no scientific evidence to back these claims.

Still, as NPR's Rhitu Chatterjee reports, some sleep researchers in Switzerland have decided to take another look.

RHITU CHATTERJEE, BYLINE: Christian Cajochen is a sleep biologist at the Psychiatric Hospital of the University of Basel.

CHRISTIAN CAJOCHEN: We were sitting outside a pub, actually, and we were looking at the full moon. And we asked ourselves whether the full moon really had some influence on human sleep.

CHATTERJEE: He says his colleagues who work in sleep clinics say patients often blame their sleep troubles on a full moon, and so Cajochen had an idea. He and his colleagues had done a previous study, where they'd collected detailed information on the sleep patterns of healthy adults in their sleep lab. They knew the dates of the experiment which took place over a long period.

So Cajochen dug up that data, and looked to see if there was any association between the phases of the moon and changes in people's sleep patterns. To his surprise...

CAJOCHEN: People who entered the lab during full moon slept, on average, 20 minutes less than people who came in during the new moon phase.

CHATTERJEE: They also found that deep sleep was reduced by 30 percent during a full moon. Cajochen says he didn't believe what he was seeing.

CAJOCHEN: I was like, wow, this can't be.

CHATTERJEE: After all, other scientists who looked at this have failed to find a link. Cajochen says he's still skeptical, even though he's just published his findings in the journal Current Biology.

At least one sleep researcher says it's just a statistical fluke. But Frank Scheer, who studies circadian rhythms at Harvard Medical School, is less dismissive.

FRANK SCHEER: Well, I think it is an interesting finding. It's a small study. It's performed in 33 subjects. So it will need to be replicated in a larger study to see if these findings hold true.

CHATTERJEE: Scheer says, to date, researchers haven't been able to find any evidence that the moon influences our bodies, not even the menstrual cycle.

SCHEER: Of course, it is the first thing one would think about, because it also has a rhythm that is somewhat close to the lunar cycle.

CHATTERJEE: But, he says there's no evidence that it's in sync with the moon. On the other hand, Scheer says, we do know that the sun influences the human body in many ways.

SCHEER: So we understand quite a bit about workings of the circadian clock, the biological system that regulates our day-night rhythms and physiology and behavior.

CHATTERJEE: And that could mean that if the moon does have an effect, it may be too subtle to detect, given the much greater effect of daylight.

Now, it's a different story for some animals. Scientists have found lunar - or circa-lunar clocks, as they call them - in a range of marine animals.

TOBIAS KAISE: It was shown in corals, in worms, in insects, in fish.

CHATTERJEE: Tobias Kaise is at the University of Vienna. He says many marine animals synchronize their feeding and reproduction with the lunar cycle. And that makes sense, because their lives are dependent on tides, which are controlled by the moon. But Kaiser says even these circa-lunar clocks are poorly understood.

KAISE: You know, the great mystery of circa-lunar clocks is that nobody knows anything about the molecular basis of the circa-lunar clock. It's not understood in any organism.

CHATTERJEE: And as for the great mystery of the moon's effect on humans, that seems like something we just have to sleep on.

Rhitu Chatterjee, NPR News.

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