Many myths have told of the powers of a full moon, from werewolves to sudden madness to unexplained seizures, but new research suggests an impact close to home: Sound sleep may be harder to come by when the moon is in its full glory.
The study suggests that the human body is cued not only to the daily rising and setting of the sun, which regulates circadian rhythms, but also to the phases of the moon.
Published in the journal Current Biology, the idea behind the finding was dreamed up in a bar one night as the Swiss researchers were having a drink.
"A lot of people complain that they have bad sleep around a full moon. These are anecdotes, but you hear it from a lot of different people," said Silvia Frey, a neurobiologist at the University of Basel. "So, we were sitting there on a full moon night and just discussing this, and we thought maybe we can go back in our data and align it with the date of the full moon," Frey said. "It was just pure curiosity."
Frey and her co-authors took a second look at sleep data they had collected for a previous study.
That study included 33 healthy people who each spent three and a half days in the carefully controlled environment of a sleep lab. They couldn't see clocks or outside light. Temperature and humidity were held constant. Scientists kept tabs on their hormones and on brain activity. They also recorded how long it took people to fall asleep, as well as their total sleep time.
For the first two nights of the study, participants were allowed to fall asleep naturally and sleep as they normally would.
Scientists took all the data collected on those two nights of normal sleep, a total of 64 nights, and they compared it to the phases of the moon.
In the four days before and after a full moon, study participants saw significant decreases in sleep quality. They also got an average of about 20 minutes less sleep on those nights. The amount of time they spent in deep sleep decreased by about a third. And they made less melatonin, the hormone that makes people feel drowsy after dark.
Researchers admit the study has some important limitations. Because they used preexisting data, they weren't able to get a sense of how sleep might change for a single individual who was followed for an entire month. But they say the retrospective nature of the study might also be a positive. Because nobody, not even the researchers, knew they might be interested in the question of how moon cycles affected sleep, it couldn't have biased their findings.
"We didn't expect to find such huge differences, I must admit," Frey said. "The drop in melatonin was really unexpected. It points really towards a physiological thing that is happening. Like a circalunar clock," she said.
Biologists have shown that some kinds of animals, especially animals that live near the ocean, such as fiddler crabs, have circalunar clocks that are synced to the moon.
In tune with the moon
And a recent study found that heart surgery patients spent less time in the hospital and had lower death rates if they were operated on around the time of a full moon.
Researchers admit they're not sure why human biology might be in tune with the moon. Frey said it's probably an evolutionary relic, left over from a time when it might not have been safe for humans to sleep deeply under the light of a bright moon, making them vulnerable to night-time predators.
An expert who wasn't involved in the research said the observations were intriguing.
"A significant amount of research has been done on circadian rhythms in humans, looking at the light and dark cycles within a 24-hour day and how they influence melatonin production and the overall body clock," said Shelby Harris, director of the behavioral sleep medicine program at Montefiore Medical Center in New York City. "This study is highly unusual in that it looks at circalunar cycles in humans," she noted.
"This is the first study to establish this periodicity in healthy adults as well," Harris said.
"More research is needed to establish a solid foundation to this finding, and it is quite possible that more findings similar to this could very well influence sleep treatment in the future," she added.
For more information on circadian rhythms and the body's internal clock, head to the U.S. National Institute of General Medical Sciences.