If you have trouble falling asleep around the same time each month, the moon might be to blame.
Scientists say they've found evidence that human sleep patterns are timed to the phases of the moon, and that people sleep 20 minutes less on average during a full moon.
"The lunar cycle seems to influence human sleep, even when one does not 'see' the moon and is not aware of the actual moon phase," study co-author Christian Cajochen, a chronobiologist at the University of Basel in Switzerland, said in a statement.
Cajochen says his team's findings, detailed in the July 25 issue of the journal Current Biology, could be evidence of a biological "circalunar clock" ticking inside of humans.
Similar to the circadian clock that helps humans and other animals sync their physical and behavioral changes to a 24-hour day-night cycle, a circalunar clock would somehow be synchronized with the changing phases of the moon.
Evidence of a circalunar clock has been found in insects and reptiles, but not yet in humans. Cajochen stressed that there could be other ways to explain his findings.
"This is just an interpretation of the results," he said in an interview Thursday.
Malcom von Schantz, a sleep and circadian researcher at the University of Surrey in the U.K., called the new findings "fascinating" because they run counter to the results of several other studies that failed to find a link between the moon and human behavior.
"Essentially, every report published to date has failed to show significant associations between the phase of the moon and any number of behavioral and physiological parameters," von Schantz, who was not involved in the study, said in an email.
"This is the very first report that suggests an association with one behavior, sleep, and of course it's a behavior that in our species normally occurs at night."
In their new study, Cajochen and his team studied 33 volunteers in the lab while they slept. While they slept, the subjects' brain patterns, eye movements, and hormone secretions were monitored.
The analysis showed that around the full moon, the volunteers slept less and their brain activity related to deep sleep dropped by 30 percent.
They also took about five minutes longer to fall asleep and showed diminished levels of melatonin, a hormone known to regulate sleep and wake cycles.
The study participants also reported feeling that their sleep was poorer when the moon was full.
The volunteers' sleep patterns were originally studied as part of another research project, and the effects of the moon were not analyzed until much later.
"We just thought of it after a drink in a local bar one evening at full moon, years after the [original] study was completed," the authors write.
This fact makes it difficult to come up with alternative explanations for the findings, von Schantz said.
"The subjects were sleeping in windowless rooms, and although they may have been aware of the phases of the moon, neither they nor even the investigators knew that lunar phase would be a parameter that would be considered in the study," he added.
Kristin Tessmar-Raible, a sleep researcher at the University of Vienna in Austria, agreed. "This pretty convincingly excludes possible biases ... At the present time their explanation is at least the most plausible one," said Tessmar-Raible, who also did not participate in the research.
Cajochen said that the circalunar rhythm in humans could be a relic from a past in which the moon could have synchronized our ancestors' behaviors for reproduction or other purposes.
For example, Cajochen said, early humans might have been primed to sleep lightly during the full moon, "when there is more light at night and potential danger from predators is more likely."
The origins of our circalunar clock might be far older, dating back to the very dawn of mammals, von Schantz speculated. "We have to remember that mammals evolved through what is called the nocturnal bottleneck. When dinosaurs roamed the Earth by day, the night represented a window of opportunity for a new group of vertebrates to evolve," von Schantz said.
If humans do possess a circalunar clock, then a major unanswered question is the external cue that allows it to stay in sync with the moon, scientists say.
"Just like the circadian clock, [the circalunar clock] will need to be reset based on external time cues to stay in sync. Now what I can't get my head around is, what would that cue be?" von Schantz said.
"It is hard to imagine that our light receptors are able to specifically filter out the light from a full moon amongst all the other light signals we receive today. We are not physically exposed to tides. And the gravitational pull of the moon is really quite weak ...
"This study identifies a lot of fascinating questions for further investigations."