The Moon and Your Sleep June 05, 2014

We are all, quite literally, lunatics—and I mean that in the nicest way possible. It is the moon, after all, that is responsible for the luna part of that word—and the moon has always made us at least a little crazy. Over our long history we have been charmed by it, spooked by it, seduced by it. We kiss by the moon, go to war by the moon, we spent $25 billion—in 1960s money, no less—to go to the moon. So it’s hardly a surprise that the moon is in some very real ways inside of us all.

The human menstrual cycle is the best-known example of the way our bodies—over millions of years of evolution—have synchronized themselves to the rhythms of the moon. Less well-known is the lunar link to the electrochemistry of the brain in epileptic patients, which changes in the few days surrounding a new moon, making seizures more likely. And then there are the anecdotal accounts of the effects the moon has on sleep

People have long reported that it is harder to get to sleep and remain asleep when the moon is full, and even after a seemingly good night’s rest, there can be a faint sluggishness—a sort of full-moon hangover—that is not present on other days. If you’re sleeping on the prairie or in a settler’s cabin with no shades, the simple presence of moonlight is an inescapable explanation. But long after humans moved indoors into fully curtained and climate-controlled homes, the phenomenon has remained. What’s never been clear is whether it’s the real deal—if the moon really does mess with us–or if it’s some combination of imagination and selective reporting, with people who believe in lunar cycles seeing patterns where none exist. Now, a report in the journal Current Biology suggests that the believers have been right all along.

For a research paper that was just released today, the initial work took place an awful long time ago. In 2000, a team of investigators from the University of Basel, the Swiss Federal Institute of Technology and the Switzerland Centre for Sleep Medicine, recruited 33 volunteers and studied them in a sleep lab on and off over the course of three years. The investigators gathered a range of data—brain wave activity during sleep as measured by electroencephalograms (EEG); levels of melatonin, a sleep-related hormone; the amount of time it took subjects to fall asleep and the amount of time they spent in deep sleep; and their subjective reports of how rested they felt the next day. All of it was intended to learn more about human sleep patterns in a general way and, more specifically, how they are affected by age and gender. Only a decade later did the investigators realize that they may be able to re-crunch the data to learn about the moon.

“The aim of exploring the influence of different lunar phases on sleep regulation was never a priori hypothesized,” they wrote in a wonderfully candid passage in their paper. “We just thought of it after a drink in a local bar one evening at full moon.”

Thus should all great science be done, since as it turned out, the second look revealed intriguing patterns. On average, the subjects in the study took five minutes longer to fall asleep on the three or four nights surrounding a full moon and they slept for 20 fewer minutes. In addition, EEG activity related to deep sleep fell 30%, melatonin levels were lower and the subjects reported feeling less refreshed the next day than on other days. The subjects slept in a completely darkened lab with no sight of the moon, and none of them—at least from what was known—appeared to have given any thought at all to lunar cycles. And since the moon was not an experimental variable in the original study, it was never mentioned either to the subjects or even among the investigators.

In terms of scientific reliability, all of this is both good and not so good. A study can’t get more effectively double-blind than if no one is even thinking about the thing you wind up testing for, which makes the findings uniquely objective. On the other hand, the ideal moon study would have been carefully set up to give equal weight to every night in the lunar cycle. This study—while capturing most of the nights in the month—did so in a less rigorous way.

“The a posteriori analysis is a strength and a weakness,” concedes lead author Christian Cajochen, head of the University of Basel’s Centre for Chronobiology, in an e-mail to TIME. “The strength is that investigators and subject expectations are not likely to influence the results, yet the weakness is that each subject was not studied across all lunar phases.”

Even if the moon has as significant an effect on sleep as the study suggests, what’s less clear is the mechanism behind it. Dark labs eliminate the variable of light, so that can’t be it. And before you ask, no, it’s not gravity either. The authors stress that while lunar gravity does indeed raise tides in the oceans, it doesn’t on lakes and even many seas. Those bodies are simply too small to feel the effects—to say nothing of human bodies.

Rather, the answer is simply that we, like every other species on Earth, evolved on a particular planet with a particular set of astronomical cycles—day and night, full moons and less full—and our circadian systems adapted. It’s hard to say where the internal clock is in, say, a flowering plant, but in humans, it’s likely in the suprachiasmatic nuclei, a tiny region of the brain near the optic nerve involved in the production of melatonin, certain neurotransmitters and other time-keeping chemicals, all in a rhythm consistent with both its terrestrial and cosmic surroundings. Physically, human beings may be creatures of just this world, but our brains—and our behavior—appear to belong to two.