“That was super surprising,” he says. Perhaps, he speculates, ancient human hunter-gatherers evolved an as-yet-unknown way to sense the lunar cycle to stay alert and active right before the full moon, when they’d have more light during the first half of the night for attaining resources or social activities. (Learn more about the fascinating science of sleep.)

Another unexpected result: Many subjects in both study groups also slept less around the new moon, the lunar phase during which the moon generally isn’t visible. 

Evidently, there’s more to the story than moonlight. De la Iglesia hypothesizes the moon’s gravitational forces, which are strongest on the full and new phases, could also influence sleep patterns. That’s when the sun, Earth, and moon stand in one line, maximizing the gravitational pull on Earth from both sides.

Yet there’s no evidence so far that humans—or any animal for that matter—can detect such minute changes in gravity, says Tessmar-Raible, who is also at the Alfred Wegener Institute for Polar and Marine Research and the University of Oldenburg in Germany.