At a bar one night during a full moon, sleep researchers had a thought: Could it be true that the cycles of the moon still influences sleep?

The team from the University of Basel, the Swiss Federal Institute of Technology and the Switzerland Center for Sleep Medicine already had the data, according to Time.

From 2000 until 2003, they had recorded the sleep habits of 33 subjects in a laboratory. At the time, they’d been investigating the effects of age and gender.

PHOTOS: Ways Not Getting Enough Sleep Messes You Up

But when they analyzed the data and compared it to the moon cycles at the time, patterns emerged. People took five minutes longer to fall asleep and they slept for about 20 minutes less on the three or four nights surrounding a full moon.

The volunteers also felt as though their sleep had been poorer during full moons and they showed lower levels of melatonin, a hormone that regulates sleep and wake cycles.

“This is the first reliable evidence that lunar rhythm can modulate sleep structure in humans,” Prof. Christian Cajochen of the Psychiatric Hospital of the University of Basel said in a press release.

NEWS: Lack of Sleep Linked to Early Death

Lead author Christian Cajochen, head of the University of Basel’s Center for Chronobiology, conceded in an e-mail to TIME that analyzing data that was not designed for the study was both a strength and a weakness.

“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.”

The research was published in Current Biology.

Photo: Corbis