Under the leadership of Horacio de la Iglesia, Ph.D., a professor of biology at the University of Washington (U.W.), the research team began by studying three Toba-Qom Indigenous communities in northern Argentina. Each one had a different level of access to electricity and modern amenities: One was based in an urban area, one had limited sources of lighting and electricity, and one was completely off the grid.

The research team figured that individuals in these last two groups would be the ones whose sleep was affected on full moon evenings.

“Our hypothesis was that if we did find an effect of the moon on sleep, it would only be present in these communities that had no access to electric light, or very limited access, because they would be the ones taking advantage of the light of the moon,” Leandro Casiraghi, the study’s lead author, tells mbg via Zoom.

The urban group was meant to act as a sort of “cultural control” and demonstrate that the further removed we become from natural cycles, the less they affect us.

After equipping participants with wrist monitors (think highly sophisticated Fitbits) and monitoring their sleep over the course of one to two moon cycles, the researchers did indeed find that those who lacked electricity went to bed later in the evening and slept for a shorter period of time in the three to five days leading up to the full moon. But here’s the kicker: The same pattern emerged among urban dwellers, too, challenging their original theory.

“That was pretty surprising,” Casiraghi recalls. “We looked at [the data] like 10 times before we said ‘OK this is actually happening.'”