Jan. 27 (UPI) -- Werewolves aren't the only creatures influenced by the lunar cycle -- it turns out regular folks, unbitten and otherwise healthy, are made restless by the full moon, too.
When scientists at Yale University and the University of Washington teamed up with researchers in Argentina to track the sleep patterns of hundreds of people, they were surprised to find oscillations synced across the 29.5-day lunar cycle.
The results of the novel study were published Wednesday in the journal Science Advances.
"We were totally stunned to see that the timing and duration of sleep could vary anywhere from 30 to 90 minutes across the moon cycle, for most participants," lead study author Leandro Casiraghi told UPI in an email.
"That was way outside of our predictions, as we were expecting very mild variations, if any," said Casiraghi, a postdoctoral researcher who studies human biological rhythms at the University of Washington.
Researchers recruited a few thousand participants to wear wrist monitors to track sleep patterns.
The volunteer base represented young and old, as well as rural. It also included a few hundred college students living in Seattle, as well as a several dozen members of the Toba community, or Qom people, one of the largest indigenous group in Argentina.
"We have been studying these communities for several years, and we have an ongoing relationship with them," co-author Horacio de la Iglesia told UPI in an email.
"Some of the members in the communities help us as field assistants, and they are all extremely collaborative," said de la Iglesia, a professor of biology at the University of Washington.
The study was at least partially borne of conversations with the Qom people, some of whom had mentioned moonlight nights were traditionally used for hunting and fishing, as well as for community camaraderie.
"Because there was a lot of research on the effects of the moon on animal behavior, but only ... a few laboratory sleep studies had suggested that human sleep may be affected by the moon, we decided to give it a try and study it in real-life conditions," Casiraghi said.
While researchers expected to observe slightly shortened sleep cycles on moonlit nights, they were surprised to find people actually slept the least on the nights leading up to a full moon -- not the full moon itself.
"We believe this is because the availability of moonlight during the first half of the night is maximal during these nights, but not necessarily in the nights that follow the full moon -- because the moon rises later every night," said de la Iglesia.
Data from the wrist monitors showed the lunar cycle had the largest effect on the sleep patterns of members of the Toba community, most of whom live without electricity.
But researchers were surprised to find the sleep patterns of college students in Seattle -- who are regularly exposed to blue-light emitting screens and urban light pollution -- were also affected by the waxing and waning of the moon.
Access to electricity and exposure to artificial light quieted some of the moon's influence on sleep, but the circadian rhythms of urban residents still oscillated across the 29.5-day lunar cycle.
"We still don't know why we still see these changes in our sleep, and that question is even more difficult to answer because we still cannot say what is the mechanism that is driving them," Casiraghi said.
"What is clear is that it is not moonlight itself that is driving these changes, as otherwise we wouldn't have detected these effects in communities with ubiquitous artificial light -- and which probably are very little aware of the moon phases. There has to be a different cue from the moon; maybe gravitational pull cycles," Casiraghi said.
Researchers said they hope follow up studies will help determine what kinds of moon-related signals, like subtle changes in gravity, are driving these sleep patterns.
Scientists also want to figure out why sleep is delayed: Is it because a person's body clock shifts? Or is it delayed because sleep pressures change and people don't feel as tired?
"We have found a very robust phenomenon, that human sleep timing is synchronized with moon phases, now we want to know how this happens," said de la Iglesia.