Moonstruck madness has long been relegated to the annals of folklore, but new findings raise questions about if the moon may actually hold some sway over human sleep patterns. At the very least, it would be a promising explanation for why you’re tired today.

A new study finds that around the full moon humans get less shut-eye and their slumber is not as deep, even if sleep is restricted to windowless rooms free of environmental and time-based cues—such as those found in a sleep lab. The findings, published today in Current Biology, suggest that restful sleep takes a hit during a full moon as well as a few days before and after the phase. Still, no one has any idea why that would occur or what biological mechanism could be at work. The authors found that during and for the few days around the full moon—the period in its monthly phase cycle when it is brightest and appears in the sky from sunset to sunrise—it takes about five minutes longer to fall asleep, sleep duration is reduced by 20 minutes and slumber is not as deep.

Researchers from the Psychiatric University Hospital (UPK) at the University of Basel in Switzerland studied 33 volunteers in a laboratory environment while they slept. The group was split evenly among men and women and included both young adults between 20 and 31 years old and individuals ages 57 to 74. The same types of sleep patterns were witnessed in both groups, although some impacts were more pronounced in the young because older adults typically sleep less deeply than their younger counterparts. (The analysis did not include individuals in their 30s and 40s merely because it is challenging to find people willing to live in a sleep lab for close to a week, says study author Christian Cajochen, the head of the Center for Chronobiology at the UPK.)

The researchers performed their analysis retrospectively, tapping data gathered for another study. Because the research was originally looking at a different research question, neither the investigators nor participants were primed to be focused on the moon phase, even if they may have been aware of it. Still, retrospective analysis can sometimes be a double-edged sword, potentially giving rise to statistical correlations that are somewhat spurious. Either way, the researchers caution that they would like to see some prospective analysis in the future that would back up their findings.

Their work remains the first to suggest any strong correlation between lunar cycles and sleep deprivation. While the study participants slept, their brain patterns were monitored along with their eye movements and hormone secretions. The researchers found that brain activity related to deep sleep dropped by 30 percent and the study participants also showed reductions in their levels of melatonin, a light-sensitive hormone that helps control the natural sleep–wake cycle. The participants also reported in a subjective questionnaire that, well, they felt tired right around the full moon. On average, their sleep quality was about 20 percent worse during the full moon as compared with the new moon, according to the data crunchers.

Earlier work with marine creatures has found that the cycle of the moon is linked to some behaviors in those species and is an essential driving force behind reproductive timing for certain worms, but there has been no strong science demonstrating any impact of the moon on human sleep or behavior. One earlier prospective field study in humans did find that individuals slept a mean of 19 minutes less on nights with a full moon compared with those during a new moon, but that study hinged on daily sleep logs kept by 31 people over six weeks and did not have the same sorts of quantifiable sleep parameters present in this study, including measurement of cerebral cortex activity and hormone level measurement.

A lunar-sleep connection in humans may point to an “echo of our evolutionary past,” says Michael Hastings, a research scientist at the University of Cambridge who studies circadian rhythms. “If you were a hunter–gatherer on the African savanna, you may want to be out hunting at the full moon,” he says.  Still, the lunar-sleep relationship was a surprise, says Cajochen, adding that future research is needed to strengthen his findings.

“Lunar rhythms are not as evident as circadian rhythms and are thus not easy to document—but they exist,” Cajochen wrote in the Current Biology article. The impacts of the moon are often masked by influences in our environment like artificial light so most people may not sense them, but exactly how—or if—they are connected with circadian rhythms remains an area to be explored, he says. The authors also note that the moon’s effects may vary across the population. So although some individuals could theoretically point to the moon as a potential cause for the dark circles under their eyes, in other individuals sleep may not be lost.

During the study, participants stayed in the sleep laboratory for four nights but the researchers just looked at data on the sleep patterns for a middle night in order to exclude the “weekend effect” on night number one. Participants in the study were required to keep a very regular sleep-wake rhythm for at least a week prior to coming to the sleep lab, but the researchers acknowledge that there may still have been some carryover from exposures to artificial light sources and the environment, which may have somehow influenced the results.

Still, next time you are feeling particularly sluggish, the findings suggest it might be worth an extra look at the calendar. Or that’s what I’ll do if I get caught dozing at the monitor.