Here’s a good reason to howl at the moon: It might be keeping you awake at night. A new Swiss study published in Current Biology suggests the phases of the moon – especially the full moon – significantly impact both sleep length and quality.

Researchers found that on average, the full moon cut 20 minutes from overall sleep time and delayed sleep onset by 5 minutes. In addition, the study’s subjects, who were unaware of the study’s focus or the phase of the moon, reported poorer sleep quality around the full moon, even when light exposure was strictly controlled in the researchers’ lab.

The subjects’ perception of poor sleep was backed up by the study’s objective data. Researchers used electroencephalograms (EEGs) to track brain waves during sleep and found that subjects’ deep sleep, a phase of sleep thought to be responsible for its restorative effects, decreased by 30% when the moon was full. Finally, evening levels of melatonin, which helps to regulate the body’s internal clock and cause bedtime drowsiness, were significantly decreased around the full moon.

The reason the moon has such an impact on our nighttime rest is unclear, but the effect appears to be automatic and inherent. The study’s authors suggest that the moon’s cyclical influence on our sleep functions as a “circalunar clock” similar to circadian (daily) and seasonal rhythms that our bodies also adhere to. What advantage, if any, this circalunar clock might provide is unknown, though a prior study of iguanas showed that the more precise the animals’ circalunar clocks were, the more likely they were to survive periods of stress. Perhaps there’s something behind all that full-moon folklore after all