(CNN) -- You don't have to be a werewolf to feel restless when the full moon rises.
A new study in the journal Cell Biology suggests that people tend to get lower quality sleep around the time of full moons, snoozing an average of 20 minutes less than they do during a new moon.
"If you ask people, at least in Switzerland, about 40% report feeling the moon during sleep, or they blame the full moon for bad sleep," said lead study author Christian Cajochen of the Psychiatric Hospital of the University of Basel in Switzerland.
That's why he and his colleagues decided to investigate.
The study included 33 healthy volunteers, between ages 20 and 74. Participants slept under strictly controlled conditions in a laboratory with no windows, so they had no way of seeing the moon. They stayed in the laboratory for 3½ days. Humidity and temperature were controlled.
Neither the participants nor the researchers knew, at the time of the experiment, that the phase of the moon would become part of the study. This decision reduced any bias that either group may have introduced regarding the moon -- but also presented the drawback that the study didn't look at all phases of the moon's cycle.
The data come from an experiment done 10 years ago; Cajochen and colleagues didn't analyze the results in terms of lunar patterns until several years after they did the study and waited to publish until now.
The full moon was associated with a 20-minute reduction of total sleep time, the study authors found.
Researchers also found that it took about five minutes longer for participants to fall asleep around a full moon than around a new moon. Deep sleep was, on average, 30% decreased around the time of a full moon.
People sleeping in the lab nearer to the day of a full moon also had lower evening levels of melatonin, a hormone important to circadian rhythm that drives the body's cycles of day and night and, therefore, wakefulness and sleep.
"We have evidence that the distance to the nearest full-moon phase significantly influences human sleep and evening melatonin levels when measured under strictly controlled laboratory conditions, where factors such as light and personal moon perception can be excluded," the study authors wrote.
The number of participants in the study was small so the results may not apply to wider population. Also, the researchers didn't control what volunteers were exposed to in the week before the study; their individual environments could have influenced their sleep habits.
Generally, the methods and analyses in this experiment are solid, said Philip Gehrman, assistant professor of psychiatry at the University of Pennsylvania, who was not involved in the study.
In graduate school, Gehrman analyzed data about the sleep habits of older adults with Alzheimer's disease in nursing homes to see if lunar cycles had an effect. He didn't find one, but "the nurses would swear that the patients became more agitated and slept worse during a full moon," he told CNN in an e-mail.
The current Cell Biology study wasn't set up to find out why the full moon may interfere with sleep, but Cajochen speculated the human brain may have an internal clock that is somehow synchronized with the moon. Scientists already know about circadian rhythms. There may also be a clock that's driven by lunar cycles.
Some marine species have been shown to have reproductive patterns that sync up with lunar cycles, Cajochen noted.
"We don't know whether humans still have it and why," he said.
Further research would be necessary to confirm these findings -- for instance, functional magnetic resonance imaging could help scientists figure out what's going on in the brain during sleep at various stages of the lunar cycle.
To find more proof than this study, the experiment would need to be conducted over on a longer period of time, Cajochen said. Already this study required participants to be observed in the lab for 3½ days; to build on this research, people would have to stay in the controlled setting for at least 30 days to cover an entire lunar cycle.
"That would be the ultimate study, but it's going to be very expensive," he said.