A full moon day can prevent a good night's sleep, say Swiss researchers.
The results of a study on 33 volunteers published in the journal Current Biology showed that people took longer to fall asleep at night and slept for a shorter time during a full moon.
The volunteers were monitored on two separate nights, and were not aware of the cycle of the moon during their time in the sleep lab.
When there was a full moon, people slept an average of 19 minutes less and took five minutes longer to fall asleep than they did when there was a new moon.
"The lunar cycle seems to influence human sleep, even when one does not 'see' the moon and is not aware of the actual moon phase," said Christian Cajochen of the Psychiatric Hospital of the University of Basel.
Study participants also reported feeling less well rested, and showed lower levels of melatonin, a hormone that regulates sleep and wake cycles, during the full moon.
"To our knowledge, this is the first report of a lunar influence on objective sleep parameters," the study said.
Researchers said more studies may reveal whether the moon has a measurable effect on moods and mental performance as well.