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For centuries people have thought that the moon, especially a full moon, influenced human biology or behavior. Conditions and events thought to be affected by the moon include births, heart attacks, deaths, suicides, violence, psychiatric hospital admissions, surgery outcomes, epileptic seizures, and more. For the most part, scientific studies have disproven any connection between these things and the lunar phases. In the case of aggressive animals, menstruation, and pet injuries, there has been some evidence for a connection.

People have also long reported not being able to fall asleep easily and/or not sleeping as well during a full moon. As far back as 1999 it was suggested in the Journal of Affective Disorders that before electric lighting, that the light of a full moon could have affected sleep cycles and that the resulting sleep deprivation could have set off maniacal behavior in bipolar patients or seizures in people with seizure disorders. But, a paper published last year in Current Biology by researchers at the Psychiatric University Hospital at the University of Basel in Switzerland made a very convincing link between human sleep patterns and the full moon that has nothing to do with illumination.

Interestingly, the research featured in the paper was actually conducted back in 2000 and never intended to study the effect of the moon on sleep. Rather, it was done to learn more about human sleep patterns in general and how they may differ by age and gender. Only more than a decade later did the researchers realize they could take a look at the data to see if the full moon had an impact on sleep. In the paper they say, "The aim of exploring the influence of different lunar phases on sleep regulation was never a priori hypothesized. We just thought of it after a drink in a local bar one evening at full moon." As it turned out, this was partly a good thing, since during the study, no thought was given to the lunar phase by the subjects or the researchers, so it did not influence their actions or observations. But, it was also partly a negative. "The a posteriori analysis is a strength and a weakness," wrote lead author Christian Cajochen, head of the University of Basel's Centre for Chronobiology, "The strength is that investigators and subject expectations are not likely to influence the results, yet the weakness is that each subject was not studied across all lunar phases."

The study focused on 33 volunteers who slept in a windowless sleep lab for four nights surrounding a night of a full moon. The number of men and women in the study was almost equal. The youngest subject was 20 years old and the oldest was 74. Unfortunately, there were no participants between 32 and 56 simply because it is so difficult to find volunteers willing to live in a sleep lab for almost a week. The same types of sleep patterns were observed in both the young and old groups.

While the subjects of the study slept, their brain patterns, eye movements and hormone secretions were studied. It was found that the full moon appeared to cause the subjects to have more difficulty getting asleep, to sleep 20 minutes less than normal, and to lose about 30% of their deep sleep time. Levels of melatonin, a light-sensitive hormone that helps control the sleep-wake cycle were lower and the participants reported feeling less refreshed the next day than on the other days.

How does the moon impact sleep patterns? That is not completely clear. Since the study was conducted in a dark lab, it proves that light or the absence of light was not an influence. It has long been speculated, that because the moon's gravitational pull influences the tides, that it may affect the human body, too, because it is made up of mostly water. But the moon doesn't affect bodies of water such as lakes that are smaller than the oceans, but very large in relation to the human body. So, why would it affect the body?

The lunar influence on sleep may be an "echo of our evolutionary past" says Michael Hastings, a 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.