We all know a full moon is supposed to bring werewolves and vampires out into the open. Belief in its power to drive us mere mortals a little mad is ancient and widespread. Such notions even gave rise to the word lunacy.
Speak to police officers or staff in hospital emergency departments, and some will insist there are more accidents, violent incidents and psychiatric admissions when the Moon is full. In 2007, the police force in the British seaside resort of Brighton even went as far as to employ extra officers during full moons.
The ability of the moon to influence our behaviour is undoubtedly an idea with broad appeal. Beyond folk tales and anecdotal evidence, the theory has been the subject of hundreds of studies. Just this summer there was new research which found that people spending the night in a sleep laboratory rated their sleep quality as 15 per cent lower when there was a full moon, even though they couldn’t see it or any extra light it produced, and they took on average five minutes longer to get to sleep.
This study got a lot of publicity, however there were only 33 people in it and even the authors were cautious about inferring too much from the results. Combining the results of multiple pieces of research in a meta-analysis is one way to ensure findings have stronger statistical foundations. US psychologists James Rotton and Ivan Kelly took this approach in 1985, combining the findings of 37 studies on the effects of the lunar cycle. They concluded it was unrelated to the numbers of psychiatric admissions, murders, car accidents, suicides and crimes.
When they looked at the individual studies that did find links, they found there were often other explanations, such as the full moon happening to coincide with a holiday or a weekend when more trouble occurs anyway. For every study that revealed more problems when the Moon was full, another showed there were fewer. Rotton and Kelly found that if you were trying to use these statistics to predict people’s behaviour, the strength of any association was very weak. Knowing the Moon’s phase only made their predictions marginally better – to the tune of 1%.
Since then, further studies have been carried out, again with mixed results. A 1992 review of 20 studies on the relationship between the phase of the Moon and the number of people contemplating suicide concluded there was no evidence for a link. Once again researchers who believed they had identified a link had often failed to take account of variations linked to the days of the week on which they happened to fall.
One difficulty that may have helped give the influence of the full moon unwarranted legitimacy is the so-called “file drawer” problem, where journals are more likely to accept studies for publication where an effect was found, than those where it wasn’t. So no one knows how many studies which found the Moon had no influence might be languishing in filing cabinets.
Then there’s the question of how the Moon could influence our behaviour. One theory is that just as it affects the tides, it exerts its influence on the water in our bodies. But the Moon is smaller than the Earth, so its gravitational pull is correspondingly less powerful. What’s more it exerts the same force on us regardless of whether it’s new or full. Others have proposed that it’s the light from a full moon that affects people, yet it has the luminance of just a quarter of that of a candle.