The origins of this idea date back at least to the Ancient Greeks, with Aristotle 2,300 years ago claiming that the Moon could influence the human mind. But madness wasn’t the only outcome: the original meaning of ‘lunatic’ referred to epilepsy rather than insanity.
The Roman naturalist Pliny the Elder argued that the Moon was able to exert its influence via the water content of the brain – rather like the tides. This is now known to be nonsense: the forces involved are far too small.
Even so, the belief persists and has been investigated scientifically many times. A 1985 review of dozens of studies of alleged links between the Moon and everything from psychiatric issues to criminal behaviour found no compelling evidence. But it still remains an active research topic.
Earlier this year, the respected journal BMJ Open published a study claiming to show that a full Moon actually makes murders less likely, although the author admits that the reason isn’t clear, and that other factors may play a role.
So how did the idea of a lunar influence on humans get traction in the first place? One suggestion is that in ancient times a bright full Moon was more likely to disturb sleep – and sleep deprivation is known to exacerbate mood disorders in some people.