The Mood-Altering Power of the Moon

August 2, 2019

The 35-year-old man sitting in David Avery’s psychiatric clinic was an engineer: “He liked to solve problems,” Avery recalls. And the problem perplexing him when he was admitted to the Seattle psychiatric ward where Avery worked in 2005 were his moods, which swung violently from one extreme to another – sometimes involving suicidal fantasies or seeing and hearing things that weren’t there. The man’s sleep pattern was similarly erratic, veering from near total insomnia to getting 12 hours per night.

Being a problem-solver, the man had been keeping meticulous records of these patterns, trying to make sense of it all. Avery closely studied these records and scratched his head: “It was the rhythmicity of it that intrigued me,” he says. To him, it looked very much like the patient’s mood and sleep patterns were tracking the waxing and waning of the Moon.

Avery initially dismissed his hunch as lunacy. Even if the man’s mood cycles were in synch with the Moon, he had no mechanism to explain it, nor any ideas about what to do about it. The patient was prescribed drugs and light therapy to stabilise his mood and sleep, and eventually discharged. Avery slipped the man’s notes into the proverbial file drawer and closed it.

Twelve years later, a renowned psychiatrist called Thomas Wehr published a paper describing 17 patients with rapid-cycling bipolar disorder – a form of the illness where people switch between depression and mania more quickly than usual – who, like Avery’s patient, showed an uncanny regularity in their episodes of illness.

“The thing that struck me about these cycles was that they seemed uncannily precise in a way that one would not necessarily expect of a biological process,” says Wehr, an emeritus professor of psychiatry at the National Institute of Mental Health in Bethesda, US. “It led me to wonder if there was some kind of external influence that was operating on these cycles – and [because of the historical belief that the Moon affects human behaviour] the obvious thing to consider was whether there was some lunar influence.”

For centuries, people have believed that the Moon affects human behaviour. The word lunacy derives from the Latin lunaticus, meaning “moonstruck”, and both the Greek philosopher Aristotle and the Roman naturalist Pliny the Elder believed that madness and epilepsy were caused by the Moon. Pregnant women are also rumoured to be more likely to give birth on a full Moon, but any scientific evidence for this, gleaned by looking back over birth records during different lunar phases, is inconsistent.

So too, is evidence that the lunar cycle increases violence among psychiatric patients or prison inmates – although one recent study suggested that outdoor criminal activity – incidents occurring on streets, or in natural settings like beaches – may be higher when there is more moonlight.

There is, however, some evidence that sleep varies across the lunar cycle. For instance, a 2013 study conducted under the highly-controlled conditions of a sleep laboratory found that people took five minutes longer to fall asleep on average, and slept for 20 minutes less overall, around a full Moon, compared to during the rest of the month – even though they weren’t exposed to any moonlight. Measurement of their brain activity, meanwhile, suggested that the amount of deep sleep they experienced dropped by 30%. Even so, a follow-up study failed to replicate the findings.

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