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 rise and fall of the Earth's oceans, which are driven by the gravitational pull of the moon.

"There seemed to be high tides occurring during the night when the sleep duration was short," says Avery. He 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.

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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.”