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From witches' covens to werewolves, from the menstrual cycle to the stock market, the moon has long been thought to exert a mysterious power over human behaviour. Although many of these associations are fanciful at best, a Swiss study suggests that when it comes to sleep, the lunar effect may indeed be grounded in scientific fact.

On the night of a full moon, six scientists meet in a bar to discuss the subject of their latest experiment. It sounds like the opening to a horror story. But in fact, this nocturnal gathering – which took place in Basel, Switzerland in 2013 – was the starting point for a scientific study that suggests centuries of folk tales may actually have some basis in truth. The data it uncovered seems to confirm a common intuition: that the quality of our sleep is directly related to the phases of the moon.


Published in Current Biology, the research by Christian Cajochen and his team analysed data taken from a previous sleep study. This earlier experiment did not focus on the moon’s influence, thus ensuring that participants had not simply imagined its effects. The results show changes to sleep throughout the moon’s 29.5-day cycle, and significant increases to sleep disruption during the period immediately surrounding the full moon.


The original experiment had been conducted with 33 volunteers: half of them aged 20-31, and the others 57-74. All were in good health and had previously been identified as good sleepers. Over a period of three years, these subjects took part in a series of laboratory sleep sessions – with their brain pattern, eye movements and hormone secretion all monitored while they slept. They were also asked to assess their sleep quality for themselves.


When Cajochen and his team correlated these earlier results with the phases of the moon, they made an extraordinary discovery. The experiment subjects’ sleep underwent significant changes throughout the lunar cycle, with disruptions to sleep peaking around the full moon. It took people longer to fall asleep as the full moon approached – an average of 5 minutes longer on full moon nights – and they also spent 30% less time in deep sleep. Their overall sleep time dropped to its lowest levels on full moon nights, when they also reported their lowest sleep quality.


So if you have ever felt that your own sleep suffers when the moon is full, this Swiss study suggests that it’s not just your imagination. Yet no one knows why the moon should be exerting this influence over us. Explanations based on gravity – similar to the tides – seem unlikely, given the weakness of the moon’s gravitational pull. Alternatively, our bodies may incorporate some kind of “counter” that keeps track of the phases of the moon. The reasons for its existence are probably lost in the mists of our own prehistory. Yet its effects make themselves felt as regular as clockwork, every 29.5 days!