The belief that the moon exerts an influence on human affairs has survived rather obstinately through history. Hippocrates wrote that "no physician should be entrusted with the treatment of disease who was ignorant of the science of astronomy."1Even when, in the 17th century, Johannes Kepler caused the disciplines of astrology and astronomy to diverge with his discovery that the motions of the planets followed mathematical laws, the belief in the moon's influence lingered. And lingered it has to this day. A study by Rotton and Kelly in 1985 showed that 50% of university students believed that people act strangely during a full moon.2 In 1995, Vance reported that as many as 81% of mental health professionals believed that the full moon alters individual behaviour.3
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All this being said, the association between lunar phases and human behaviour preoccupies us less today than in the past. One obvious explanation is that, before the advent of gas lighting at the beginning of the 19th century, the light of the moon permitted outdoor activities that were otherwise impossible. Full-moon nights are 12 times brighter (under a clear sky) than at first or last quarter, and therefore it is likely that people stayed up later and slept less than the rest of the time.6 Even partial sleep deprivation over the course of a single night can induce mania,6 and it is plausible that sleep disturbance during a full moon may function as a positive feedback once a manic episode has begun in a predisposed individual. Perhaps this lies at the origin of the association between madness and the full moon.
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Modern thought on the correlation between the lunar cycle and human behaviour is divided between those who give credence to a "lunar effect" (i.e., that more "lunacy" occurs during certain phases of the moon) and those who debunk this idea as a "Transylvania effect" (i.e., fanciful thinking).8 In a way, the debate has not changed much in the last 2000 years. In the first century AD, Pliny the Elder thought the full moon gave birth to especially heavy nocturnal dew and caused the brain to become "unnaturally moist," leading to both madness and epileptic attacks.6 This was echoed in the 20th century by Arnold Lieber's notion of "biological tides" by which "the moon, via the effects of gravitational forces on the human organism, causes cyclic changes in water flow among the fluid compartments of the body (extracellular, intravascular, and intraluminal)."9 This theory took shape in the 1970s with Lieber's finding of a statistically significant lunar periodicity for homicides over 15 years, with a peak at full moon and just after the new moon.9 The results for aggression were similar, and the size of the gravitational force was found to be directly proportional to qualitative differences in individual acts of violence: the stronger the force, the more heinous the crimes. Lieber's book How the Moon Affects You became a best-seller and is still in print, although his results have not been replicated.10
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The relation between the full moon and aggression was studied at 5 inpatient settings in Sydney, Australia.16 The "Morrison's hierarchy of violence and aggression" was used as a behavioural scale, and the lunar phases were defined with Poisson regression over 105 weeks. No relation was found, as the trend of increasing violence across the 4 phases did not reach significance. There was also no modification of patterns when weekends and public holidays were taken into account. The investigators concluded that the severity of violent behaviour did not vary with phases of the moon.
Why, then, is the belief in a "lunar effect" still so strong? Explanations include: a lack of understanding of physics, psychological biases (e.g., selective recall or selective perception), sensationalism, and the entertainment value of a belief in lunar influence. The philosopher and poet George Santayana once observed, "Men become superstitious, not because they have too much imagination, but because they are not aware that they have any." Health professionals should carefully consider whether a belief in "lunar mental effects" influences their own professional behaviours. Something to keep in mind when you end up on call under the light of the full moon.
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