The belief that the full moon affects human behaviour is one of the most persistent ideas in human culture. Hospital staff swear emergency rooms fill up during full moons. Police officers report more disturbances. Teachers say children are wilder. Parents notice their babies sleep worse. The word "lunatic" itself derives from luna, the Latin word for moon, encoding the assumption that the moon can drive people mad.
But assumptions are not evidence. Over the past fifty years, researchers have tested nearly every claimed lunar effect with varying degrees of rigour. The results are more interesting than a simple "myth busted" or "science confirms" headline would suggest.
Sleep: The Strongest Signal
This is where the evidence is most compelling. A 2013 study published in Current Biology by Cajochen et al. at the University of Basel found that around the full moon, subjects in a controlled laboratory took five minutes longer to fall asleep, slept twenty minutes less and showed 30% less deep sleep (as measured by EEG). These subjects had no view of the sky and no knowledge that the study was examining lunar effects. The data was originally collected for a different purpose and only later analysed for lunar correlations, reducing the risk of confirmation bias.
A much larger 2021 study published in Science Advances by de la Iglesia et al. examined sleep data from over 500 people across three communities: Indigenous Toba/Qom communities in Argentina (with varying access to electric light) and university students in Seattle. All three groups showed the same pattern: bedtimes were later and sleep duration was shorter in the three to five days before the full moon. The effect was present even in the urban environment with full electric lighting.
The proposed mechanism is moonlight itself. In the days before the full moon, the moon rises in the evening hours, providing ambient light during the time when humans historically would have been winding down for sleep. The researchers suggest this may be an evolutionary adaptation: on moonlit evenings, staying active longer was advantageous for hunting, socialising or travelling.
Verdict: Probable real effect. Multiple independent studies using different methodologies and populations have found consistent, small but measurable changes in sleep patterns around the full moon.
Crime and Aggression: Mostly Myth
Police officers are among the most vocal believers in the lunar effect on crime. Surveys consistently show that a majority of law enforcement officers believe full moons increase criminal activity, violent incidents and general disturbance calls. The belief is deeply embedded in professional culture.
The data does not support it. A comprehensive meta-analysis by Rotton and Kelly in 1985, examining 37 studies covering police records, crisis calls, psychiatric admissions and other indicators, found no consistent relationship between moon phase and human behaviour. A 2019 study of 17,966 cases across 15 years by Schafer et al. found no significant correlation between lunar phases and crime rates.
Why do officers believe it so firmly? Confirmation bias is the most cited explanation. A busy night during a full moon is attributed to the moon. An equally busy night during a quarter moon receives no such attribution. Over a career, the confirming cases accumulate into a vivid but statistically unsupported pattern. The full moon is also visible and memorable, creating a cognitive anchor that ordinary nights lack.
Verdict: Not supported by data. Despite strong anecdotal belief among professionals, systematic studies consistently fail to find a correlation between moon phase and crime rates.
Emergency Room Visits: No Pattern
The "full moon emergency room" effect is perhaps the most widely claimed in medicine. Nurses and emergency physicians frequently report fuller waiting rooms and stranger cases during full moons. The belief is pervasive enough that some hospitals have historically staffed more heavily during full moon shifts.
Study after study has failed to confirm it. A 2004 analysis of 150,999 emergency room records at an Indian hospital found no correlation between admission numbers and lunar phase. A 2011 study of 735,000 ER visits across multiple Australian hospitals found the same. The most generous interpretation of the existing literature is that if any effect exists, it is too small to be distinguishable from normal daily and weekly variation in ER traffic.
Verdict: Not supported. The belief is real. The phenomenon is not.
Birth Rates: Lunar Fertility Myths
The association between the moon and fertility is ancient, partly because the average human menstrual cycle (29.5 days) closely matches the synodic lunar month (29.53 days). This numerical coincidence has fuelled speculation about a biological connection for millennia.
Modern studies have found no meaningful correlation between moon phase and birth rates. A 2005 study by Arliss et al. examining 564,039 births across 62 lunar cycles found no significant variation by moon phase. A 2013 study of all births in North Carolina over a year reached the same conclusion. The occasional study that finds a small correlation has generally not been replicated.
The menstrual cycle length is likely coincidental rather than causal. Other primates have cycle lengths ranging from 21 to 37 days with no consistent relationship to the lunar month. The human average of 29.5 days falls within this range and shows substantial individual variation (cycles commonly range from 24 to 38 days), making any lunar synchronisation implausible at a population level.
Verdict: Not supported. The numerical coincidence between menstrual and lunar cycles does not translate into a measurable effect on birth timing.
Mental Health and Psychiatric Admissions
The "lunatic" connection between the moon and mental illness is the oldest of all lunar effect claims, dating back to at least Hippocrates and formalised in Roman and English law (the Lunacy Act of 1845 in England directly referenced lunar influence). Some historical treatments for mental illness were timed to moon phases.
Modern research has found no consistent correlation. A 2006 meta-analysis by Raison et al. examined all available studies on psychiatric admissions, suicides and crisis calls relative to lunar phases and found no reliable relationship. A 2017 study of 17,507 medical records at a Swiss psychiatric hospital found no association between moon phase and admissions, diagnosis type or length of stay.
One important nuance: while the full moon does not appear to trigger psychiatric crises, sleep disruption can exacerbate existing mental health conditions, particularly bipolar disorder. If the full moon genuinely disrupts sleep (see above), there is a plausible indirect pathway through which some vulnerable individuals might experience worsened symptoms around the full moon. This would be a sleep effect, not a direct lunar effect, but the subjective experience would feel like a moon effect.
Verdict: Direct effect not supported. Indirect effect through sleep disruption is plausible for individuals with sleep-sensitive conditions but has not been demonstrated in controlled studies.
Surgical Outcomes
A belief persists in some traditional and alternative medicine communities that surgery performed during certain moon phases carries greater risk of bleeding or complications. Some patients request scheduling around the lunar calendar. Similar timing traditions exist for haircuts and agricultural activities.
A 2009 study published in Anesthesiology examined 18,039 surgical cases and found no relationship between moon phase and surgical complications, blood loss or outcomes. A 2013 study of cardiovascular surgeries similarly found no lunar effect. The most thorough investigation, a 2011 review of all available evidence, concluded that moon phase is not a relevant variable in surgical planning.
Verdict: Not supported. No evidence that surgical outcomes vary with moon phase.
Animal Behaviour: The Control Group
While most human lunar effects have failed replication, the moon's effects on animal behaviour are well documented and scientifically robust. Coral spawning across the Great Barrier Reef is triggered by the full moon. Doodlebugs (antlion larvae) dig larger traps during new moons. Wildebeest calving in the Serengeti peaks during the full moon. Lion attacks on humans in Tanzania increase in the days following the full moon (when the moon rises later, leaving dark early evening hours when lions hunt but humans are still active).
These effects are generally mediated by light levels rather than gravitational forces. Animals that need darkness for predation, foraging or mating respond to the presence or absence of moonlight. The mechanisms are well understood and do not invoke mysterious forces.
The contrast between robust animal effects and absent human effects makes sense. Humans have spent millennia buffering themselves from environmental cycles with clothing, shelter, artificial light and climate control. The same lunar light that triggers mass coral spawning barely registers in a bedroom with curtains.
The moon probably affects human sleep, modestly and through the simple mechanism of ambient light during evening hours. It does not appear to affect crime, emergency room visits, birth rates, mental health or surgical outcomes by any measurable amount. The gulf between popular belief and scientific evidence is vast, and it persists because confirmation bias is powerful, because the moon is visible and memorable, and because the idea that a celestial body influences our behaviour satisfies something deep in the human need for pattern and meaning. The moon's phases are real. Their effect on human chaos is, by the best available evidence, not.