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The term lunar effect refers to the belief that there is correlation between specific stages of the Earth's lunar cycle and behavior in animals, including humans, that cannot simply be explained by variation in light levels. A considerable number of studies have examined the belief: by the late 1980s, there were at least 40 published studies on the purported lunar-lunacy connection, and at least 20 published studies on the purported lunar-birthrate connection. Several extensive literature reviews and meta-analyses have found no correlation between the lunar cycle and human biology or behavior. One study with strong experimental controls indicates a possible connection between sleep quality and lunar phases, but there has been no independent confirmation of these results to date.
Claims of a lunar connection have appeared in the following contexts:
It is widely believed that the moon has a relationship with fertility due to the corresponding human menstrual cycle, which averages 28 days. However, no connection between lunar rhythms and menstrual onset has been conclusively shown to exist, and the similarity in length between the two cycles is most likely coincidental.
California Grunion fish have an unusual mating and spawning ritual during the spring and summer months. The egg laying takes place on four consecutive nights, beginning on the nights of the full and new moons, when tides are highest. However, this is a well understood reproductive strategy that is more related to tides than it is to lunar phase. It happens to correlate with the lunar phase because tides are highest when the Sun, Earth, and Moon are aligned, i.e., at new moon or full moon.
Three studies carried out between 1959 and 1973 reported a 1 percent increase in births in New York following a full moon. However, multiple studies have found no connection between birth rate and lunar phases. A 1957 analysis of 9,551 births in Danville, PA, found no correlation between birth rate and the phase of the moon. Records of 11,961 live births and 8,142 natural births (not induced by drugs or cesarean section) over a 4-year period (1974-1978) at the UCLA hospital did not correlate in any way with the cycle of lunar phases. Analysis of 3,706 spontaneous births (excluding births resulting from induced labor) in 1994 showed no correlation with lunar phase. The distribution of 167,956 spontaneous vaginal deliveries, at 37 to 40 weeks gestation, in Phoenix, AZ, between 1995 and 2000, showed no relationship with lunar phase. Analysis of 564,039 births (1997 to 2001) in North Carolina showed no predictable influence of the lunar cycle on deliveries or complications. Analysis of 6,725 deliveries (2000 to 2006) in Hannover revealed no significant correlation of birth rate to lunar phases. A 2001 analysis of 70,000,000 birth records from the National Center for Health Statistics revealed no correlation between birth rate and lunar phase. An extensive review of 21 studies from 7 different countries showed that the majority of studies reported no relationship to lunar phase, and that the positive studies were inconsistent with each other. A review of 6 additional studies from 5 different countries similarly showed no evidence of relationship between birth rate and lunar phase.
It is sometimes claimed that surgeons used to refuse to operate on the full moon because of the increased risk of death of the patient through blood loss.[not in citation given] One study found a statistically significant correlation between lunar phase and hospital admissions due to gastrointestinal bleeding, but only when comparing full moon days to all non-full moon days lumped together. The statistical significance of the results disappears if one compares day 29 of the lunar cycle (full moon) to days 9, 12, 13, or 27 of the lunar cycle, which have an almost equal number of hospital admissions. Researchers acknowledged that the wide variation in the number of admissions throughout the lunar cycle limited the interpretation of the results.
In October 2009, British politician David Tredinnick asserted that during a full moon "[s]urgeons will not operate because blood clotting is not effective and the police have to put more people on the street.". A spokesman for the Royal College of Surgeons said they would "laugh their heads off" at the suggestion they could not operate at the full moon.
Two studies found evidence that those with mental disorders generally exhibit periods of increased violent or aggressive episodes during the full moon, but a more recent study found no such correlation. An analysis of mental-health data found a significant effect of moon phase, but only on schizophrenic patients. Such effect are not necessarily related directly to the behaviour of the moon. A study into epilepsy found a significant negative correlation between the mean number of seizures and the fraction of the moon illuminated by the sun, but this correlation disappeared when the local clarity of the night sky was controlled for, suggesting that it was the brightness of the night that influenced the occurrence of epileptic seizures.
A 1978 review of the literature found that lunar phase and human behavior are not related.
Senior police officers in Brighton, UK announced in June 2007 that they were planning to deploy more officers over the summer to counter trouble they believe is linked to the lunar cycle. This followed research by the Sussex Police force that concluded there was a rise in violent crime when the moon was full. A spokeswoman for the police force said "research carried out by us has shown a correlation between violent incidents and full moons". A police officer responsible for the research told the BBC that "From my experience of 19 years of being a police officer, undoubtedly on full moons we do seem to get people with sort of strange behavior - more fractious, argumentative."
Police in Ohio and Kentucky have blamed temporary rises in crime on the full moon. In January 2008, New Zealand's Justice Minister Annette King suggested that a spate of stabbings in the country could have been caused by the lunar cycle.
A reported correlation between moon phase and the number of homicides in Dade County was found, through later analysis, not to be supported by the data and to have been the result of inappropriate and misleading statistical procedures.
A July 2013 study carried out at the University of Basel in Switzerland suggests a correlation between the full moon and human sleep quality. Professor Cajochen and colleagues presented evidence that a lunar rhythm can modulate sleep structure in humans when measured under the highly controlled conditions of a circadian laboratory study protocol without time cues. Studying 33 volunteer subjects, the researchers found that subjective and objective measures of sleep varied according to lunar phase and thus may reflect human circalunar rhythmicity. Stringently controlled laboratory conditions, in a cross-sectional setting, were employed to exclude confounding effects such as increased light at night or the potential bias in perception. Measures of lunar influence on sleep structure, electroencephalographic activity during non-rapid eye movement sleep (NREM), and secretion of the hormones melatonin and cortisol, were retrospectively analyzed. At no point, during and after the study, were volunteers or investigators aware of the posteriori analysis relative to lunar phase. Around full moon it was found that electroencephalogram (EEG) delta activity during NREM sleep, an indicator of deep sleep, decreased by 30%, time to fall asleep increased by five minutes, and EEG-assessed total sleep duration was reduced by 20 minutes. These changes were associated with a decrease in subjective sleep quality and diminished endogenous melatonin levels. Cajochen said: "The lunar cycle seems to influence human sleep, even when one does not 'see' the Moon and is not aware of the actual moon phase." 
A meta-analysis of thirty-seven studies that examined relationships between the moon's four phases and human behavior revealed no significant correlation. The authors found that, of twenty-three studies that had claimed to show correlation, nearly half contained at least one statistical error. Similarly, in a review of twenty studies examining correlations between Moon phase and suicides, most of the twenty studies found no correlation, and the ones that did report positive results were inconsistent with each other.
Believers in the lunar theory suggest several different mechanisms by which the behaviour of the moon could influence the behaviour of human beings. A common suggestion is that, since the moon affects large bodies of water such as the ocean (a phenomenon known as "tidal force"), the moon should be expected to have an analogous effect on human beings, whose bodies contain a great deal of water. However, this is a misconception that fails to take into account differences in scale. The tidal force is in fact very weak and should be expected to exercise no more gravitational pull on the human body than a mosquito. Besides this, the "suggestion" failed to account for the dependence of tides from both the phase of the moon and the time of day. A further suggestion is that positive ions increase in abundance during a full moon and that this should be expected to influence human behavior. However, this is a pseudo-scientific claim. Not only is the increase in frequency extremely slight (much smaller than that caused by air conditioning and air pollution), but ionic charge—positive or negative—has no effect on human behavior, and no physiological effect other than static electric shock.
Believers (David Tredinnick being a prominent example) often support their claims by noting that many police officers, teachers, and nurses have observed a lunar effect in the course of their work. To the extent that nurses and police officers do indeed claim to observe patterns, this is most likely to be explained in terms of confirmation bias: People notice if something dramatic happens during a full moon, but do not notice when nothing dramatic happens; furthermore, dramatic occurrences that do not occur during full moons are typically not counted as evidence against the belief. Believers are further bolstered in their belief through communal reinforcement: The more people talk about the effect, the more people notice spurious relationships.
‘Nocturnal carnivores are widely believed to have played an important role in human evolution, driving the need for nighttime shelter, the control of fire and our innate fear of darkness. We performed an extensive analysis of predatory behavior across the lunar cycle on the largest dataset of lion attacks ever assembled and found that African lions are as sensitive to moonlight when hunting humans as when hunting herbivores and that lions are most dangerous to humans when the moon is faint or below the horizon.’
— C. Packer, A. Swanson, D. Ikanda, and H. Kushnir (2011). Fear of Darkness, the Full Moon and the Nocturnal Ecology of African Lions. PLoS ONE 6(7): e22285. doi:10.1371/journal.pone.0022285
For some 3 - 4 million years, bipedal hominins in the East African Rift valley were evolving in potential conflict and competition with fearsome carnivores including sabre-toothed cats equipped with excellent night-vision. Using the largest data set ever recorded – 1,000 lion attacks on humans across Tanzania between 1988 and 2009 – Craig Packer and his colleagues showed that there is a peak of attacks by lions upon humans during the evening dark hours following full moon. According to Packer, this may help explain why so many myths and superstitions attribute fearsome dangers and nightmarish potencies to the moon. While not all archaeologists accept that lunar periodicity was ever relevant to human evolution, those favouring the idea include Curtis Marean, who heads excavations at the important Middle Stone Age site of Pinnacle Point, South Africa. Marean argues that anatomically modern humans around 165,000 years ago – when inland regions of the continent were dry, arid and uninhabitable – became restricted to small populations clustered around coastal refugia, reliant on marine resources including shellfish whose safe harvesting at spring low tides presupposed careful tracking of lunar phase. Against this background, if Marean is right, humans who ignored or misread the moon might frequently have been drowned.
"With gradual offshore platforms during spring low tides, substantial areas of the intertidal zone are revealed, and these are the most productive and safest shellfish collecting times... Foragers should schedule visits to coastal residential sites at times during the lunar month when spring tides are present and then move slightly inland during neaps to broaden the size of the exploitable terrestrial area."
— Marean, C. 2010. Pinnacle Point Cave 13B (Western Cape Province, South Africa) in context: The Cape Floral kingdom, shellfish, and modern human origins. Journal of Human Evolution 59: 425e-443 .