Lunar effect

The lunar effect is a purported unproven correlation between specific stages of the roughly 29.5-day lunar cycle and behavior and physiological changes in living beings on Earth, including humans. In some cases the purported effect may depend on external cues, such as the amount of moonlight. In other cases, such as the approximately monthly cycle of menstruation in humans (but not other mammals), the coincidence in timing reflects no known lunar influence.
A considerable number of studies have examined the effect on humans. 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. This has allowed several extensive literature reviews and meta-analyses to be produced, which have found no correlation between the lunar cycle and human biology or behavior.
Contexts.
Claims of a lunar connection have appeared in the following contexts:
Fertility.
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.
Birth rate.
Multiple studies have found no connection between birth rate and lunar phases. A 1957 analysis of 9,551 births in Danville, Pennsylvania, 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, Arizona, 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 seven 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 six additional studies from five different countries similarly showed no evidence of relationship between birth rate and lunar phase.
Blood loss.
It is sometimes claimed that surgeons used to refuse to operate during the full Moon because of the increased risk of death of the patient through blood loss. One team, in Barcelona, Spain, reported a weak 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. This methodology has been criticized, and 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 all have an almost equal number of hospital admissions. The Spanish team 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 on the full Moon.
Human behavior.
Epilepsy.
A study into epilepsy found a significant negative correlation between the mean number of epileptic seizures per day and the fraction of the Moon that is illuminated, but the effect resulted from the overall brightness of the night, rather than from the moon phase per se.
Law and order.
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.
Motorcycle fatalities.
A study of 13,029 motorcyclists killed in nighttime crashes found that there were 5.3% more fatalities on nights with a full moon compared to other nights. The authors speculate that the increase might be due to visual distractions created by the moon, especially when it is near the horizon and appears abruptly between trees, around turns, etc.
Stock market.
Several studies have argued that the stock market's average returns are much higher during the half of the month closest to the new moon than the half closest to the full moon. The reasons for this have not been studied, but the authors suggest this may be due to lunar influences on mood. Another study has found contradictory results and questioned these claims.
Meta-analyses.
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. A 1978 review of the literature also found that lunar phases and human behavior are not related.
Sleep quality.
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."
There are suggestions that the 2013 Cajochen study is faulty because of a relatively small (n=33) sample size and inappropriate controls for age and sex. A 2014 study with larger sample sizes (n1=366, n2=29, n3=870) and better experimental controls found no effect of the lunar phase on sleep quality metrics. A 2015 study of 795 children found a three-minute "increase" in sleep duration near the full moon, but a 2016 study of 5,812 children found a five-minute "decrease" in sleep duration near the full moon. No other modification in activity behaviors were reported, and the lead scientist concluded: "Our study provides compelling evidence that the moon does not seem to influence people's behavior." A study published in 2021 by researchers from the University of Washington, Yale University, and the National University of Quilmes showed a correlation between lunar cycles and sleep cycles. During the days preceding a full moon, people went to bed later and slept for shorter periods (in some cases with differences of up to 90 minutes), even in locations with full access to electric light.
In animals.
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.
Correlation between hormonal changes in the testis and lunar periodicity was found in streamlined spinefoot (a type of fish), which spawns synchronously around the last Moon quarter. In orange-spotted spinefoot, lunar phases affect the levels of melatonin in the blood.
Evidence for lunar effect in reptiles, birds and mammals is scant, but among reptiles marine iguanas (which live in the Galápagos Islands) time their trips to the sea in order to arrive at low tide.
In insects, the lunar cycle may affect hormonal changes. The body weight of honeybees peaks during new Moon. The midge "Clunio marinus" has a biological clock synchronized with the moon.
Spawning of coral "Platygyra lamellina" occurs at night during the summer on a date determined by the phase of the Moon; in the Red Sea, this is the three- to five-day period around the new Moon in July and the similar period in August. "Acropora" coral time their simultaneous release of sperm and eggs to just one or two days a year, after sundown with a full moon.
A relationship between the moon and the birth rate of cows was reported in a 2016 study.
In 2000, a retrospective study in the United Kingdom reported an association between the full moon and significant increases in animal bites to humans. The study reported that patients presenting to the A&E with injuries stemming from bites from an animal rose significantly at the time of a full moon in the period 1997–1999. The study concluded that animals have an increased inclination to bite a human during a full moon period.
In plants.
Serious doubts have been raised about the claim that a species of Ephedra synchronizes its pollination peak to the full moon in July.